Electric motor 8

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Selecting Low Voltage Motor Starters

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Electrical File Reference: EEX21608

For additional information on this subject, contact W.A. Roussel on 874-1320

Engineering Encyclopedia

Electrical Selecting Low Voltage Motor Starters

CONTENTS

PAGE

PHYSICAL ARRANGEMENTS OF MOTOR STARTER ENCLOSURES ...........1 Introduction........ ...........................................................................................1 Common Enclosure Components..................................................................1 Disconnecting Means ...........................................................................1 Lock and Tag Features .........................................................................1 Interlocks and Latches ..........................................................................2 Open Panel Type Enclosures.........................................................................3 Description............................................................................................3 Saudi Aramco Applications ..................................................................4 Enclosed Type Motor Starter Enclosures ......................................................5 Single (Wall-Mount).............................................................................5 Group (Wall-mount) .............................................................................6 Motor Control Centers (MCC’s)...........................................................8 Saudi Aramco Applications ................................................................11 NEMA ENCLOSURE CLASSIFICATION SYSTEM...........................................13 NEMA 1 - General Purpose Enclosures......................................................17 NEMA 12 - Dust-Tight Industrial Enclosures.............................................19 NEMA 3R - Rain-Resistant Enclosures ......................................................20 NEMA 4/4X - Water, Dust-Tight and Corrosion-Resistant Enclosures ......21 NEMA 7 - Hazardous Location Enclosures ................................................22 SELECTING A LOW VOLTAGE MOTOR O/L RELAY.....................................24 Introduction........ .........................................................................................24 Motor Data........ ..........................................................................................24 Full-Load Amperes.............................................................................24 Service Factor .....................................................................................26 Bi-Metallic O/L Relays ...............................................................................27 Components ........................................................................................27 Saudi Aramco DeskTop Standards



Operating Principles ...........................................................................29 Solder-Pot O/L Relays.................................................................................30 Components ........................................................................................30 Operating Principles ...........................................................................31 Solid-State O/L Relays................................................................................32 Components ........................................................................................32 Operating Principles ...........................................................................34 Classes.........................................................................................................36 Class 10 ..............................................................................................37 Class 20 ..............................................................................................37 Class 30 ..............................................................................................37 Types...........................................................................................................37 Type A ................................................................................................37 Type B ................................................................................................38 Temperature Compensation Criteria ...........................................................39 Environmental Conditions ..................................................................39 Ambient ..............................................................................................40 Non-Ambient ......................................................................................40 Pole Arrangements ......................................................................................41 Single-Pole .........................................................................................41 Three-Pole ..........................................................................................41 Other Considerations...................................................................................42 Single-Phasing ....................................................................................42 Process Criticality...............................................................................43 Remote Access Sites...........................................................................43 SELECTING A LOW VOLTAGE MOTOR CONTACTOR .................................44 Motor Contactor Types ...............................................................................44 Air-Magnetic ......................................................................................44 Vacuum...............................................................................................46

Saudi Aramco DeskTop Standards



NEMA Motor Contactor Sizing Criteria .....................................................48 Horsepower.........................................................................................48 Motor Voltage.....................................................................................50 Continuous Current.............................................................................50 Special Criteria ...................................................................................51 Motor Contactor Auxiliary Devices ............................................................53 Contacts ..............................................................................................53 Interlocks ............................................................................................54 Motor Contactor Coil Voltage Ratings........................................................55 SELECTING A LOW VOLTAGE MOTOR DISCONNECT/FAULT PROTECTIVE DEVICE.........................................................................................57 Types...........................................................................................................57 Disconnect Switch With Fuses ...........................................................57 Molded Case Circuit Breakers (MCCBs) ...........................................58 Low Voltage Power Circuit Breakers (LVPCBs) ...............................60 Ratings.........................................................................................................61 Disconnect Switch and Fuses .............................................................61 Molded Case Circuit Breakers (MCCBs) ...........................................64 Low Voltage Power Circuit Breakers (LVPCBs) ...............................68 Combination Motor Starters ...............................................................70 Fuse T/C Characteristics..............................................................................72 Log-Log T/C Paper.............................................................................72 Non-Time Delay .................................................................................72 Time Delay .........................................................................................72 Molded Case Circuit Breaker T/C Characteristics.......................................76 Phase Fault Protection ........................................................................76 Ground Fault Protection .....................................................................80 LVPCB T/C Characteristics ........................................................................81 Phase Fault Protection ........................................................................81 Ground Fault Protection (GFP) ..........................................................84 Saudi Aramco DeskTop Standards



Motor Nameplate Data ................................................................................88 Full-Load Amperes.............................................................................88 kVA Code/Locked-Rotor Amperes ....................................................88 Voltage and Horsepower ....................................................................88 Fault/Starting Currents ................................................................................89 Symmetrical Current...........................................................................89 Asymmetrical Current.........................................................................89 NEC Maximum Settings .............................................................................90 Inverse-Time MCCBs.........................................................................90 Magnetic-Only MCCBs and MCPs ....................................................90 LVPCBs..............................................................................................90 WORK AID 1: RESOURCES USED TO SELECT A LOW VOLTAGE MOTOR O/L RELAY..................................................................................91 Work Aid 1A: NEC Article 430..................................................................91 Work Aid 1B: 16-SAMSS-503 ...................................................................91 Work Aid 1C: Vendor’s Literature, Westinghouse Catalog 25-000 ...........91 Work Aid 1D: Applicable Selection Procedures.........................................91 WORK AID 2: RESOURCES USED TO SELECT A LOW VOLTAGE MOTOR CONTACTOR ..............................................................................96 Work Aid 2A: NEC Article 430..................................................................96 Work Aid 2B: 16-SAMSS-503, Chapter 4..................................................96 Work Aid 2C: Vendor’s Literature, Westinghouse Catalog 25-000 ...........96 Work Aid 2D: Applicable Selection Procedures.........................................96 WORK AID 3: RESOURCES USED TO SELECT A LOW VOLTAGE MOTOR DISCONNECT/FAULT PROTECTIVE DEVICE.......................98 Work Aid 3A: NEC Article 430..................................................................98 Work Aid 3B: 16-SAMSS-503 ...................................................................98 Work Aid 3C: Vendor’s Literature, Westinghouse Catalog 25-000 ...........98 Work Aid 3D: SAES-P-114, Chapter 6.......................................................98 Saudi Aramco DeskTop Standards



Work Aid 3E: Vendor’s Literature, Westinghouse SA-11647, Low Voltage Metal Enclosed Switchgear - Type DS...................98 Work Aid 3F: Applicable Selection Procedures .........................................98 GLOSSARY...... ...................................................................................................101




LIST OF FIGURES Figure 1.

Common Motor Starter Enclosure Components...................................2

Figure 2.

NEMA General Purpose Contactor Rating...........................................4

Figure 3.

Single Wall-Mount Enclosure ..............................................................5

Figure 4.

Group Wall-Mount Enclosure ..............................................................7

Figure 5.

Typical Low-Voltage Motor Control Center ........................................8

Figure 6.

MCC Drawout Unit ..............................................................................9

Figure 7.

Handle Mechanism Locked-Out With Padlock ..................................10

Figure 8.

NEMA Wiring Classes for Motor Control Centers.............................12

Figure 9.

Comparison of Specific Applications of Enclosures for Indoor Nonhazardous Locations ....................................................................14

Figure 10. Comparison of Specific Applications of Enclosures for Outdoor Nonhazardous Locations ....................................................................15 Figure 11. Comparison of Specific Applications of Enclosures for Indoor Hazardous Locations ..........................................................................16 Figure 12. Conversion of NEMA Type Numbers to IEC Classification Designation.........................................................................................18 Figure 13. Example of Full-Load Ampere Range for Various Sizes of Overload Relays .............................................................................25 Figure 14. Maximum Overload Relay Trip Rating Based on Motor Service Factor (S.F.)........................................................................................26 Figure 15. Bimetallic Type Overload Relay ........................................................27 Figure 16. Solder-Pot Type Overload Relay........................................................30 Figure 17. Current Sensing (Heater) Plug-In Module for Solid-State Overload Relay...................................................................................................33




Figure 18. Solid-State Overload Relay Time-Current Curve ...............................35 Figure 19. Typical Time-Current Characteristics for Class 20 and Class 30 Overload Relays .................................................................................36 Figure 20. Typical Air-Magnetic Contactor with O/L Relay ...............................45 Figure 21. Typical Vacuum Contactor.................................................................47 Figure 22. Horsepower Ratings for Three-Phase Single Speed Full-Voltage Magnetic Contactors (Controllers) for Nonplugging and Nonjogging Duty ...................................................49 Figure 23. Typical Auxiliary Contact ..................................................................53 Figure 24. Typical Auxiliary Interlocks...............................................................54 Figure 25. Example of AC Coil Voltage Ratings for NEMA Size 3 and 4 Low Voltage Contactors ...........................................................55 Figure 26. Disconnect Switch ..............................................................................57 Figure 27. Dual-Element Cartridge Fuse .............................................................58 Figure 28. Molded Case Circuit Breaker (MCCB) ..............................................58 Figure 29. Switch Nameplate...............................................................................61 Figure 30. Fuse Label ..........................................................................................61 Figure 31. Disconnect Switch Ratings .................................................................62 Figure 32. Low Voltage Fuse Ratings..................................................................63 Figure 33. MCCB Asymmetrical Factors.............................................................64 Figure 34. Typical MCCB Ratings ......................................................................66 Figure 35. Typical MCP Ratings and Settings.....................................................67 Figure 36. LVPCB Frame and Sensor Ratings ....................................................69 Figure 37. LVPCB Short-Time and Interrupting Ratings ....................................69 Figure 38. Typical Combination Motor Starter Ratings.......................................71




Figure 39. Typical Log-Log Paper.......................................................................73 Figure 40. Non-Time Delay Fuse Characteristics ................................................74 Figure 41. Time Delay Fuse T/C Characteristics .................................................75 Figure 42. Thermal Magnetic MCCB Fault Protection........................................77 Figure 43. Magnetic-Only MCCB Fault Protection.............................................78 Figure 44. MCP Fault Protection .........................................................................79 Figure 45. Ground Fault Protection With Shunt Trip ..........................................80 Figure 46. Long Time Pickup (LTPU) T/C Characteristics .................................81 Figure 47. Long Time Delay (LTD) T/C Characteristics .....................................82 Figure 48. Short Time Pickup (STPU) T/C Characteristics .................................82 Figure 49. Short Delay Time (SDT) With I2t T/C Characteristics.......................83 Figure 50. Instantaneous Trip (IT) T/C Characteristics........................................84 Figure 51. GFP With Window-Type CT..............................................................84 Figure 52. Sample GFPU Code Letters and Settings ...........................................85 Figure 53. Ground Fault Pickup (GFPU) T/C Characteristics .............................85 Figure 54. Ground Fault Time (GFT) With I2t T/C Characteristics ....................86 Figure 55. LVPCB Motor Protection ...................................................................87 Figure 58. NEC Table 430-32..............................................................................92 Figure 59. Problem 1 Motor Nameplate Data ....................................................106 Figure 60. Problem 1 One-Line Diagram...........................................................107 Figure 61. Problem 2 Motor Nameplate Data ....................................................111 Figure 62. Problem 2 One-Line Diagram...........................................................114




PHYSICAL ARRANGEMENTS OF MOTOR STARTER ENCLOSURES Introduction A major component of a motor starter is the enclosure. To properly select low voltage motor starters, it is necessary to understand the physical arrangements of motor starter enclosures. This Information Sheet explains the physical arrangements of enclosures by describing components that are common to all enclosures and by describing various types of enclosures. Common Enclosure Components Motor starter enclosures have several components that are common to all types of enclosures. These components include a disconnecting means, lock and tag features, and enclosure interlocks. Descriptions of these common components are given in the following paragraphs. Disconnecting Means A common component that is included on all types of enclosures is a means of externally operating the disconnect device that is mounted inside of the enclosure. This component is typically a flange mounted handle located on the outside of the enclosure as shown in Figure 1. The handle is mechanically fastened to an operating mechanism that is located inside of the enclosure and that attaches to the disconnecting device (disconnect switch or breaker). The handle provides for external operation of the disconnecting device, and it gives positive visual indication of its status (open or closed). Lock and Tag Features A common component of enclosures that is very important for safety is the provision to padlock the operating handle. This provision allows one or more padlocks to be inserted through a hole in the operating handle to lock it in the “Off” position. The purpose of this feature is to allow the motor starter to be locked in the de-energized position and tagged with a “Warning” tag to provide for safe inspection and maintenance of the motor. The location of this locking provision is identified for the enclosure shown in Figure 1. In addition to the capability of padlocking the operating handle, enclosures also allow padlocking of the cover to prevent access by unauthorized personnel.


1



Interlocks and Latches Another set of components that are common to all types of enclosures is the cover safety interlock. The typical enclosure has two interlocks. These interlocks are illustrated in Figure 1. One is connected between the external operating handle and the enclosure cover to prevent opening of the cover while the handle is in the “On” position. In order to open the cover, the handle must be moved to the “Off” position. However, to allow access by trained and authorized personnel for purposes of special maintenance, an interlock bypass is provided. The second interlock is designed to function when the cover is open. This interlock prevents the breaker or disconnect switch from being operated in the “On” position while the cover remains open. The one exception to the operation of this interlock is that trained and authorized personnel are provided the option of activating the interlock bypass.

Figure 1. Common Motor Starter Enclosure Components


2



Open Panel Type Enclosures Description Motor starters are typically mounted in NEMA-type enclosures. However, for some applications, motor starters are mounted on flat, open panels. In accordance with NEC Article 430-132, motor starters operated at 50 volts or more between terminals must be guarded against accidental contact by mounting in an enclosure or by locating in a controlled room, controlled balcony, or at an elevation of 8 feet or more. In older manufacturing facilities, open panel mounting was normally accomplished by mounting the motor starters on pole-supported slate or micarta panels. These panels, which are sometimes called “electric switchboards”, were also used to mount other electrical controls needed for the facility. The switchboards, which are usually supplied by open-type uninsulated bus, were typically located in a dedicated room where access was allowed only to qualified electricians and to authorized managers. For modern applications, open panel mounting of motor starters is typically accomplished by fastening the starters to a flat, painted, steel panel. The panel is then mounted in a large steel cabinet or in a separate control room. When the starters are mounted in this manner, their continuous current rating is increased in accordance with the NEMA ICS2 contactor ratings shown in Figure 2. With reference to this figure, it is noted that the ratings for open panel mounting are 110% of the ratings for enclosed mounting.


