Grounding

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Design And Application Of System Grounding

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: EEX20501

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

Engineering Encyclopedia

Electrical Design and Application of System Grounding

CONTENTS

PAGE

Locating System Grounding Information.......................................................... 1 Basis For Installation Of System Grounds In Saudi Aramco Electrical Systems.......................................................................................... 12 Determining The Appropriate Method Of System Grounding For Saudi Aramco Electrical Systems .................................................................. 21 Designing A Substation/Plant Ground Grid For Saudi Aramco Electrical Installations..................................................................................... 38 Work Aid 1: Saudi Aramco And Industry Standards For Locating System Grounding Information .................................. 55 Work Aid 2: Table Of Saudi Aramco System Grounding Methods .................................................................................... 56 Work Aid 3: Procedures And References For Designing Substation/Plant Ground Grids ................................................. 57 Glossary ......................................................................................................... 69

Saudi Aramco DeskTop Standards



LOCATING SYSTEM GROUNDING INFORMATION An Engineer should consult the following Saudi Aramco documents and the following industry codes and standards for information on system grounding: _ _ _ _

Saudi Aramco Design Practices (SADP's) Saudi Aramco Engineering Standards (SAES's) Institute of Electrical and Electronics Engineers Standards (IEEE) National Electrical Code (NEC)

Saudi Aramco Design Practices The purpose of the SADP's is to provide the background information that is needed to explain, amplify, and apply the mandatory requirements of the SAES's. An Engineer should reference a SADP when he needs tutorial or background information on the design and application of system grounding. The information in the SADP's is not mandatory, and written approval is not needed to deviate from the SADP's. Statements in the SADP's that are in capital letters are mandatory because they are taken from the SAES's. The SADP that contains information on the design and application of system grounding is SADP-P-111.


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Saudi Aramco Design Practices (Cont'd) SADP-P-111

The title of SADP-P-111 is "Grounding." This SADP includes two parts. Part one provides the rationale for technical requirements in SAES-P-111 that are not obvious. This rationale is based on Saudi Aramco experience. Part two contains fourteen chapters of tutorial information that explains the principles and application of grounding to meet the requirements of Saudi Aramco installations. An Engineer can use this information to clarify the technical requirements that are given in SAES-P-111. Figure 1 shows the table of contents for SADP-P-111. Only Chapters One, Two, Three, Four, Five, Seven, Ten, Eleven, and Thirteen contain information that is applicable to system grounding. Each of the sections that follow describes the scope of one of these chapters. Chapter One describes the general grounding requirements for Saudi Aramco

installations. This chapter also contains a list of all the references that were used to write SADP-P-111. The latest edition of the references that are listed are for use in interpreting and/or in modifying the text in SADP-P-111. Chapter Two contains the definitions of the technical terms that are related to grounds

and grounding. Chapter Three provides guidance on the selection and installation of grounding conductors for high and low voltage systems. Chapter Four provides guidance on the design of grounding electrodes. This chapter

discusses the design of all types of grounding electrodes from single rods to extensive buried grids. Chapter Five provides guidance on the design and application of the various methods

that are available to ground Saudi Aramco electrical systems. This chapter includes both generator transmission system and distribution system grounding. Chapter Seven provides guidance on the measures that Saudi Aramco uses to combat

the corrosion that is associated with grounding.


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Saudi Aramco Design Practices (Cont'd)

SADP-P-111 Table of Contents Figure 1


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Saudi Aramco Design Practices (Cont'd) Chapter Ten provides guidance on the measurement of soil resistivity for the purpose of

ground grid design. The information in this chapter is for use with the information in Chapter Four. Chapter Eleven defines ground potential rise (GPR) and other electric power parameters

that have an effect on communication systems. Chapter Thirteen provides guidance on the measurement of a ground grid's resistance

after the ground grid has been installed. Saudi Aramco Engineering Standards The SAES's contain the minimum mandatory requirements for the design and installation of electrical equipment and systems. Engineers cannot deviate from the requirements of the SAES's without written approval from the Saudi Aramco Chief Engineer (Dhahran). User/specifier requirements that exceed the minimum requirement of the SAES's need no waiver approval, even though the requirements are different. The following SAES's apply to system grounding: _ _ _

SAES-P-100 SAES-P-111 SAES-P-119

SAES-P-100

This SAES states the minimum mandatory requirements for the design and installation of electrical power systems. This standard is intended to assist Design Engineers in areas that are not specifically referenced in another Saudi Aramco standard. The only sub-section of this SAES that applies to system grounding is sub-section 4.4. Sub-section 4.4 contains a table that lists the system grounding methods that should be used according to the voltage level of the system that is being grounded.


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Saudi Aramco Engineering Standards (Cont'd) SAES-P-111

This SAES states the minimum mandatory requirements for the grounding of electrical equipment and for the design and installation of grounding and lightning protection systems. The following sub-sections apply to system grounding: _ _

Sub-section Four Sub-section Five

Sub-section Four, titled "System Design," states the minimum mandatory requirements

for the design of a ground system. Sub-section Five, titled "Materials," states the minimum mandatory requirements for the

materials that are used in the design and installation of a ground system. SAES-P-119

This SAES states the minimum mandatory requirements for the design and installation of onshore power substations. The only sub-section of this SAES that applies to system grounding is sub-section six. Sub-section six states the minimum mandatory requirements for terminating surge arrestors and overhead ground wires to the system ground grid. Institute of Electrical and Electronic Engineers Standards (IEEE) IEEE Standards provide information on how to design, test, measure, and specify electrical systems. The information in the IEEE Standards represents the consensus opinion of a group of subject matter experts. The requirements and procedures that are given in IEEE Standards are useful in the design and application of grounding systems. The following IEEE Standards apply to system grounding: _ _ _ _

IEEE 80 IEEE 81 IEEE 142 IEEE 367


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Institute of Electrical and Electronic Engineers Standards (IEEE) (Cont'd) IEEE 80

IEEE 80 is titled "IEEE Guide for Safety in AC Substation Grounding." The intent of IEEE 80 is to provide guidance and information to ensure that safe grounding practices are applied in AC substation designs. Figure 2 shows the table of contents of IEEE 80. The sections that follow provide a brief description of the scope of each chapter. Chapter One describes the purpose and scope of the standard. Chapter Two reviews the objectives of safe grounding system design and the potential

dangers that must be considered during grounding system design. Chapter Three discusses the effects of passage of an electric current through the vital

parts of a human body. The effects are discussed in terms of the electric current's frequency, amplitude, and duration. Chapter Four discusses how to determine the limits to the amount of electrical current

that can pass through the human body. Chapter Five discusses calculations involving the resistance of the human body when

the body becomes