3



Size of Contactor

Enclosed Mounting Continuous Current (amperes, rms)

Open Mounting Continuous Current (amperes, rms)

00

9

10

0

18

20

1

27

30

2

45

50

3

90

100

4

135

150

5

270

300

6

540

600 (Reference NEMA ICS2-210)

Figure 2. NEMA General Purpose Contactor Rating Saudi Aramco Applications Saudi Aramco standards do not permit the use of open panel-type enclosures. In accordance with SAES-P-114, a motor starter for a low voltage motor must be either a combination motor starter or a circuit breaker depending on the horsepower rating of the motor. With reference to a combination motor controller, it is defined by NEMA ICS2-321 as an externally operable circuit-disconnecting means and a magnetic controller mounted in a single enclosure. On the other hand, circuit breakers are by design enclosed in their own case or housing. As a result, both types of low voltage motor starters allowed by Saudi Aramco standards are enclosed.


4



Enclosed Type Motor Starter Enclosures Single (Wall-Mount) The single wall-mount enclosure is the most commonly used type of enclosure. A typical single enclosure, similar to the one illustrated in Figure 3, offers the advantage of placing individual starters at their most convenient location while still providing all of the common component features described above (i.e. disconnecting means, lock and tag features, and enclosure interlocks). Single enclosures are also designated by a NEMA-type number that indicates the environmental conditions for which they are suitable. NEMA enclosure types and classifications are described in the following Information Sheet (NEMA Enclosure Classification System). Single wall-mount enclosures are available from manufacturers in a number of sizes. The required size for an enclosure is recommended by the manufacturer and is determined by the type and size of combination controller to be housed. When needed, extra space can be requested by the user to accomodate field-mounted control components.

Figure 3. Single Wall-Mount Enclosure


5



Group (Wall-mount) A group wall-mount enclosure is essentially several single enclosures designed and manufactured as one unit but with individual internal compartments. The group-type enclosure is designed to save time, space, and expense when installing multiple control devices. Figure 4 shows an example of a group enclosure with four compartments for mounting four combination controllers. Group enclosures are typically partitioned into either four or six compartments. Each compartment is designed to hold a combination starter, incoming or feeder circuit breakers, fusible switches, or other auxiliary devices. The barriers between compartments can be removed to provide oversize spaces allowing for installation of a lesser number of larger size controllers. In addition to the barrier compartments, the group enclosure normally contains internal wiring troughs. Typically, one trough is located at the top and is fitted with power terminal straps for extension to adjoining compartments. Another wiring trough is located along the bottom for interconnecting wiring and outgoing cables. The compartments have hinged doors that are interlocked to prevent opening them when the breaker switch is in the “On” position. In addition, the disconnect operating mechanism can be padlocked in either the “On” or “Off” positions.


6



Figure 4. Group Wall-Mount Enclosure


7



Motor Control Centers (MCC’s) A motor control center (MCC) is a group of combination starters assembled into a single metal enclosure with individual compartments for each starter. Control centers are arranged in straight-line, L-shaped, U-shaped, or back-to-back configurations. Figure 5 shows a typical arrangement of a motor control center in a straight-line configuration.

Figure 5. Typical Low-Voltage Motor Control Center


8



The metal enclosure of the motor control center is built with a single steel-channel frame that has compartment-like spaces for insertion of individual combination starters. The individual compartments of the enclosure share common bus systems and wireways. With regard to the bus systems, a main horizontal bus is installed across the top of the unit to provide threephase power distribution from the incoming line or primary disconnect device to each vertical structure. A vertical bus is mounted in each vertical unit to provide distribution of the main bus power to each of the individual vertical compartments. Completing the arrangement of bus systems is a neutral bus mounted on stand-off insulators across the bottom of each vertical unit and a ground bus mounted across the top of each unit. With regard to the wireways, the enclosure has both vertical and horizontal wireways to provide for convenient servicing and controller change-outs. All wireways are provided with hinged panel covers for easy access and as a barrier to fire. For this type of enclosure, a steel compartment shell, referred to as a drawout case, is provided for each compartment. Figure 6 shows the construction of a typical drawout case. The drawout case, comprised of three sides and a base, serves as a housing for mounting of each starter. Four mounting points on the drawout case allow it to engage guide rails, located near the top of the compartment space, for easy insertion and withdrawal. A quarter turn latch located at the top of the case securely holds it in the compartment after insertion.

Figure 6. MCC Drawout Unit


9



Figure 7 shows the arrangement of a typical handle mechanism that is located on the front of a drawout case. The handle mechanism is designed to operate the controller disconnecting device located inside of the drawout case. Similar to other types of enclosures, the handle mechanism for this enclosure provides common safety features. These features include an interlock that prevents the compartment door from being opened when the handle is in the “On” position. When the compartment door is open and the handle is in the “On” position, an interlock prevents the drawout case from being removed from the compartment. In addition, the handle mechanism can be padlocked in the “Off” position to insure that individual starters are not energized accidentally or by unauthorized personnel during maintenance procedures.

Figure 7. Handle Mechanism Locked-Out With Padlock


10



In addition to the handle mechanism, a control panel is mounted on the front of the drawout case. The control panel allows mounting of pushbuttons, indicator lights and related control devices. The arrangement of mounting both the handle mechanism and the control panel on the front of the drawout case helps to make inspections and maintenance easier. A final feature of this type of enclosure is the compartment door. Each compartment of the motor control center has a separate hinged door that allows the handle mechanism and control panel to protrude through the door when it is closed. The doors are typically secured in the closed position using two quarter turn indicating type fasteners. As described above, an interlock prevents the door from being opened when the handle is in the “On” position. Saudi Aramco Applications With regard to Saudi Aramco application of enclosures, SAES-P-114 requires that a motor controller be either a combination motor starter or a circuit breaker. When the controller is a combination motor starter, the enclosure for the controller is provided by the manufacturer as an integral part of the starter. The provided enclosure is designed and assembled in accordance with NEMA Standards 250 and ICS-6 to meet specific application environmental conditions (NEMA enclosure types and classifications are described in the following Information Sheet). The enclosure provided by the manufacturer also includes the common enclosure components described above (a disconnecting means, lock and tag features, and enclosure interlocks). When the controller is a circuit breaker, the enclosure is provided by one of two means. Either the circuit breaker is designed and constructed with a self-encasing enclosure, or the breaker is designed for mounting inside of a metal-enclosed switchgear compartment. With regard to enclosures applied for low voltage motor control centers (MCC), 16-SAMSS503.4.2 requires that MCC’s be rigid, free-standing, metal-enclosed structures, consisting of vertical sections assembled into a group having a common bus and forming an enclosure to which additional sections may readily be added. The enclosures must be suitable for back-towall or back-to-back mounting. Back-to-back constructions having a common horizontal bus are not acceptable. The MCC cubicle design must be NEMA Class I, Type B, with all ventilation openings suitably filtered or screened with a specified corrosion-resistant material arranged to prevent entrance of rodents and other foreign matter.


11



NEMA Standard ICS2-322.08 describes Class I motor control centers as consisting of mechanical groupings of independent combination motor control units, feeder tap units, other units, and electrical devices arranged in a convenient assembly. The “Type” designation indicates whether wiring between motor control units is allowed and whether unit and/or master terminal blocks are required. Figure 8 shows in summary form the NEMA wiring classes for motor control centers. With reference to this figure, it is seen that Class I does not allow wiring between independent motor control units and that Type B requires that terminal blocks be provided for field wiring to the units.

Class I

Class II

(No interwiring between units.)

(Interwiring between units.)

A. No Terminal Blocks

Type A

-----

B. Unit Terminal Blocks

Type B

Type B

C. Unit and Master Terminal Blocks

Type C

Type C

Figure 8. NEMA Wiring Classes for Motor Control Centers


12



NEMA ENCLOSURE CLASSIFICATION SYSTEM NEMA Standard 250 provides a classification system for enclosures of electrical equipment. The primary purpose of the classification system is to permit potential users to determine: •

The type of enclosure appropriate for the application.

•

The features that the enclosure is expected to have.

•

The tests applied to the enclosure to demonstrate its conformance to the description.

The system provides for enclosures to be designated by a “Type” number that indicates the environmental conditions for which the enclosure is suitable. Applicable type numbers for nonhazardous application include Types 1, 2, 3, 3R, 3S, 4, 4X, 5, 6, 6P, 7, 8, 9, 10, 11, 12, and 13. Type numbers applied to enclosures for hazardous location use include Types 7, 8, 9, and 10. Enclosures covered by this classification system are nonventilated, except that Types 1, 2, and 3R enclosures may be either nonventilated or ventilated. Figures 9, 10, and 11 give a brief overview of the types of enclosures included in the NEMA classification system and the environmental conditions that they protect against. Figure 9 shows an overview comparison of enclosures used for indoor nonhazardous locations, Figure 10 shows a comparison of enclosures used for outdoor nonhazardous locations, and Figure 11 compares enclosures applied to indoor hazardous locations. Detailed descriptions for selected enclosure types are provided in the sections that follow these figures.


13



Types of Enclosures

1*

2*

4

4X

5

6

6P

12

12K

13

Incidental contact with enclosed equipment

X

X

X

X

X

X

X

X

X

X

Falling dirt

X

X

X

X

X

X

X

X

X

X

Falling liquids and light splashing

---

X

X

X

X

X

X

X

X

X

Circulating dust, lint, fibers and flyings

---

---

X

X

---

X

X

X

X

X

Settling airborne dust, lint fibers, and flyings

---

---

X

X

X

X

X

X

X

X

Hosedown and splashing water

---

---

X

X

---

X

X

---

---

---

Oil and coolant seepage

---

---

---

---

---

---

---

X

X

X

Oil or coolant spraying and splashing

---

---

---

---

---

---

---

---

---

X

Corrosive agents

---

---

---

X

---

---

X

---

---

---

Occasional temporary submersion

---

---

---

---

---

X

X

---

---

---

Occasional prolonged submersion

---

---

---

---

---

---

X

---

---

---

Provides a Degree of Protection Against the Following Environmental Conditions

* Note: These enclosures may be ventilated. (Reference NEMA Standard Publication No. 250)

Figure 9. Comparison of Specific Applications of Enclosures for Indoor Nonhazardous Locations


14



Types of Enclosures Provides a Degree of Protection Against the Following Environmental Conditions

3

3R*

3S

4

4X

6

6P

Incidental contact with the enclosed equipment

X

X

X

X

X

X

X

Rain, snow, and sleet

X

X

X

X

X

X

X

Sleet

---

---

X

---

---

---

---

Windblown dust

X

---

X

X

X

X

X

Hosedown

---

---

---

X

X

X

X

Corrosive agents

---

---

---

---

X

---

X

Occasional temporary submersion

---

---

---

---

---

X

X

Occasional prolonged submersion

---

---

---

---

---

---

X

* Note: These enclosures may be ventilated. (Reference NEMA Standard Publication No. 250)

Figure 10. Comparison of Specific Applications of Enclosures for Outdoor Nonhazardous Locations


15



Provides a Degree of Protection Against Atmospheres Typically Containing

Type of Enclosure

Type of Enclosure 9,

7 & 8, Class I Groups

Class II Groups

Class

A

B

C

D

E

F

G

10

Acetylene

I

X

---

---

---

---

---

---

---

Hydrogen, manufactured gas

I

---

X

---

---

---

---

---

---

Diethel ether, ethylene, cyclopropane

I

---

---

X

---

---

---

---

---

Gasoline, hexane, butane, naphtha, propane, acetone, toluene, isoprene

I

---

---

---

X

---

---

---

---

Metal dust

II

---

---

---

---

X

---

---

---

Carbon black, coal dust, coke dust

II

---

---

---

---

---

X

---

---

Flour, starch, grain dust

II

---

---

---

---

---

---

X

---

Fibers, flyings

III

---

---

---

---

---

---

X

---

MSHA

---

---

---

---

---

---

---

X

Methane with or without coal dust

(Reference NEMA Standard Publication No. 250)

Figure 11. Comparison of Specific Applications of Enclosures for Indoor Hazardous Locations


16



NEMA 1 - General Purpose Enclosures All enclosure types included in the NEMA classification system are intended to provide a degree of protection to personnel against incidental contact with the enclosed equipment. In addition to this common protection provided by all enclosures, each enclosure is identified by a Type number that indicates the degree of protection provided to the enclosed equipment against environmental conditions. A NEMA Type 1 enclosure is intended for general purpose indoor applications. It is used primarily to provide a degree of protection against falling dirt in locations where unusual service conditions do not exist. When properly installed, Type 1 enclosures: •

Prevent the insertion of the end portion of a straight rod of specified diameter into the equipment cavity of the enclosure.

•

Provide a degree of protection against limited amounts of falling dirt.

•

Provide suitable rust-resistance protection.

Type 1 enclosures are evaluated to demonstrate their conformance to environmental protection requirements by the following NEMA specified tests: •

Rod entry test (reference NEMA 250.6.2)

•

Rust-resistance test (reference NEMA 250.6.8)

A similar -- but different -- classification system for enclosures is provided by the International Electrotechnical Commission (IEC) in standard IEC-529. Figure 12 shows a comparison of the two enclosure classifications, and it provides for conversion from NEMAtype numbers to IEC classification designations. However, Figure 12 cannot be used to convert IEC classification designations to NEMA-type numbers. The reason Figure 12 cannot be used to convert from IEC designations to NEMA-type numbers is because the tests and evaluations between the two systems are not identical. With reference to Figure 12, it is noted that an IEC enclosure classification designation of IP10 represents a conversion of a NEMA Type 1 enclosure. This means that the NEMA Type 1 meets or exceeds the test requirements of the IEC IP10 enclosure.


17



NEMA Enclosure Type Number

IEC Enclosure Classification Designation

1

IP10

2

IP11

3

IP54

3R

IP14

3S

IP54

4 and 4X

IP56

5

IP52

6 and 6P

IP67

12 and 12K

IP52

13

IP54

(Reference NEMA Standard Publication No. 250) Notes: 1. This comparison is based on tests specified in IEC Publication 529 2. Cannot be used to convert IEC Classification Designation to NEMA-Type Numbers

Figure 12. Conversion of NEMA-Type Numbers to IEC Classification Designation


18



NEMA 12 - Dust-Tight Industrial Enclosures A NEMA Type 12 enclosure is intended for indoor applications. It is used primarily to provide a degree of protection against circulating dust, falling dirt, and dripping noncorrosive liquids. This type of enclosure is not intended to provide protection against such conditions as internal condensation. When completely and properly installed, Type 12 enclosures: •

Prevent the entrance of water under test conditions intended to simulate an environment of light splashes, seepage, and dripping of noncorrosive liquids.

•

Exclude dust under test conditions that are intended to simulate an indoor industrial environment of circulating dust, lint, nonignitable fibers, and noncombustible flyings.

•

Have no knockouts or unused openings.

•

Have doors with provisions for locking or the requirement that a tool be used to gain entry. All closing hardware is captive.

•

When intended for wall mounting, have mounting means external to the equipment cavity. When intended for floor mounting, have closed bottoms.

•

Have gaskets, if provided, that are oil-resistant.

•

Have suitable rust-resistance protection.

Type 12 enclosures are evaluated to demonstrate their conformance to environmental protection requirements by the following NEMA specified tests: •

Drip test (reference NEMA 250.6.3)

•

Circulating dust test (reference NEMA 250.6.5.1.2)

•

Rust-resistance test (reference NEMA 250.6.8)


19



NEMA 3R - Rain-Resistant Enclosures A NEMA Type 3R enclosure is intended for outdoor applications. It is used primarily to provide a degree of protection against rain and sleet and to be undamaged by the formation of ice on the enclosure. This type of enclosure is not intended to provide protection against such conditions as internal condensation or internal icing. When completely and properly installed, Type 3R enclosures: •

Prevent water from contacting live parts, insulation, and wiring under test conditions that are intended to simulate rain.

•

Are undamaged after being encased in ice under test conditions.

•

Prevent the insertion of the end portion of a straight rod of specified diameter into the equipment cavity of the enclosure.

•

Require the use of a tool to gain access to the equipment cavity or have provisions for locking.

•

Are permitted to have a conduit hub or equivalent provision to exclude water at the conduit entrance if the entrance is above the lowest live part.

•

Have provisions for drainage.

•

Have suitable rust-resistance protection.

Type 3R enclosures are evaluated to demonstrate their conformance to environmental protection requirements by the following NEMA-specified tests: •

Rod entry test (reference NEMA 250.6.2)

•

Rain test (reference NEMA 250.6.4)

•

External icing test (reference NEMA 250.6.6)

•

Corrosion protection test (reference NEMA 250.6.9.1)


20



NEMA 4/4X - Water, Dust-Tight and Corrosion-Resistant Enclosures NEMA Type 4 and 4X enclosures are intended for indoor or outdoor applications. Both types are used primarily to provide a degree of protection against windblown dust and rain, splashing water, and hose-directed water. In addition, the Type 4X enclosure is also intended to provide a degree of protection against corrosion. These types of enclosures are not intended to provide protection against such conditions as internal condensation or internal icing. When completely and properly installed, Type 4 and 4X enclosures: •

Exclude water under test conditions that are intended to simulate a hosedown condition.

•

Are undamaged after being encased in ice under test conditions.

•

Are permitted to have a conduit hub or an equivalent provision to exclude water at the conduit entrance.

•

Have mounting means, if provided, that are external to the equipment cavity.

In addition to the above features, Type 4 enclosures have suitable corrosion protection, and Type 4X enclosures, in order to provide a degree of protection against specific corrosion agents, are made of American Iron and Steel Institute Type 304 Stainless steel, polymerics, or materials with equivalent corrosion resistance. Type 4 and 4X enclosures are evaluated to demonstrate their conformance to environmental protection requirements by the following NEMA specified tests: •

External icing test (reference NEMA 250.6.6)

•

Hosedown test (reference NEMA 250.6.7)

•

Corrosion protection test (reference NEMA 250.6.9.1 for Type 4 and NEMA 250.6.9.2 for Type 4X)


21



NEMA 7 - Hazardous Location Enclosures NEMA Type 7 enclosures are intended for indoor use in hazardous locations classified as Class 1, Group A, B, C, or D, as defined in the National Electric Code. When properly installed and maintained, this type of enclosure is designed to contain an internal explosion without causing an external hazard. Type 7 enclosures are designed to be capable of withstanding the pressures resulting from an internal explosion of specified gases and to sufficiently contain the explosion to the extent that an explosive gas-air mixture existing in the atmosphere surrounding the enclosure will not be ignited. Additionally, Type 7 enclosures are designed such that heat generating devices contained within the enclosure will not cause external enclosure surfaces to reach a temperature capable of igniting explosive gas-air mixtures in the surrounding atmosphere. When completely and properly installed, Type 7 enclosures: •

Provide a degree of protection to a hazardous gas environment from an internal explosion or from operation of internal equipment.

•

Do not develop, when equipment is operated at rated load, surface temperatures that exceed prescribed limits for the specific gas corresponding to the atmospheres for which the enclosures are intended.

•

Withstand a series of internal explosion design tests that determine: a. The maximum pressure effects of the gas mixture. b. Propagation effects of the gas mixture.

•

Withstand, without rupture or permanent distortion, an internal hydrostatic design test based on the maximum internal pressure obtained during explosion tests and the specified safety factor.

•

Are marked with the appropriate Class and Group(s) for which they have been qualified.


22



Type 7 enclosures are tested and evaluated in accordance with the applicable portions of: •

ANSI/UL 698, Industrial Control Equipment for Use in Hazardous Locations.

•

ANSI/UL 877, Circuit Breakers and Circuit Breaker Enclosures for Use in Hazardous Locations, Class 1, Groups A, B, C, and D, and Class II, Groups E, F, and G.

•

ANSI/UL 886, Outlet Boxes and Fittings for Use in Hazardous Locations, Class 1, Groups A, B, C, and D, and Class II, Groups E, F, and G.

•

ANSI/UL 894, Switches for Use in Hazardous Locations, Class 1, Groups A, B, C, and D, and Class II, Groups E, F, and G.


23



SELECTING A LOW VOLTAGE MOTOR O/L RELAY Introduction Overload relays are protective devices that guard low voltage AC motors against a variety of abnormal conditions that can overheat motor windings. The overload relays are designed to accomplish this protection by reflecting the heating characteristics of the motors that they protect. The two main components of an overload relay are the relay itself and the heater element. When selecting an overload relay and its heater elements for application, several factors must be considered. These factors include the motor full-load current and service factor and the relay style, class, type, temperature compensation, and pole arrangement. This Information Sheet describes these overload relay selection factors. Note: Work Aid 1 has been developed to help the Participant select an overload relay. Motor Data Full-Load Amperes An important factor used in the selection of the overload relay is the motor nameplate fullload amperes. The amperes marked on the motor nameplate represents the amount of amperes that the motor will draw continuously when delivering its nameplate-rated horsepower at nameplate-rated voltage and frequency. When an overload relay is applied to a motor circuit, it senses the motor line currents either directly or indirectly. For the case where the overload relay senses the current directly, the motor amperes flow directly through the relay and its heater elements. For the case where the overload relay senses the current indirectly, the motor amperes flow through the primary winding of a current transformer (CT) and allow the relay to sense the current via the secondary winding of the CT. Because overload relays sense the line currents of a motor, they are sized according to the amount of amperes that they are capable of handling. Each size of relay is rated with a range of amperes that it can safely and continuously carry. Figure 13 shows an example of the ampere rating range for a few sizes of one particular manufacturer’s overload relay. When selecting an overload relay, the selected size must have a current range that covers the fullload nameplate amperes of the motor to which it is applied.


24



Motor Full-Load Amperes

Overload Relay

0.25 - 26.2

AA13P

26.3 - 45

AA23P

19 - 90

AA33P

19 - 135

AA43P

Figure 13. Example of Full-Load Ampere Range for Various Sizes of Overload Relays In addition to selecting the overload relay, the motor nameplate full-load amperes are also used to select the heater elements that are mounted in the relay block. The heater elements are in series with the power conductors of the relay, and they use the full-load amperes to generate and provide the heat that operates the bi-metallic contact in the relay. Similar to the overload relay, heater elements are sized and selected according to a range of full-load amperes for which they are designed. Note: Work Aid 1 describes the procedures for using the motor full-load amperes to select both the overload relay and its heater elements.


25



Service Factor Another factor that is used in the selection of the overload relay is the motor service factor (S.F.). In accordance with NEMA MG-1, the service factor of an AC motor is a multiplier, which when applied to the rated horsepower, indicates a permissible continuous horsepower loading for the motor. When the voltage and frequency of a motor are maintained at nameplate values, the motor may be loaded up to the horsepower obtained by multiplying the rated horsepower by the service factor. As a result of the maximum continuous horsepower load and, thus, maximum continuous amperes for a motor being affected by the service factor for the motor, the service factor is used in determining the maximum trip rating for the overload relay. In accordance with NEC Article 430, the overload relay must be selected to trip, or it must be rated at no more than the percent of motor nameplate full-load amperes shown in Figure 14. Motor Parameter

Percent of Motor Nameplate Full-Load Amperes (FLA)

Motors with S.F. > 1.15

125%

Motors with temperature rise < 40oC

125%

All other motors

115% (Reference NEC Article 430-32)

Figure 14. Maximum Overload Relay Trip Rating Based on Motor Service Factor (S.F.) Note: Work Aid 1 describes the procedures for using the motor service factor to select both the overload relay and its heater elements. Note: Saudi Aramco specifies only 1.0 S.F. motors.


26



Bi-Metallic O/L Relays Components As schematically shown in Figure 15, a bi-metallic overload relay has two basic components: the relay itself, which contains the bi-metallic actuated contact, and the heater elements. The relay is available as either a single-pole relay or a three-pole (block) relay. The heater elements are constructed of resistance wire or similar material, and they are mounted inside of the relay body. Following is a description of each of these basic overload relay components.

Figure 15. Bimetallic Type Overload Relay


27



Block-type relays are three-pole bimetallic, thermally actuated relays.

The physical construction of the block-type relay includes three sets of motor current-carrying connection terminals mounted on an insulated housing and used for connection to a threephase motor circuit. Contained within the insulated housing (body) of the relay are provisions for inserting and connecting interchangeable heater elements. The relay provides a circuit that allows motor current to flow into the relay connection terminals, through the heater elements, and back out to the motor circuit. Also contained within the insulated housing (body) of the relay is a bimetallic strip that is used to detect the heat generated by the interchangeable thermal elements. The bimetallic strip is mechanically connected to and operates a single-pole, single-throw, snap action switch. The snap-action switch is used to open the control circuit of the starter. The block-type relay is rated in accordance with the range of full-load current that it is capable of carrying, the NEMA size of contactor it connects to, and the interchangeable heater elements designed for use with it. Heater elements are constructed of resistance wire or similar material. They are designed to

be inserted into and connected to the overload relay. Each block-type relay is constructed with three individual compartments to accept three individual heating elements. The heaters are connected to the relay in an arrangement that allows the motor current or CT secondary current to flow directly through them. Individual heating elements are marked with their heater type numbers. Each manufacturer has its own form of designating the heater ranges and ratings. The precise current that a heater element is rated at depends on many factors, such as the number of heaters included in the overload relay and the type of enclosure used for the starter. However, in all cases, heaters are rated based on a range of motor amperes at which they will generate sufficient heat to cause the overload relay to operate. Typically, the heater(s) selected will provide for the overload relay to operate at 115% to 125% of heater rating at an ambient of 40oC.


28



Operating Principles With reference to Figure 15, the operation of the bimetallic type of overload relay can be described by noting that the bimetallic strip is in a straight or unflexed state when it is relatively cool (e.g. when current through the heater is below the rating of the heater). In this position, the normally closed (NC) contact mechanically connected to the bimetallic strip is in its normal (closed) state. With the terminals of the heater connected to the motor circuit, motor current flows through the heater. As current flows, the power consumed by the heater (I2R) is converted to heat that acts directly on the bimetallic strip. In accordance with the inverse time versus current curve for the relay, when the motor current becomes excessive for a sustained period of time, the heat from the heater element will cause the bimetallic strip to deflect and operate the NC contact. Opening the contact, in turn, opens the coil circuit to the starter.


29



Solder-Pot O/L Relays Components Solder-pot overload relays are thermally responsive relays that contain two basic component: a ratchet mechanism that operates a NC contact and a heater element as schematically shown in Figure 16. Following is a description of these basic components.

Figure 16. Solder-Pot Type Overload Relay


30



Ratchet Mechanism - With reference to Figure 16, it is noted that the ratchet mechanism is

comprised of several parts. One part is a small cylinder that contains an alloy (e.g. solder) that will melt due to heat produced by excessive current flow. Within this cylinder is a portion of a shaft that is prevented from turning by the holding action of the alloy. The other end of the shaft is connected to a toothed ratchet wheel that interlocks with a pawl and holds a spring loaded actuator in the loaded position. At the end of the actuator travel path is an NC contact that is operated when the actuator is released and allowed to reach the end of its travel path. Heater - The heater element for this relay is designed in the form of a resistance wire coil that

mounts around the cylinder containing the alloy. Similar to the heater elements used for the bi-metallic type relay, the heater elements for the solder-pot relay are designed to produce a precise amount of heat in direct proportion to the motor current that flows through them. The heater elements are rated in accordance with a range of motor current that will cause the overload relay to operate when excessive motor current flows for a specified period of time. The characteristics of the heater cause the overload relay to operate with an inverse timecurrent characteristic. Operating Principles With reference to Figure 16, the operation of the solder-pot relay can be described by first noting, when the overload relay is connected for operation, that its heater terminals are connected to the motor circuit to allow motor current to flow through the heater. Prior to an excessive flow of current, the alloy in the cylinder is in a solid state allowing the ratchet to hold the actuator in place. When an excessive amount of current flows through the heater for a specific amount of time, the heat generated by the heater element acts directly on the alloy film, melting it at a precise temperature. Once the alloy is converted to a liquid state, the shaft within the cylinder is released allowing it to turn and rotate the ratchet wheel. Rotation of the wheel releases the pawl, which in turn releases the spring-loaded actuator. The released actuator then travels to the NC contact, and operates it to open the coil circuit of the starter.


31



Solid-State O/L Relays Solid-state overload relays monitor motor line current and use semiconductor circuits to determine the heating effects that the level of current will have on the motor and conductors. Components The basic components that make up a solid-state relay are the main body (or block) and a selection of current sensing and special function plug-in modules. Following is a description of these components. Block - The main body (or block) of the solid-state overload relay is physically constructed to

hold three sets of motor current-carrying connection terminals mounted on an insulated housing. When placed in operation, the terminals are connected to the motor circuit to allow motor current to flow through the relay. Contained within the relay body are built-in current transformers that are used to monitor the motor line currents and to translate them into logic level signals. Also contained within the body of the relay is a semiconductor circuit that represents a thermal model of the motor. The thermal model is typically calibrated to have an exponential function with NEMA overload relay Class 10 characteristics. The main body of the relay provides for mounting of selected plug-in modules to build in the amount and type of protection desired. The selection of plug-in modules include current sensing modules and special function modules. The main body of the relay also houses an electromechanical relay contact that is used for opening the coil circuit of the starter. This contact is normally provided as a single-pole single-throw (SPST) NC contact that is closed when the relay is energized and that opens when the relay trips or when control power is removed. In addition to the above features, the solid-state overload relay is ambient-compensated, has both manual and automatic reset capabilities, and indicates overload trip operations through use of light emitting diodes (LEDs). Modules - In place of the type of heater elements used by thermally actuated overload relays,

the solid-state relay uses a plug-in module, shown in Figure 17, that is identified as a current sensing module. This module, sometime referred to as a “heater” module, receives the logic level signals that represent the motor line current, and it determines the relative heating effect.


32



Figure 17. Current Sensing (Heater) Plug-In Module for Solid-State Overload Relay Although the current sensing plug-in module receives logic level signals but does not receive actual motor amperes, it is still rated in units of motor line amperes. Nominal ratings for the current sensing plug-in module range from 0.54 amperes to 150 amperes. When a current sensing module for the solid-state relay is selected, the selection is made in accordance with the percent of full-load current desired to trip the overload relay. Similar to thermal type relays, the solid-state overload relay normally provides for trip operation at 115% to 125% of motor full-load amperes at 40oC. In addition to the current sensing plug-in module that is required for operation of the solidstate overload relays, several special plug-in modules are available for optional selection to provide additional types of protection for the motor. These modules are physically pluggedin, adjacent to, or in tandem with, the current sensing module. The special function modules available for selection include: a phase unbalance module that trips the solid-state relay when line currents are unbalanced, an overtorque protection module that trips the relay when overtorque conditions exist for specific periods of time, a long acceleration module that permits extra acceleration time beyond NEMA Class 10 characteristic, and an underload protection module that senses loss of motor load and, then, trips the relay.


33



Operating Principles Operation of the solid-state overload relay with a properly sized plug-in current sensing module follows the inverse time-current curve shown in Figure 18. Based on this curve, the relay will trip after 7 seconds at 600% full-load amperes for “cold” starts, after 4 seconds at 600% full-load current for “hot” starts, and ultimately at 115% of full-load current for long periods of time. A principle advantage of the solid-state relay over the thermally actuated type is that the solidstate relay operates with a one percent accuracy. The thermal type relay is not as accurate because small variations in tolerances in the mechanical elements of a thermal relay result in large variations in performance. On the other hand, solid-state overload relays are more expensive than thermal types, which make them less popular for smaller, less critical motors and loads. Operation of the solid-state relay is accomplished with the CTs monitoring all three phases of the motor current. The current signals from the CTs are transposed, via solid-state circuits, to a logic level signal and then transmitted to the current sensing plug-in module. The plug-in module, which also contains solid-state circuitry, receives the logic signals and, using the thermal model circuit built into the relay, it determines the corresponding heating effects on the motor. When the current sensing module determines that the flow of current is excessive for a specified period of time (in accordance with Figure 18), it sends a trip signal to the NC electromechanical relay contact in the main relay, operating the contact and thus opening the external coil circuit of the starter.


34



Figure 18. Solid-State Overload Relay Time-Current Curve


35



Classes Inverse-time overload relays are described by time-current characteristics, and, in accordance with NEMA ICS-2, they are designated with a class number indicating the maximum time in seconds at which they will operate (trip) when carrying a current equal to 600% of their current rating. The class number applies to the relay under the condition that overcurrents are balanced in all three phases. NEMA overload relay classes include Classes 10, 20, and 30. Figure 19 shows typical time-current characteristics for Class 20 and Class 30 overload relays. A description of each class follows.

Figure 19. Typical Time-Current Characteristics for Class 20 and Class 30 Overload Relays


36



Class 10 A NEMA Class 10 overload relay operates (trips) in 10 seconds or less when carrying a balanced overload current of 600% of its current rating. Class 20 A NEMA Class 20 overload relay operates (trips) in 20 seconds or less when carrying a balanced overload current of 600% of its current rating. Class 30 A NEMA Class 30 overload relay operates (trips) in 30 seconds or less when carrying a balanced overload current of 600% of its current rating. Types Thermally actuated bi-metallic overlay relays are available as one of two types, either Type A or Type B. Following is a description of each type. Type A The Type A overload relay is designed to protect industrial motors against overload conditions. Using a block-type, bi-metallic design, this relay provides Class 20 operation in either single or three-phase applications. Type A relays are provided with field selectable manual or automatic reset modes. The relay is typically supplied from the manufacturer set for manual reset operation. However, it may be adjusted in the field for automatic reset by loosening the hold-down clamp at the base of the relay, repositioning the reset rod, and re-tightening the clamp. The Type A relay has an inverse time delay trip with adjustable trip rating of the heater element over a + 15% range (approximately 85% to 115%) of its rating. This feature permits adjustment of the desired protection level and is accomplished by turning an adjustment knob located on the relay body. Positive visual indication of a trip operation of the relay is provided by a trip indicator that projects out of the relay. The relay is provided with a standard SPST NC snap-action contact for control of the contactor coil circuit or circuit breaker trip circuit. SPDT NO-NC contacts are available as a factory option. Another contact option for the Type A relay is a factoryavailable alarm contact.


37



The Type A relay is available as either ambient-compensated or non-compensated. Ambientcompensated relays have the advantage of providing the same trip characteristics in ambient temperature from -40oC to +77oC. Compensated and non-compensated relays are generally identified by the color of their reset rod. For the Type A overload relays, interchangeable thermal heater elements for single-pole and block-type relays are available to cover motor full-load currents from 0.29 to 133 amperes in approximately 10% steps. Type B Using a block-type, bi-metallic design that provides Class 20 operation in either single or three-phase applications, the Type B overload relay is similar to the Type A overload relay in that it is also designed to protect industrial motors against overload conditions. Additional similarities of the Type B with the Type A relay include: available ambientcompensated and non-compensated models, inverse time delay trip operation, standard SPST NC snap-action control contact, factory-available SPDT NO-NC contacts, visual trip indicator and available interchangeable thermal heater elements rated to cover motor full-load currents from 0.29 to 133 amperes in approximately 10% steps. The basic differences of the Type B relay with respect to the Type A relay is that Type B relays are furnished only with manual reset capabilities, they have no trip adjustment knob, they provide a mechanical trip bar to manually check the contact operation of the relay, and they use a different reset-bar color code to indicate compensated and non-compensated relays.


38



Temperature Compensation Criteria Environmental Conditions In the selection of overload relays, it is important to note and consider temperature environmental conditions. Following are conditions that should be considered. Motor-Ambient - In accordance with NEMA MG-1, the ambient temperature rating of the

motor is the maximum temperature of the medium and gases surrounding the motor that the motor is designed to operate in and to meet the ratings of its nameplate. Increased ambient temperature will cause an increase in motor operating temperature, which in turn presents a risk to the motor. In the selection of overload relays, NEC Article 430 addresses the consideration of motor temperature rise and thus motor ambient temperature by requiring that overload relay trip ratings be limited based on rated motor temperature rise. In accordance with NEC Article 430, overload trip settings are to be limited to a maximum of 115% of motor full-load current for motors with a temperature rise greater than 40oC. Starter-Ambient - The ambient operating temperature of the starter should also be considered.

Starters operating in a constant ambient temperature that is within the rating of the overload relay will allow the relay to operate properly. This operation will provide for consistent and acceptable protection of the motor. For this condition, it is not a requirement to use a temperature compensated overload relay. Severe Environments - For some cases, a starter and its overload relay may be located in one

area where the ambient temperature varies, while the motor is located in a different area where the ambient is constant. The varying ambient temperature at the starter can result in improper operation of the overload relay. This operation will cause the protection of the motor to be affected. For this condition and similar conditions, where ambient operating temperature for the starter and the overload relay vary, it is important to use an overload relay that has ambient temperature compensation.


39



Ambient In accordance with NEMA ICS-2, an overload relay identified as ambient temperaturecompensated indicates that the ultimate current that causes the relay to trip remains essentially unchanged over a designated range of ambient temperatures. The important feature of an ambient-compensated overload relay is that motor overload protection is provided with substantially the same trip characteristics in ambient temperatures that vary. Overload relays typically provide ambient temperature compensation for temperatures ranging from -40oC to +75oC. In thermally actuated overload relays, temperature compensation is typically accomplished through use of a compensating bi-metal that is responsive only to heat generated by motor current that is passing through the heater element. The bi-metal maintains a constant “travel to trip” distance that is independent of ambient conditions. In this way, the operation of the relay remains essentially unchanged by any change in ambient temperature. The compensating feature is fully automatic, and no adjustments are required for its use when it is supplied with the overload relay. An ambient-compensated overload relay should be used whenever the control is located in a varying ambient temperature area and whenever the motor that it protects is in a constant ambient temperature. Non-Ambient Non-ambient compensated overload relays are relays that do not have built-in features to automatically compensate for varying ambient temperatures. Whenever the overload relay is located in an area with a constant temperature, or whenever it is located in the same area as the motor, compensation may not be necessary. Note: 16-SAMSS-503.5 requires overload relays to be ambient temperature-compensated.


40



Pole Arrangements Single-Pole Thermally activated overload relays are available as single-pole or three-pole arangements. Single-pole overload relays can be used for application on single-phase circuits, or three individual single-pole units can be combined for use on a three-phase application. The single-pole unit works as an independent overload relay with its own heater element and its own NC contact to open the starter coil circuit. Selection of a single-pole unit is accomplished in the same manner as selection of a three-pole block unit, with the selected relay rating and heater rating being based on full-load current. When three single-pole units are applied to a three-phase application, the individual NC contacts of the three units are connected in series to allow any one of the three to open the starter coil circuit. The major advantage of selecting three single-pole units for a three-phase application is that the arrangement provides improved protection against a single-phasing condition, where one phase of the three-phase circuit becomes open. The disadvantages of using three single-pole units for a three-phase application in place of a single three-pole block are increased cost and increased space requirements. Note: 16-SAMSS-503.5 requires thermally actuated overload relays to be three-pole block type. Three-Pole The use of a single three-pole overload relay for three-phase applications is the arrangement that is commonly used. This arrangement provides for the three current carrying poles of the relay to be mounted in the same insulated housing. The relay contains only one NC contact for use in opening the starter coil on a relay trip. With the three-pole arrangement, overload relays can be designed to work with one, two, or three heater elements. Most modern thermally activated overload relays are designed to use three separate heater elements. The body of the relay is designed to allow mounting and connection of each heater in its own compartment, with the heat generated by all three heaters acting on the bi-metallic strip that operates the relay NC contact. The advantages of the three-pole arrangement are that it is compact, economical, and efficient. The disadvantage is that it is not able to provide reliable protection against a singlephasing condition.


41



Other Considerations Single-Phasing Single-phasing is a conditions that occurs when one phase of a three-phase circuit supplying a motor becomes open and allows the motor to operate as a single-phase motor. For this condition, the current in the phase that opens goes to zero while the current in the other two phases increases. Operating in this unbalanced condition results in overheating of the motor and can lead to damage or failure of the insulation if not detected quickly enough. The potential problem when this condition occurs and a single three-pole block-type thermal overload relay is connected in the circuit is that the relay may not be able to detect the condition and operate. The operation of the relay depends on the combined heat generated by all three heater elements. With the relay operating with one phase open, the heater in the open phase will not generate any heat, and even though the current in the other two phases has increased, the increased heat of the two elements may not be sufficient to result in a total amount of heat that will activate the bimetallic strip and operate the relay. Alternatively, if the single-phasing (or open-phase) condition occurred in a circuit using three single-pole relays, each of the relays in the two conducting phases would immediately detect the increase in current and, in accordance with its time-current curve, cause its relay to operate. The three-pole solid-state relay does not have the same problem as the thermal relay in detecting an unbalanced current condition. For the solid-state relay, a special function plug-in module is available to detect unbalanced current conditions. Modules are available to trip on detecting either a maximum of 10% current unbalance or 20% current unbalance.


42



Process Criticality When overload relays are selected and applied to nominally non-critical operating processes, the usual choice is to select the less accurate, but more economical thermally activated type of overload relay. For these applications, an occasional false trip due to the less accurate relay will not result in significant expense or loss of production. However, when an overload relay is selected for a critical process, where priority must be given to maintaining the process operational, the selection should not be made on economy. For this type of application, the more expensive but more accurate solid-state overload relay should be selected. In this case, owing to the accuracy of the overload relay, the advantage will be a minimum of false trips. Solid-state relays, owing to their more complex design using solid-state components, are more expensive to purchase than are the more simply constructed thermal relays. However, the solid-state overload relay operates with greater accuracy than does the thermal type. Remote Access Sites Another consideration when selecting relays is to determine whether the overload relay will be placed in a local or in a remote location. This determination can contribute to whether a manual or automatic reset is selected for the relay. When overload relays are placed in an area that is local and accessible, the normal and accepted practice is to select a manual reset for the relay. Selection of a manual reset provides an opportunity, following a relay trip, for an operator to inspect a motor installation to determine the cause of the trip, and to establish that conditions are safe and acceptable for resetting and restarting. However, there may be cases, where the overload relay is placed at a remote and inconvenient-to-reach location. In addition, operating conditions for the motor may be such that no danger or hazard is presented for an automatic restart following both a relay trip and a cooling period. Under these conditions, it may be an advantage to select an overload relay with an automatic reset. Note: 16-SAMSS-503.5 requires that overload relays be of the manual type reset unless otherwise specified.


43



SELECTING A LOW VOLTAGE MOTOR CONTACTOR A major component in all motor starters is the contactor. The contactor is essentially an onoff switch that is operated by electromechanical means and that controls the flow of current to the motor. When selecting a contactor for application in a motor starter, several factors must be considered. These factors include the type of contactor to be selected (air-magnetic or vacuum), the size of contactor required for the application, the need of contactor auxiliary devices for operation of the control circuit, and the proper contactor coil voltage rating. This Information Sheet describes these contactor selection factors. Note: Work Aid 2 has been developed to help the Participant select a contactor. Motor Contactor Types Air-Magnetic The air-magnetic contactor is the most common type of contactor selected for motor starter applications. Figure 20 shows a typical NEMA air-magnetic contactor with an overload relay connected to its load terminals. This type of contactor is generally selected because it is economical and easy to maintain and because it has a versatile design that provides for accommodating a great many variations in the method of control. The electrical portion of the contactor consists of an electromagnet, a coil, and a moving armature or crossbar. Moving and stationary contacts, arranged in sets or poles, carry the motor current. Air-magnetic contactors are often provided with three poles or sets of contacts. However, other configurations, such as two, four, or five poles are available. When power is applied to the contactor coil, magnetic flux is created in the electromagnet. The magnet then attracts the armature, pulling the moving contacts into the stationary contacts and allowing power to flow through the contacts to the motor. The air-magnetic contactor must be able to close, carry, and open normal motor current. As a result, the contactor is rated in accordance with the size of load that it must control. NEMA standards provide two methods of rating the air-magnetic contactor. One is a rating based on horsepower and the other is a rating based on motor full-load and locked-rotor current. Low-voltage air-magnetic type contactors are designated by NEMA (and available from manufacturers) in sizes 00 to 9 with horsepower ratings from 1.5 hp to 1600 hp. Note: 16SAMMSS-503.4.4 requires that air-magnetic contactors be selected based on horsepower rating.


44



Figure 20. Typical Air-Magnetic Contactor with O/L Relay


45



Vacuum When selecting the type of contactor to use in a low-voltage motor starter, another choice is a vacuum type. Figure 21 shows a typical three-pole vacuum contactor. The vacuum type contactor offers several advantages. These advantages include a compact, lightweight design and a long service life. The most important of these advantages to consider is the long service life. With respect to air-magnetic contactors, service life is typically measured in tens-ofthousands of operations. But in the case of vacuum contactors, service life is typically measured in hundreds-of-thousands of operations. However, the comparision of a vacuum contactor with an air-magnetic contactor of the same rating, reveals that the vacuum contactors cost more. The vacuum contactor is constructed with its main contacts sealed inside ceramic tubes from which all air has been evacuated (i.e. the contacts are in a vacuum). No arc boxes are required, because any arc formed between opening contacts in a vacuum has no ionized air to sustain it. The arc simply stops when the current goes through zero as it alternates at line frequency. The arc usually does not survive beyond the first half-cycle after the contacts separate. As a result of the vacuum’s limiting the amount of arcing, the rate of contact wear is reduced and contact life is increased. The ceramic tube with the moving and stationary tubes enclosed is called a vacuum interrupter, or bottle. There is one bottle for each pole of the contactor. A two-pole contactor has two bottles, and a three-pole contactor has three bottles. A metal bellows (like a small, circular accordion) allows the moving contact to be closed and pulled open from the outside without letting air into the vacuum chamber of the bottle. Both the bellows and the metal-toceramic seals of modern bottles have been improved to the point that loss of vacuum is no longer a cause for excessive concern. Aside from the difference in contact and interrupting medium (vacuum versus air) design, the vacuum contactor is used and applied in the same manner as an air-magnetic contactor. As a result, low-voltage vacuum contactors are designated by NEMA according to the same tables as used to size and rate air-magnetic contactors. NEMA sizing and rating criteria are described in the following section.


46



Figure 21. Typical Vacuum Contactor


47



NEMA Motor Contactor Sizing Criteria Horsepower When selecting a contactor, an important selection factor to consider is the size and rating of the contactor required for the application. In accordance with NEMA ICS-2, contactors (controllers) are rated by means of two methods. One rating is based on horsepower, and the other rating is based on motor full-load and locked-rotor current. The method of rating contactors based on horsepower is the one more rating that is commonly used and the one rating that is required by 16-SAMSS-503.4.4. Because both the full-load and locked-rotor currents are a function of the horsepower rating at a specified voltage, motor contactors (controllers) are rated for the maximum horsepower that they can safely handle at these voltages. The motor contactors (controllers) are classified by a size number, and they are rated in horsepower. Figure 22 shows the maximum horsepower ratings for three-phase, single-speed full-voltage magnetic contactors for nonplugging and nonjogging duty as designated by NEMA. As the NEMA size classification increases, so does the physical size of the contactors (controllers), because larger contacts are needed to carry and break the higher motor currents, and heavier mechanisms are required to open and close the contacts. The NEMA size horsepower ratings shown in Figure 22 are based on the mechanical and electrical requirements for starting a NEMA design B or C motor that has normal acceleration time and normal start/stop duty. If greater than normal duty is required such as motor jogging, long acceleration time, or dynamic braking, a controller of larger than normal size is used. Tables showing the recommended sizes and horsepower ratings for greater than normal duty are given in NEMA ICS-2.321.


48



Maximum Horsepower NEMA Size

Cont. Current Rating, Amps

200 V @ 60 HZ

230 V @ 60 Hz

460 V or 575 V @ 60 Hz

Service-Limit Current Rating Amperes

00

9

1.5

1.5

2

11

0

18

3

3

5

21

1

27

7.5

7.5

10

32

2

45

10

15

25

52

3

90

25

30

50

104

4

135

40

50

100

156

5

270

75

100

200

311

6

540

150

200

400

621

7

810

-----

300

600

932

8

1215

-----

450

900

1400

9

2250

-----

800

1600

2590

(Reference: NEMA Standard ICS-2-321)

Figure 22. Horsepower Ratings for Three-Phase Single Speed Full-Voltage Magnetic Contactors (Controllers) for Nonplugging and Nonjogging Duty


49



Motor Voltage Another factor that must be considered when selecting a contactor is the voltage rating required for the contactor. Low voltage contactors are designed for service on circuits rated to 600 VAC. However, for a given NEMA size contactor, the horsepower rating for the contactor is dependent on the voltage level at which the contactor is applied. With reference to the table of horsepower ratings shown in Figure 22, it is seen that for a given NEMA size contactor, its horsepower rating is reduced when applied at the lower voltage levels. For example, a NEMA size 1 contactor is rated to control an AC induction motor with a maximum nameplate rating of 10 horsepower at a nameplate voltage rating of 460V or 575V. However, the same NEMA size 1 contactor, when it is operated at a voltage of 200 VAC or 230 VAC, is rated to control only a 7.5 horsepower motor. When selecting a contactor, it is necessary to use both the motor nameplate voltage and the motor nameplate horsepower for the selection process. Continuous Current When a contactor is being selected, another factor to consider is the continuous current rating of the contactor. In accordance with NEMA ICS-2-321, each NEMA size contactor is designated with a continuous current rating. This rating represents the maximum rms current, in amperes, which the contactor (controller) is permitted to carry on a continuous basis without exceeding the temperature rises permitted for the contactor. For example, with reference to Figure 22, it is seen that the maximum rated continuous current for a NEMA size 1 contactor is 27 amperes. When selecting a contactor, this value should be compared with the continuous full-load current rating for the motor. One exception for the continuous current rating of the contactor, is the “service-limit current rating”. The service-limit current represents the maximum rms current, in amperes, which the contactor is permitted to carry for protracted periods in normal service. At service-limit current ratings, temperature rises are permitted to exceed those ratings that are obtained by testing the contactor at its continuous current rating. For example, with reference to Figure 22, it is seen that the service-limit current rating for a NEMA size 1 contactor is 32 amperes. This service-limit current rating implies that the contactor may be used at this current level for reasonable periods during normal service (i.e., the high-current intervals of load cycles, long acceleration times, short periods of dynamic braking, etc.), however, it is expected that the temperature rise of the contactor will exceed its continuous current temperature rise.


50



Special Criteria When selecting a contactor, there are other special factors that may exist for the intended application. When these factors exist, they must also be given consideration. Special factors that may exist for an application include long-acceleration times, dynamic braking duties, greater than normal starting duties, and greater than normal contact wear. When these conditions are found to exist, the condition must be examined to determine the expected level of duty above normal rating for the contactor and a larger size contactor, capable of handling the increased duty, must be selected. As a general rule, when one or more of these conditions is known to exist, a contactor that is one size larger than normal is selected. Following is a brief description of the above identified special factors that may require consideration when selecting a contactor. Long-Acceleration Time - One special factor that may exist for a contactor application is a

longer than normal motor acceleration time. During the period that a motor accelerates from standstill to full speed, the motor draws a current that is greater than nameplate full-load amperes. Typical initial starting current levels for induction motors are 600% of full-load nameplate amperes. In accordance with NEMA ICS-2-321.41, a new Class A contactor (air-magnetic contactor, 600 volts or less) with overload relays, must be capable of withstanding for the time necessary for its overload relays to trip the thermal stresses caused by a current flow of 6.4 times the current rating of its highest rated overload relay. Also, in accordance with NEMA ICS-2-321.41, a new Class A contactor without overload relays, must be capable of withstanding for 20 seconds the thermal stresses caused by a current that is 8 times the current corresponding to the horsepower rating of the contactor. When a contactor is being selected, the required acceleration time and level of current flow for the intended application should be reviewed in accordance with the above referenced NEMA requirements. When the expected time and current exceed the above specified NEMA limits, a larger size contactor with the required capability should be selected.


51



Dynamic Braking - Another special factor that may exist for a motor application is the

requirement of the motor to be used for dynamic braking. When used for this purpose, the motor may be required to carry higher than normal nameplate current for a period of time. When a contactor is being selected and if it is known that a motor will be used for this type of service, an inspection should be made to determine the level and duration of current in regard to NEMA requirements for a Class A contactor. When the NEMA limits are exceeded by the application requirements, a larger size contactor should be selected. Starting Duties - Under normal starting conditions, an AC induction motor is expected to draw

approximately 6 times normal current for the starting period. However, when a motor is accelerated from standstill to full speed with its shaft mechanical load fully applied, the current drawn by the motor can be larger. When it is determined, during the process of selecting a contactor, that a motor must be started with its full mechanical load being applied, then an inspection should be made to determine the duration of the load and the maximum current for the starting period. For this condition, as for the conditions of dynamic braking and long acceleration time, a larger contactor size should be selected in accordance with the NEMA limits for a Class A contactor. Contact Life

Another special factor that may exist for a contactor application is the requirement for greater than normal interrupting duty. Under this condition, contact wear will exceed normal wear rates and contact service life will be shortened. The service life of the interrupting contacts on a contactor is directly related to the amplitude of arcing current interrupted and the number of times interruption is required. When greater than normal interrupting service is expected for the contactor, a larger NEMA size contactor should be selected. The general rule is to select one NEMA size larger than normal.


52



Motor Contactor Auxiliary Devices During the selection of a contactor, auxiliary devices are available for selection and inclusion on the contactor. Consideration should be given to the contactor application and the control circuit arrangement to determine if these items are needed and if they should be selected. Two auxiliary items that may be considered are auxiliary contacts for the contactor and interlocks. Following is a description of these items. Contacts Depending on the complexity of the control circuit to be used for the contactor being selected, additional auxiliary contacts may be required in addition to the standard ones provided with the contactor. For this case, manufacturers typically offer one or more types and sizes of auxiliary contacts that can be added to the contactor. Some of these auxiliary contacts can be assembled to the contactor in the field while others may require factory assembly. Figure 23 shows one manufacturer’s offering of one type of auxiliary contact that can be added to size 00 through size 1 contactors at the factory or in the field. The auxiliary contact shown in Figure 23 can be provided as a NO or NC contact, and it can be selected with either an 18 ampere or a 27 ampere continuous current rating.

Figure 23. Typical Auxiliary Contact


53



Interlocks When more than one contactor is to be selected for a single purpose application, such as reversing or multi-speed applications, it is necessary to provide interlocks between the contactors to prevent one from closing before the other has opened. When contactors are selected for these types of applications, the manufacturer normally installs the required mechanical and/or electrical interlocks at the factory. However, for some applications it may be necessary to separately select either mechanical and/or electrical interlocks for installation at the factory or installation in the field. Figure 24 shows examples of a typical mechanical interlock and a typical electrical interlock.

Figure 24. Typical Auxiliary Interlocks


54



Motor Contactor Coil Voltage Ratings When a low voltage motor contactor is being selected, another important factor to consider is the voltage rating of the coil for the contactor. This rating is the voltage that must be applied to the contactor coil in order to operate the contactor. The coil voltage rating is selected to be equal to the voltage rating of the motor starter control circuit. Because there are many different voltage rating for control circuits, low voltage contactors are available with a wide selection of AC and DC coil voltage ratings. With reference to contactors with AC voltage coils, manufacturers typically offer coil voltage ratings from 24 volts AC to 600 volts AC in a number of steps. As an example, Figure 25 shows the standard AC coil voltage ratings offered by a typical manufacturer for NEMA size 3 and 4 contactors. Other voltage ratings are usually available as a special order. In accordance with NEMA Standard ICS 2-110, these alternating current-operated contactors must be able to withstand 110 percent of their rated voltage continuously without injury to the operating coil, and they must close successfully at a minimum of 85 percent of their rated voltage. NEMA Contactor Size

Cont. Rating Amperes

AC Coil Volts

3

90

120

3

90

208

3

90

240

3

90

480

3

90

600

4

135

120

4

135

208

4

135

240

4

135

480

4

135

600

Figure 25. Example of AC Coil Voltage Ratings for NEMA Size 3 and 4 Low Voltage Contactors


55



With reference to contactors with DC voltage coils, manufacturers typically offer coil voltage ratings from 24 volts DC to 250 volts DC in a selection of steps. Voltage ratings offered for DC coils typically include 24, 48, 125, and 250 VDC. In accordance with NEMA Standard ICS 2-110, DC-operated contactors must be able to withstand 110 percent of their rated voltage continuously without injury to the operating coil, and they must close successfully at a minimum of 80 percent of their rated voltage.


56



SELECTING A LOW VOLTAGE MOTOR DISCONNECT/FAULT PROTECTIVE DEVICE Types There are basically three types of low voltage motor disconnect/fault protective types permitted by the National Electric Code (NEC). •

disconnect switch with fuses.

•

molded case circuit breakers (MCCBs).

•

low voltage power circuit breakers (LVPCBs).

Note: Although fuses are permitted by the NEC, SAES-P-114 specifies MCCBs or LVPCBs for low voltage motor disconnect/fault protection. Disconnect Switch With Fuses The disconnect switch serves the NEC Article 430, Section I, purpose of opening all ungrounded conductors of the motor. Figure 26 describes a motor disconnect (safety) switch. Switch disconnect ratings will be discussed later in the Module.

Figure 26. Disconnect Switch


57



The fuses provide the short circuit and ground fault protection for the motor branch circuit. Figure 27 describes a dual-element (DE) fuse used for protection of low voltage motors.

Figure 27. Dual-Element Cartridge Fuse Molded Case Circuit Breakers (MCCBs) The MCCB gets its name from the material (plastic) and manufacturing process (molded) used to make the frame (case) of the breaker. Figure 28 describes a MCCB. The MCCB serves both as the disconnect and the fault protection for a motor.

Figure 28. Molded Case Circuit Breaker (MCCB)


58



Inverse-Time (Thermal-Magnetic) MCCBs have a thermal-magnetic tripping action. The current

path within the breaker is through a bimetallic strip. A bi-metal consists of two strips of metal that is bonded together. Each strip has a different thermal rate-of-heat expansion. As the current passes through the bi-metal, the bi-metal strip heats up and bends. Greater current passing through the bi-metal will generate more heat, resulting in faster bending of the strip. The bi-metal continues to bend until it moves far enough to mechanically unlatch the breaker mechanism, allowing the breaker to open. This thermal action is called an inverse-time characteristic (as the current increases, the time to trip is less). For high fault currents, the thermal action is too slow to protect the downstream devices, therefore a magnetic trip action is used. The magnetic trip action functions by use of an electromagnet in series with the load current. When a short circuit occurs, the fault current passing through the circuit causes the electromagnet in the breaker to attract the armature, initiating an unlatching action. This magnetic trip response is instantaneous. By definition instantaneous means “no intentional time delay.” The magnetic action is usually adjustable within a range (5-10x) for large frame MCCBs, where x is the breakers’ ampere trip (AT) rating. Magnetic Only MCCBs are identical to the inverse-time MCCB except that the thermal trip

action is eliminated. Magnetic trip MCCBs are often used for motor fault protection because the NEC also requires a separate device to provide overload protection for the motor. Motor Circuit Protectors (MCPs) are identical to magnetic-only MCCBs except for the ratings

label. Magnetic-only MCCBs have an interrupting rating that is the same as the rating for thermal-magnetic breakers (e.g., 14 kA, 25 kA, 65 kA). MCPs do not have a stand-alone interrupting rating; they are rated as an assembly, which is called a combination motor starter, consisting of overloads, a contactor, and a fault/disconnect device. The other minor difference is that the MCP adjustments must be listed in amperes, whereas the magnetic-only MCCB adjustments are typically listed in multiples of the trip (continuous current) rating.


59



Low Voltage Power Circuit Breakers (LVPCBs) Low-voltage power circuit breakers are more rugged and more flexible and generally have higher ratings than the molded-case circuit breakers. The designation power circuit breaker was adopted after the introduction of the molded-case breaker to differentiate between the two types. The term unfortunately does not adequately describe these breakers because all breakers in reality are power breakers in that they make and break power circuits. The term power presumably was chosen because these breakers can be used to handle large blocks of power up to 4000 amperes at 600 volts, three-phase, whereas the molded-case breakers originally could only handle loads up to 600 amperes. Note: The National Electrical Manufacturers Association defines the low-voltage power circuit breaker as one for use on circuits rated 1000 volts alternating current and below, or 3000 volts direct current and below, but not including molded-case breakers. The power circuit breaker has an open-type heavy steel frame upon which the components are mounted, making them more readily accessible. These breakers tend to be heavier, larger, and more costly than molded-case breakers. Fixed breakers are available for mounting in individual enclosures, but generally the breakers are of the drawout type for mounting in metal-enclosed switchgear. See Work Aid 3E (Handout 8) for a physical description of a LVPCB. The breakers are closed by means of the two-step, stored-energy spring mechanism. Manual operation is accomplished by first compressing a heavy spring by means of the operating handle. With the closing spring compressed, the breaker can be closed at any time by pushing the close button mounted on the breaker faceplate, which mechanically releases the spring. Electrical operation uses an electric gear motor to compress the spring. The breaker is then closed by electrically activating a small closing solenoid, which releases the closing spring. The breakers are opened manually by pressing the separate trip button mounted on the breaker faceplate, which mechanically unlatches the breaker, allowing the opening springs to rapidly force the main contacts apart. The breakers are opened electrically by energizing a shunt trip coil from a remote pushbutton, which then similarly unlatches the breaker contacts. The trip units used with power circuit breakers are today almost universally of the solid-state type. These units have replaced the mechanical dual-magnetic trip units that were the standard for many years. The solid-state trip units consist of three components: (1) current sensors, (2) the solid-state unit itself, and (3) a separate shunt-trip mechanism.


60



Ratings Disconnect Switch and Fuses Disconnect switches have voltage, continuous current, and horsepower ratings, whereas fuses have voltage, continuous current, and interrupting ratings. Figure 29 shows a typical nameplate of a disconnect (safety) switch and Figure 30 shows a typical label of a low voltage fuse.

SAFETY SWITCH 200 Amperes Cat No 324HD Voltage Max Horsepower 240 VAC 60 hp 480 VAC 125 hp 600 VAC 150 hp

Figure 29. Switch Nameplate

ABC Fuse CLASS RK-1 Int Rating

MKN-RK 100 Amp 250 VAC or Less 200,000 A rms sym

Figure 30. Fuse Label


61



Disconnect Switch - The disconnect switch is rated in amperes, voltage, and horsepower. Figure

31 describes the ratings of a typical disconnect switch.

Poles

Amps

Horsepower Rating AC Std (1) Maximum (2) 480V 500V 480V 600V 20 15 7.5 6 3-Pole 30 50 30 15 15 4-Wire SN 60 75 60 30 25 100 150 125 60 50 200 350 250 125 100 400 500 400 200 150 600 (1) Applies when standard Class H fuses are used. (2) Applies when time delay fuses are used.

Figure 31. Disconnect Switch Ratings


62



Fuses - General characteristics common to low voltage fuses include:

•

fuses should carry 110% of their rating continuously.

•

fuses 0-60 A should open within 1 hour at 135% of their rating.

•

fuse 61-600 A should open within 2 hours at 135% of their rating.

•

fuses above 600 A should open within 4 hours at 150% of their rating.

•

fuses with different current and voltage ratings should have specified physical dimensions, which will prevent interchangeability.

•

fuses with an interrupting capability greater than 10 kA should have their interrupting ratings marked on the fuse.

•

fuses must be tested at short-circuit power factors of 20% or less. They are typically tested at power factors of 15%, which implies an X/R ratio of 6.6 and an A.F. (Mm) that is equal to 1.331.

•

current limiting fuses clear faults in one-half cycle or less.

Low voltage fuses are UL class fuses. Class H are non-current limiting, whereas Classes G, J, K, R, and L are current limiting. Figure 32 lists the ratings of fuses. UL Class

Voltage (Volts)

Continuous Amperes

RMS Sym Interrupting (Kiloamperes)

G

300

0-60

100

H

250, 600

0-600

10

J

600

0-600

200

K

250, 600

0-600

50, 100, 200

250, 600

0-600

200

601-6000

200

(K1, K5) R (RK1, RK5) L

600

Figure 32. Low Voltage Fuse Ratings


63



Molded Case Circuit Breakers (MCCBs) Molded case circuit breakers are rated as follows: •

Frame sizes (AF) of 100 A, 225 A, 400 A, ..., 6000 A.

•

Trip ratings (AT) of 15 A, 20 A, 25 A, ..., 6000 A; NEC Article 240-6 lists all 37 standard AT ratings.

•

Amperes interrupting capability (AIC) ratings of 10 kA, 14 kA, 18 kA, ..., 100 kA; no standard exists for AIC typical ratings.

•

Voltage ratings of 120 V, 240 V, 277 V, 480 V, and 600 V. 0

Inverse-Time (Thermal-Magnetic) - The interrupting rating or short circuit rating at a 40 C

ambient temperature is commonly expressed in root mean square (rms) symmetrical amperes. The interrupting capability of the breaker may vary with the applied voltage. For example, a breaker applied at 480 volts could have an interrupting rating of 25,000 amps at 480 volts, but the same breaker applied at 240 volts may have an increased interrupting rating of 65,000 amps. All MCCBs operate instantaneously at currents well below their interrupting rating. Nonadjustable MCCBs will usually operate instantaneously at current values approximately five times (5x) their trip rating. Low voltage breaker contacts separate and interrupt the fault current during the first cycle of short circuit current. Because of this fast operation, the momentary and interrupting duties are considered to be the same. Therefore, all fault contribution from generators, motors, and the dc components of the fault waveform must be considered. Some MCCB manufacturers only list the symmetrical interrupting rating. If an asymmetrical rating is not given, assume the following (Figure 33):

Interrupting Rate 10,000 A and less 10,001 A to 20,000A Above 20,000A

Test Circuit Power Factor .45-.50 .25-.30 .15-.20

X/R Ratio 1.98-1.73 3.87-3.18 6.59-4.90

A. F. Multiplier (Ma) 1.02 to 1.01 1.09 to 1.07 1.17 to 1.13

Figure 33. MCCB Asymmetrical Factors


64



Magnetic-Only MCCB ratings are identical to the thermal-magnetic MCCB. Figure 34 lists the

typical ratings for both thermal-magnetic and magnetic-only MCCBs. Line No.

Frame

Rated

Size

Continuous

(amps)

Current

(AF)

(amps)

Interrupting Current Rating (AIC) (amps) 240 Volts

480 Volts

600 Volts

Sym

Asym

Sym

Asym

Sym

Asym

18,000

20,000

14,000

15,000

14,000

15,000

75,000

25,000

30,000

18,000

20,000

1

100

(AT) 10-100

2

100

10-100

65,000

3

100

10-100

100,000

4

225

125-200

22,000

25,000

18,000

20,000

14,000

15,000

5

225

70-225

25,000

30,000

22,000

25,000

22,000

25,000

6

225

70-225

65,000

75,000

35,000

40,000

25,000

30,000

7

225

70-225

100,000

--

8

225

70-225

35,000

40,000

9

400

200-400

65,000

10

400

200-400

100,000

11

400

200-400

42,000

12

600

300-600

100,000

13

800

300-800

42,000

50,000

30,000

35,000

22,000

25,000

14

800

300-800

65,000

75,000

35,000

40,000

25,000

30,000

15

800

600-800

100,000

16

1000

600-1000

42,000

50,000

30,000

35,000

22,000

25,000

17

1200

700-1200

42,000

50,000

30,000

35,000

22,000

25,000

--

75,000 -50,000 --

--

100,000

100,000 25,000 35,000 100,000 30,000 100,000

100,000

--

--

100,000

100,000

--

--

30,000

22,000

25,000

40,000

25,000

30,000

-35,000 --

--

100,000 22,000 100,000

100,000

-25,000 --

--

Figure 34. Typical MCCB Ratings


65



MCPs are recognized components in UL480 listed control assemblies, which includes

contactors and overload relays. They are sized to correspond with NEMA starter sizes (0, 1, 2, 3, 4, 5, 6), and, as mentioned previously, their adjustments must be labeled in amperes. MCPs are tested in combination with a specific contactor and overload relays to establish the maximum symmetrical interrupting capability. Typical ratings are 65 kA at 480 V, increasing to 200 kA when combined with fuse limiters. MCPs are voltage-rated up to 600 V, with typical continuous current ratings ranging from 3 to 600 A. Figure 35 lists the ratings and adjustments of a typical family of MCPs.


66



Figure 35. Typical MCP Ratings and Settings


67



Low Voltage Power Circuit Breakers (LVPCBs) LVPCBs are designed and marked with the maximum voltage at which they can be applied. They can be used on any system where the voltage is lower than the breaker rating. The applied voltage will effect the interrupting rating of the breaker. Standard maximum voltage ratings for LVPCBs are 635 volts, 508 volts, and 254 volts. LVPCBs are usually suitable for both 50 and 60 hertz. The rated continuous current of a LVPCB is the designated limit of rms current at rated frequency that is required to carry continuously without exceeding the temperature limitations based on a 400C ambient temperature. The temperature limit on which the rating of circuit breakers are based is determined by the characteristics of the insulating materials and the metals that are used in the current carrying components and springs. Standard frame size ratings for low voltage power circuit breakers are 800, 1600, 2000, 3200, and 4000 amps. Some manufacturers may have additional frame sizes. These breakers will all have either an electro-mechanical trip or a solid state trip that is adjustable or interchangeable from a minimum rating up to the ampere rating of the frame. The interrupting rating of an LVPCB is the symmetrical current rating of the circuit breaker. The asymmetrical interrupting rating is implied, and it is based on an X/R ratio of 6.6 for unfused breakers and an X/R ratio of 4.9 for fused breakers. An X/R ratio of 6.6 corresponds to an A.F. (Ma) of 1.17. Most low voltage systems have an X/R ratio of less than 6.6. Therefore, if an asymmetrical interrupting rating is not listed by the manufacturer, assume that the asymmetrical rating is 1.17 times the symmetrical rating. The short-time current rating of a LVPCB specifies the maximum capability of the circuit breaker to withstand the effects of short circuit current flow for a stated period, typically 30 cycles or less. The short-time delay on the breaker’s trip units corresponds to the short-time current rating. This delay provides time for downstream protective devices closer to the fault to operate and isolate the circuit. The short-time current rating of a modern day LVPCB without an instantaneous trip characteristic is usually equal to the breaker’s short circuit interrupting rating. By comparison, MCCBs usually do not have a short-time rating. Figure 36 lists the frame and sensor ratings of a typical LVPCB, and Figure 37 lists the shorttime and interrupting ratings of a typical LVPCB.


68



Frame Size (amps)

Available Sensor Ratings (amps) 50, 100, 150, 200, 300, 400, 600, 800

800 1600

100, 150, 200, 300, 400, 600, 800, 1200, 1600

2000

100, 150, 200, 300, 400, 600, 800, 1200, 1600, 2000

3200

2400, 3200

4000

4000

Figure 36. LVPCB Frame and Sensor Ratings

Frame Size amps

Interrupting Ratings, RMS Symmetrical Amperes Short Time Ratings - 30 cycles With Instantaneous Trip (With Short-Delay) 208-240 V 480 V 600 V 208-240 V 480 V 600 V

800

42,000

30,000

30,000

30,000

30,000

30,000

1600

65,000

50,000

42,000

50,000

50,000

42,000

2000

65,000

65,000

50,000

65,000

65,000

50,000

3200

85,000

65,000

65,000

65,000

65,000

65,000

4000

130,000

85,000

85,000

85,000

85,000

85,000

Figure 37. LVPCB Short-Time and Interrupting Ratings


69



Combination Motor Starters When the disconnect and/or branch circuit fault protective device and the controller (starter) are at the same location, they can be combined into a single enclosure called a combination motor starter. The combination motor starter is a much more compact unit that saves both space and installation costs and that increases safety, because the cover of the starter is interlocked with the protective/disconnect to prevent opening unless the disconnecting means is in the “off” position. The rating of combination motor starters is based on maximum horsepower, voltage, NEMA starter size, continuous amperes, and interrupting amperes. The standard interrupting rating of MCPs and magnetic-only breakers is 65 kA. Optional units are available on the market that increase the interrupting capacity to 100 kA by adding current limiters to the breaker. Figure 38 lists the ratings of typical combination motor starters up to a maximum rating of 100 hp. Note: Higher rated (greater than 100 hp) combination starters are available, but they are not listed in Figure 38 because SAES-P-114 limits their application to low voltage motors rated 100 hp and below.


70



Max hp

Motor Volts

NEMA Size

Continuous Amperes

Breaker Amperes

1 27 30 200 7.5 230 7.5 460 10 575 10 10 200 2 45 50 15 230 25 460 25 575 25 200 3 90 100 30 230 50 460 50 575 40 200 4 135 150 50 230 100 460 100 575 75 200 5 270 400 100 230 Notes: a. For motor horsepower ratings less than 7.5 hp, NEMA Size 0 starters are available. b. Combination starter interrupting ratings are 65 kA; 100 kA ratings are also available if current limiter attachments are installed. Figure 38. Typical Combination Motor Starter Ratings


71



Fuse T/C Characteristics Log-Log T/C Paper The response curves of all protective devices are plotted on common graphs so that they may be compared at all current and time points. The standard method used to plot device T/C characteristics is to plot the devices on log-log graph paper (Figure 39). Standard log-log graphs show 4.5 cycles on the horizontal scale representing current. The current axis ranges from 0.5 to 10,000 amperes. The vertical axis, representing time, ranges from 0.01 to 1000 seconds and/or .6 to 60,000 cycles. Because current limiting fuses and molded case circuit breakers may operate in less than 0.5 cycles (.00835 seconds), manufacturers of these devices may reproduce T/C characteristic curves with 6 cycle vertical scales and times ranging from .001 to 10000 seconds (.06 to 600,000 cycles). The horizontal current scale is also often “shifted” for a particular plot by multiplying the current scale by a factor of 10, 100, or 1000 (x10, x100, x1000). Non-Time Delay Non-time delay fuses are typically single-element fuses that are particularly suited for short circuit protection of components in circuits without inrush currents, such as lighting loads. If used on circuits with inrush currents (motors and transformers), they must be often oversized, which sacrifices certain levels of current limitation. Figure 40 describes typical T/C characteristic curves of both a 30 and 400 ampere non-time delay fuse. Time Delay Time delay fuses are typically dual-element fuses providing both overload and short circuit protection. They are typically applied on circuits with motor loads (temporary inrush current). They do not offer excellent short circuit protection as non-time delay fuses. However, they provide excellent overload protection because they can be closely sized to fullload motor currents. Figure 41 describes typical T/C characteristics of both a 20 and 400 ampere time delay fuse.


72



Figure 39. Typical Log-Log Paper


73



Figure 40. Non-Time Delay Fuse Characteristics


74



Figure 41. Time Delay Fuse T/C Characteristics


75



Molded Case Circuit Breaker T/C Characteristics Phase Fault Protection Saudi Aramco (SAES-P-114) permits the following three types of molded case circuit breakers (MCCB) to be used for motor phase fault protection. •

inverse-time (thermal-magnetic)

•

magnetic only

•

motor circuit protectors (MCPs)

Inverse-Time (Thermal-Magnetic) MCCBs are permitted by SAES-P-114 for low voltage motors

rated 1.0 hp or less. The NEC also permits their use as long as their continuous current rating does not exceed 250 percent of the motor’s full-load amperes (IFLA) as listed in NEC Table 430-150. Although most codes and standards permit use of inverse-time MCCBs, these MCCBs are typically not used because of nuisance tripping caused by high motor starting inrush currents (typically 4-6 IFLA). Figure 42 shows the T/C characteristics of the MCCB protecting a 1.0 hp motor. Note: Although Figure 42 shows, and the NEC permits, the MCCB providing both overload and short circuit protection, it is not recommended practice. Magnetic-Only MCCBs are also permitted by SAES-P-114 for low voltage motors rated 1.0 to

100 hp. The NEC also permits their use as long as their rating (setting) does not exceed 700 percent of the motor’s full-load amperes (IFLA) as listed in NEC Table 430-150 and if the MCCB is part of a listed combination controller. Figure 43 shows the T/C characteristics of a magnetic only MCCB protecting a 100 hp motor. Motor Circuit Protectors (MCPs) , like magnetic-only MCCBs, are permitted by SAES-P-114 for

low voltage motors rated 1.0 to 100 hp. NEC Article 430-52 also permits their use as long as they are part of a listed combination controller and are set at not more than 1300 percent of the motor’s full load amperes (IFLA) as listed in NEC Table 430-150. Figure 44 shows the T/C characteristics of an MCP protecting a 100 hp motor.


76



Figure 42. Thermal Magnetic MCCB Fault Protection


77



Figure 43. Magnetic-Only MCCB Fault Protection


78



Figure 44. MCP Fault Protection


79



Ground Fault Protection SAES-P-114 requires ground fault protection for all low voltage motors rated 30 hp or greater. The ground fault relay (GFR, ANSI Device 50GS) used to provide this protection is an instantaneous, ground overcurrent sensor, with an adjustable pickup, connected to a window-type CT. The MCCB or MCP must also be modified to include a shunt trip device. When the shunt trip solenoid is energized, a plunger hits the trip mechanism and the breaker opens. The shunt trip solenoid is activated by the ground fault relay (sensor). The shunt trip requires an auxiliary power source, and it is available in a wide range of voltages. A control power transformer is typically used for voltages above 240 V. A typical shunt trip capable of operating at voltages as low as 55 percent should be selected because ground faults cause a severe drop in the system voltage. Figure 45 is a simple one-line diagram showing application of a ground fault relay. Note: Although shunt trip devices usually require an auxiliary power source, 16-SAMSS-503 does not permit their use in Saudi Aramco industrial facilities. Saudi Aramco applications require special flux transfer shunt trips, which do not require auxiliary sources of power.

Figure 45. Ground Fault Protection With Shunt Trip


80



LVPCB T/C Characteristics SAES-P-114 requires use of LVPCBs for fault protection (phase and ground) for all low voltage motors rated above 100 hp. Modern solid-state trip (SST) units are typically available with the following trip functions. •

Long Time/Instantaneous (LI)

•

Long Time/Short Time (LS) with or without I2t

•

Long Time/Short Time/Instantaneous (LSI)

•

Long Time/Instantaneous/Ground (LIG) with or without I2t

•

Long Time/Short Time/Ground (LSG)

•

Long Time/Short Time/Instantaneous/Ground (LSIG)

Phase Fault Protection Long Time Pickup (LTPU) or long delay settings are adjustable from 0.5 - 1.0 times the plug

rating (In). The plug rating is also a function of the sensor rating (current transformer), which establishes the continuous current rating of the breaker. Typical tolerances for a modern day SST (i.e. Westinghouse Digitrip RMS) are -0%, + 10% (Figure 46).

Figure 46. Long Time Pickup (LTPU) T/C Characteristics


81



Long Time Delay (LTD) or long delay time settings are adjustable from 2 - 24 seconds at six

times the (Figure 47).

plug

rating

(6In).

Typical

tolerance

are

+0%,

-

33%

Figure 47. Long Time Delay (LTD) T/C Characteristics Short Time Pickup (STPU) or short delay pickup settings are adjustable from two to six times the plug rating (2 - 6In) plus two variable settings of S1 (8In) and S2 (10In). Typical tolerances are +10%, -10% (Figure 48).

Figure 48. Short Time Pickup (STPU) T/C Characteristics


82



Short Time Delay (STD) or short delay time settings are available with five flat responses of 0.1,

0.2, 0.3, 0.4 and 0.5 seconds. Typical tolerances are variable depending on the setting (Figure 49). I2t Function settings are available in three responses of 0.1*, 0.3*, and 0.5* seconds, and they

revert back to a flat response at 8In (Figure 49). Note: The asterisk (*) refers to I2t settings.

Figure 49. Short Delay Time (SDT) With I2t T/C Characteristics


83



Instantaneous Trip (IT) settings are adjustable from two to six times the plug rating (2 - 6In)

plug two variable settings of M1 (8In) and M2 (12In). Typical tolerances are +10%, -10% (Figure 50).

Figure 50. Instantaneous Trip (IT) T/C Characteristics Ground Fault Protection (GFP) SAES-P-114 requires ground fault protection that uses window-type current transformers (CTs) that are similar to the BYZ CT shown in Figure 51. The tripping function, unlike its MCCB or MCP counterpart, is part of the same SST unit.

Figure 51. GFP With Window-Type CT


84



Ground Fault Pickup (GFPU) settings have eight discrete adjustments (A, B, C, D, E, F, H, K),

which are a function of the plug ratings (In). Figure 52 shows a sample listing of the settings for plug ratings of 100, 200, 250, and 300 amperes. Typical tolerances are +10%, -10% (Figure 53).

In

A

B

C

D

E

F

H

K

100

25

30

35

40

50

60

75

100

200

50

60

70

80

100

120

150

200

250

63

75

88

100

125

150

188

250

300

75

90

105

120

150

180

225

300

Figure 52. Sample GFPU Code Letters and Settings

Figure 53. Ground Fault Pickup (GFPU) T/C Characteristics


85



Ground Fault Time (GFT) or ground fault delay time settings, like the SDT settings, are

available with five flat responses of 0.1, 0.2, 0.3, 0.4, and 0.5 seconds. Typical tolerances are variable depending on the setting (Figure 54). I2t Function settings are also available in three responses of 0.1*, 0.2*, and 0.3* seconds, and

they revert back to a flat response at 0.625 In (Figure 54).

Figure 54. Ground Fault Time (GFT) With I2t T/C Characteristics


86



Figure 55 shows the T/C characteristics of a low voltage power circuit breaker protecting a 200 hp low voltage motor that uses a Westinghouse Digitrip RMS trip unit.

Figure 55. LVPCB Motor Protection


87



Motor Nameplate Data Motor nameplate data was previously discussed in Module EEX 216.02. This Module will briefly review the following nameplate data to be used in selecting a low voltage motor disconnect/fault protective device: •

full-load amperes

•

kVA code/locked rotor amperes

•

voltage and horsepower

Note: Work Aid 3 has been developed to help the Participant select a motor disconnect/fault protective device. Full-Load Amperes The protective device’s continuous current rating should not exceed the motor’s full-load amperes (IFLA) as listed in NEC Table 430-150. 16-SAMSS-503 specifies that the continuous current ratings of MCCBs (or MCPs) shall not be less than 125% IFLA unless the MCCB is 100% rated. If a LVPCB is being used, 16-SAMSS-503 specifies a continuous current rating no less than 115% IFLA. kVA Code/Locked-Rotor Amperes The code letters marked on motor nameplates show motor input kVA under locked-rotor (starting) conditions. The code letters for determining motor branch-circuit short-circuit and ground fault protection are explained in NEC Article 430-52 and Table 430-152. Voltage and Horsepower The protective device’s voltage rating is based on the system’s nominal voltage rating and not on the motor’s nameplate voltage rating. The motor’s nameplate horsepower rating is used to determine the kVA input under locked-rotor conditions (see previous paragraph), and to determine the motor’s full-load and locked-rotor amperes in accordance with NEC Tables 430-150 and 430-151.


88



Fault/Starting Currents Under fault conditions a motor supplies the same magnitude of current that it draws under locked-rotor (starting) conditions (refer to Figure 7 of Module EEX 216.02). The protective device’s interrupting rating must be greater than the symmetrical fault current available in the system, and its trip rating must be greater than the asymmetrical starting current available. Symmetrical Current MCCBs and LVPCBs, like most other electrical equipment, are rated based on their symmetrical interrupting capability (Isym). Combination motor controllers using magneticonly MCCBs and MCPs are also rated (listed) on a symmetrical current basis. Asymmetrical Current Although MCCBs and LVPCBs are rated symmetrically, they have an implied asymmetrical rating (Iasy) as well. LVPCB asymmetrical ratings are 1.17 times their symmetrical rating (Iasy = 1.17 Isym). MCCB asymmetrical ratings are 1.02, 1.09, and 1.17 times their symmetrical rating based on symmetrical interrupting ratings of 10kA or less, 10.001 to 20 kA, and greater than 20kA respectively.


89



NEC Maximum Settings NEC Table 430-152 specifies maximum settings to provide motor branch-circuit short-circuit and ground-fault protection. Inverse-Time MCCBs NEC Table 430-152 specifies a setting not to exceed 250% IFLA for inverse time MCCBs. Because a MCCB will trip on its instantaneous function at 5 times its trip rating, the NEC in effect limits the trip rating under motor starting conditions to 12.5 (5 X 2.5). If this setting nuisance trips the motor under starting conditions, NEC Article 430-52 permits increasing the rating to 400%, where IFLA is 100 amperes or less, and 300%, where IFLA is greater than 100 amperes. Magnetic-Only MCCBs and MCPs NEC Table 430-152 specifies a maximum setting of 700% IFLA and an absolute maximum of 1300% IFLA if, under motor starting conditions, the protective device nuisance trips. LVPCBs The NEC does not explicitly specify LVPCB settings for motor protection. For NEC purposes, if the LVPCB is used as an inverse time breaker, it must comply to the same rules as an inverse time MCCB. If the LVPCB is used as a magnetic-only breaker, it must comply to the same rules as a magnetic-only breaker or MCP.


90



WORK AID 1:

RESOURCES USED TO SELECT A LOW VOLTAGE MOTOR O/L RELAY

Work Aid 1A: NEC Article 430 For the content of NEC Article 430, refer to Handout 1. Work Aid 1B: 16-SAMSS-503 For the content of 16-SAMSS-503, refer to Handout 2. Work Aid 1C: Vendor’s Literature, Westinghouse Catalog 25-000 For the content of Westinghouse Catalog 25-000, refer to Handout 3. Work Aid 1D: Applicable Selection Procedures 1. Collect the following information from the motor nameplate: •

full-load amperes nameplate (FLA) -

•

service factor (S.F.) -

Note: Saudi Aramco specifies only 1.0 S.F. motors. 2. Determine the operating ambient temperatures for the motor environment and for the controller environment.


91



3. Using NEC Article 430-32 (Work Aid 1A, Handout 1), determine the trip current, or rating, for the overload relay to be selected. Note: NEC Article 430-32(a) states that maximum trip current, or rating, of an overload relay, when protecting a continuous-duty motor, is determined in accordance with Figure 58. Motor Parameter

Percent of Motor Nameplate Full-Load Amperes (FLA)

* Motors marked with SF > 1.15

125%

* Motors marked with temperature rise < 40oC

125%

All other motors

115%

* Note: If the nameplate is not marked with the service factor (S.F.) or temperature rise, the stated condition does not apply to the selection of the trip current, or rating, of the overload relay. Figure 58. NEC Table 430-32 4. Determine the style of overload relay to be selected. Consider bimetallic or solid-state styles. Note 1: 16-SAMSS-503.5.1.3 (Work Aid 1B, Handout 2) allows the overload relay to be thermally actuated, bimetallic (block-type) style, or solid-state style. Note 2: Choose the more economical thermally actuated, bi-metallic style, unless the special accuracy of the solid-state style is specified or required for the application. 5. Determine the class of overload relay to be selected: Class 10, 20, or 30. Note: 16-SAMSS-503.5.1.2 (Work Aid 1B, Handout 2) requires that overload relays be Class 20 unless otherwise specified. For example, if motor acceleration time is known to exceed 20 seconds, Class 30 should be specified and selected.


92



6. Determine the type of overload relay to be selected: Type A or Type B. Per 16-SAMSS503, select Type A. Note 1: Type A overload relays are furnished with manual and automatic reset capabilities, and they allow heater element trip ratings to be adjusted over a range of approximately 85% to 115% of the heater element’s respective rating. Type B overload relays are furnished with only manual reset capability, and they do not provide for adjustment of heater element trip ratings. Note 2: 16-SAMSS-503.5.1.3 (Work Aid 1B, Handout 2) requires that overload relays be of the manual-reset type unless otherwise specified, and to have field-adjustable trip settings with a minimum range of 85% to 100% of the heater’s element rating. 7. Determine if the overload relay to be selected is to be compensated or non-compensated for ambient temperature. Note: 16-SAMSS-503.5.1.3 (Work Aid 1B, Handout 2) requires that thermally actuated, bi-metallic overload relays be temperature compensating from 0oC to 75oC. 8. Determine the pole arrangement for the overload relay to be selected. Choose either a single three-pole block overload relay or three single-pole overload relays. Note: 16-SAMSS-503.5.1.3 (Work Aid 1B, Handout 2) requires that thermally actuated overload relays be block-type with bi-metallic type heater elements. By definition, a block-type relay is a single three-pole block overload relay.


93



9. Using the through 8:

overload

relay

selection

parameters

determined

•

full-load nameplate amperes (FLA) -

•

service factor (S.F.) -

•

motor operating ambient temperature -

•

controller operating ambient temperature -

•

overload relay style -

•

overload relay class -

•

overload relay type -

•

overload relay ambient temperature compensation -

•

overload relay pole arrangement -

in

steps

1

Select an overload relay from the Westinghouse Catalog 25-000, pages 469 - 472 (Work Aid 1C, Handout 3). Note: If current is to be supplied to the overload relay through a current transformer (CT), the selection parameter of motor full-load amperes (FLA) must be divided by the ratio of the CT before it is used to select the overload relay. For example, if the overload relay is intended to monitor the motor current through a 300/5 (or 60/1) CT, the parameter used to select the overload relay must be (FLA)/60.


94



10. To complete the selection of the overload relay, select the appropriate heater elements for the relay. Select the heater elements from Westinghouse Catalog 25-000, page 473 (Work Aid 1C, Handout 3), by using the following information: •

NEMA size contactor used for starter

•

full-load nameplate amperes (FLA) (from step 1)

•

service factor (S.F.) (from step 1)

•

ultimate trip current for overload relay (from step 3)

•

temperature compensation requirement (from step 7)

•

overload relay type (from step 6)

Note: When heater elements are to be selected for applications where the motor and overload relay are in different ambients and where the overload relay being used is noncompensating, adjustments must be made to the value of motor currrent used to select the heater in accordance with manufacturer’s instructions. 11. Note: 16-SAMSS-503 (Work Aid 1B, Handout 2) requires that only combination controllers be used for motors rated 600 V and below and 1 to 100 horsepower. As a result, overload relays are supplied by the manufacturer as an integral component of the combination controller. These selection procedures provide for verifying the selection of overload relays provided with combination controllers.


95



WORK AID 2: Work Aid 2A:

RESOURCES USED TO SELECT A LOW VOLTAGE MOTOR CONTACTOR NEC Article 430

For the content of NEC Article 430, refer to Handout 1. Work Aid 2B:

16-SAMSS-503, Chapter 4

For the content of 16-SAMSS-503, Chapter 4, refer to Handout 2. Work Aid 2C:

Vendor’s Literature, Westinghouse Catalog 25-000

For the content of Westinghouse Catalog 25-000, refer to Handout 3. Work Aid 2D:

Applicable Selection Procedures

1. Collect the following information from the motor nameplate: •

motor horsepower (hp) -

•

motor voltage (VM) -

•

number of phases, 1 or 3 -

2. Determine the type of contactor to be selected. Note 1: Types of contactors typically considered for low voltage starters include airmagnetic and vacuum. Note 2: 16-SAMSS-503.4.4.1 (Work Aid 2B, Handout 2) requires that motor controllers be 600 V, three-pole, general purpose, NEMA Class A, air-magnetic-type motor controllers that are rated in horsepower, and that conform to the requirements of NEMA ICS 2-321.


96



3. Determine the minimum size contactor to select. Note 1: In accordance with NEC Article 430 (Work Aid 2A, Handout 1), the general requirement is that the controller must have a horsepower rating that is not lower than the horsepower rating of the motor at the application voltage. Note 2: When special application criteria such as long-acceleration time, dynamic breaking, or above average starting duty is specified, a larger NEMA size contactor must be selected in accordance with the rating tables provided in NEMA ICS 2-321. In general, these special application criteria require the selection of a contactor that is one NEMA size larger. 4. Determine whether a reversing or non-reversing contactor is to be selected. Note: This selection parameter is determined from the operating conditions for the starter. 5. Determine the coil voltage rating for the contactor to be selected. The coil voltage rating must be equal to the control circuit voltage rating. Per 16-SAMSS-503, select a 120 V coil rating. 6. Using the contactor selection parameters determined in steps 1 through 5: •

motor horsepower (hp) -

•

motor voltage (VM) -

•

number of phases, 1 or 3 -

•

type of contactor -

•

reversing or non-reversing -

•

contactor coil voltage rating -

Select a low voltage contactor from the Westinghouse Catalog 25-000, pages 356 - 359 (Work Aid 2C, Handout 3). 7. Note: 16-SAMSS-503 (Work Aid 2B, Handout 2) requires that only combination controllers be used for motors rated 600 V and below and 1 to 100 horsepower. As a result, contactors are supplied by the manufacturer as an integral component of the combination controller. These selection procedures provide for verifying the selection of contactors provided with combination controllers.


97



WORK AID 3: RESOURCES USED TO SELECT A LOW VOLTAGE MOTOR DISCONNECT/FAULT PROTECTIVE DEVICE Work Aid 3A: NEC Article 430 For the content of NEC Article 430, refer to Handout 1. Work Aid 3B: 16-SAMSS-503 For the content of 16-SAMSS-503, refer to Handout 2. Work Aid 3C: Vendor’s Literature, Westinghouse Catalog 25-000 For the content of Westinghouse Catalog 25-000, refer to Handout 3. Work Aid 3D: SAES-P-114, Chapter 6 For the content of SAES-P-114, Chapter 6, refer to Handout 4. Work Aid 3E:

Vendor’s Literature, Westinghouse SA-11647, Low Voltage Metal Enclosed Switchgear - Type DS

For the content of Westinghouse SA-11647, refer to Handout 8. Work Aid 3F: Applicable Selection Procedures 1. Collect the following data from the motor nameplate (if available): horsepower (hp) full-load amperes nameplate (FLA) voltage (V) service factor (S.F.) kVA code letter Note: Saudi Aramco specifies only 1.0 S.F. motors. 2. Collect the following data, based on motor horsepower and voltage, from NEC Tables 430-150 and 430-151 (Handout 1): NEC full-load amperes (FLAN) NEC locked-rotor amperes (LRAN) -


98



3. Collect the maximum symmetrical short circuit current available (SCA) from the system one-line diagram: Short circuit current available (SCA) 4. Calculate a required breaker interrupting rating 105 percent greater than the maximum SCA: Notes: 1) Saudi Aramco design practices require that all electrical equipment interrupting and withstand ratings be equal to 105 percent of SCA. 2) Magnetic-only and MCP-interrupting ratings are part of the listed combination controller ratings. Breaker interrupting rating in amperes - Iint = 1.05 x SCA 5. If using MCP fault/disconnect protection, select the next standard size MCP (including magnetic trip ranges) from Westinghouse Catalog 25-000 (Work Aid 3C, Handout 3), pages 127 or 128, that equals or exceeds the voltage (V), NEC full-load amperes (FLAN) breaker interrupting rating (Iint), and locked rotor amperes (LRAN) from steps 1, 2, and 4 above. Note: All Westinghouse MCP interrupting ratings are 65 kA. If higher ratings are required, which is considered unlikely for refinery operations, an MCP must be selected with a current limiter attachment that increases the rating to 100 kA. 6. If using a magnetic-only breaker, follow the same procedures as in step 5. Note: This Module limits selection to MCPs for motors less than or equal to 75 kW (100 hp). 7. If using a LVPCB controller, select the next standard size from Westinghouse SA-11647 (Work Aid 3E, Handout 8), page 7, that equals or exceeds V, 1.15 FLAN, and Iint from 1, 2, and 4 above. Note: SAES-P-114 (Work Aid 3D, Handout 4) requires LVPCBs used as motor starters to have continuous current ratings 115 percent greater than FLAN.


99



8. Select ground fault protection for motors above 30 hp. a. If using MCP fault/disconnect protection, select a ground fault protection device from pages 480 and 481 of Westinghouse Catalog 25-000 (Work Aid 3C, Handout 3). Note: SAES-P-114 requires zero sequence, window-type CTs for motor ground fault protection. SAES-P-114 also requires that the ground fault protection device operate without auxiliary power. Therefore, this ground fault device must be selected using flux transfer shunt trips. b. If using LVPCB controllers, select the ground fault function (G) when selecting the breakers trip functions. Note: SAES-P-114 requires zero-sequence, window-type CTs for motor ground protection. 9. Alternative to selecting the individual fault/disconnect protective device, select from the vendor’s list an enclosed combination motor starter (O/L relay, contactors, fault/disconnect device, and enclosure). Therefore, if using this option, select a combination motor starter from Westinghouse Catalog 25-000 (Work Aid 3C, Handout 3), pages 406, 407, 415 and 416. The combination starter ratings must equal or exceed V, FLAN, and Iint from steps 1, 2 and 4 above. 10. Verify that the MCP or LVPCB selected complies with SAES-P-114 (Work Aid 3D, Handout 4), and 16-SAMSS-503 (Work Aid 3B, Handout 2) criteria.


100



GLOSSARY air magnetic breaker

A type of medium voltage circuit breaker with its contacts in air. An electromagnet built into the arc chutes aids in extinguishing the arc.

ambient temperature

The temperature of the medium such as air, water, or earth into which the heat of the equipment is dissipated.

American National Standards Institute (ANSI)

An organization whose members approve various standards for use in American industries.

asymmetrical (current)

The combination of the symmetrical component and the direct-current component of the current.

combination starter

A complete motor starter consisting of a disconnect device, a magnetic contactor, and protective devices for short circuit and overload. All devices are assembled in a single enclosure.

contactor

A magnetic device that has sufficient capability to connect and disconnect the electric circuit of a motor under normal and overload conditions.

continuous rating

The maximum constant load that can be carried continuously without exceeding established temperature-rise limitations under prescribed conditions of test and within the limitations of established standards.

control circuit

The circuit that carries the electric signals directing the performance of the controller but does not carry the main power current.

control relay

A component that is used in a motor starter’s control circuit to interface between a pilot device and the circuit that the pilot device controls.

control power transformer (CPT)

A transformer used to draw control power from the main power circuit of a motor starter.


101



current-limiting (fuse)

A fuse that, when it is melted by a current within its specified current-limiting range, abruptly introduces a high arc voltage to reduce the current magnitude and duration. Note: The values specified in standards for the threshold ratio, peak letthrough current, and I2t characteristic are used as the measures of current-limiting ability.

disconnect switch

A switch intended for isolating an electric circuit from the source of power.

duty (rotating machinery)

A variation of load with time, which may or may not be repeated, and in which the cycle time is too short for thermal equilibrium to be attained.

fault current

A current that results from the loss of insulation between conductors or between a conductor and ground.

fault current, low-level (as applied to a motor branch circuit)

A fault current that is equal to or less than the maximum operating overload.

full-voltage starter

A type of motor starter that applies full voltage to the motor terminals during the starting period.

horsepower (shaft) (hp)

The mechanical output (shaft) rating of a One (1) hp equals 746 watts. See kilowatt (shaft).

induction motor

An alternating-current motor in which a primary winding on one member (usually the stator) is connected to the power source and in which a polyphase secondary winding or a squirrel-cage secondary winding on the other member (usually the rotor) carries induced current.

instantaneous (relay)

A qualifying term applied to a relay indicating that no delay is purposely introduced in its action.

Institute of Electrical and Electronics Engineers (IEEE)

A worldwide society of electrical and electronics engineers


motor.

102



interrupting capability

The maximum value of current that a contact assembly is required to successfully interrupt at a specified voltage for a limited number of operations under specified conditions.

inverse-time

A qualifying term applied to a relay indicating that its time of operation decreases as the magnitude of the operating quantity increases.

jogging

The quickly repeated closure of the circuit to start a motor from rest for the purpose of accomplishing small movements of the driven machine.

kilowatt (shaft) (kw)

The mechanical output (shaft) rating of a motor. See horsepower (hp).

locked-rotor (rotating machinery)

The condition existing when the circuits of a motor are energized, but the rotor is not turning.

locked-rotor current

The steady-state current taken from the line with the rotor locked and with rated voltage (and rated frequency in the case of alternating-current motors) applied to the motor.

locked-rotor indicating code letter

Code letters marked on a motor nameplate to show motor kVA per hp under locked-rotor conditions.

low voltage

Voltage levels below 1000 volts usually called utilization level outages.

manual starter

A simple type of motor starter that provides full-voltage, on-off type operation for small single-phase and three-phase motors.

motor circuit protector(MCP)

A magnetic-only molded case circuit breaker used in low voltage combination starters. This device has only instantaneous functions to protect the motor, starter, and branch circuit from short circuit and ground fault currents.


103



National Electric Code (NEC)

An electrical safety code developed and approved every three years by the National Fire Protection Association (NFPA).

National Electrical Manufacturers (NEMA)

A nonprofit trade association of manufacturers of electrical apparatus and supplies, whose members Association are engaged in standardization to facilitate understanding between users and manufacturers of electrical products.

operating overload

The overcurrent to which an electric apparatus is subjected in the course of the normal operating conditions that it may encounter.

overload relay

A device that is used to sense an overload on a motor circuit. The most common type uses a heater that heats a bi-metallic strip that operates a set of contacts.

overload protection

The effect of a device operative on excessive current, but not necessarily on short circuit, to cause and maintain the interruption of current flow to the governed device.

relay

An electrically controlled, usually two-state, device that opens and closes electrical contacts to effect the operation of other devices in the same or another electric circuit.

replica temperature relay

A thermal relay whose internal temperature rise is proportional, over a range of values and durations of overloads, to that of the protected apparatus or conductor.

reversing starter

A type of motor starter that provides for reversing the direction of rotation of the motor.

service factor (S.F.)

A multiplier that, when applied to the rated power, indicates a permissible power loading that may be carried under the conditions specified for the service factor.

single-phasing (motor)

An abnormal operation of a polyphase machine when its supply is effectively single-phase.


104



starter (motor)

An electric controller for accelerating a motor from rest to normal speed and for stopping the motor.

starting current (rotating machinery)

The current drawn by the motor during the starting period. It is a function of speed or slip.

symmetrical (current)

A periodic alternating current in which points one-half a period apart are equal and have opposite signs.

temperature rise (rotating machinery)

A test undertaken to determine the temperature rise above ambient of one or more parts of a machine under specified operating conditions. Note:The specified conditions may refer to current, load,etcetera.

three-wire control

The most common type of control used to start and stop a motor.

two-wire control

This type of control automatically starts and stops a motor depending on the set points of a pilot device.

time-current characteristics

The correlated values of time and current that designate the performance of all or a stated portion of the functions of a protective device. Note: The time-current characteristics of a protective device are usually shown as a curve.

time-overcurrent relay

An overcurrent relay in which the input current and operating time are inversely related throughout a substantial portion of the performance range.

total current

See asymmetrical current.


105

Electric motor 8

Recommend Documents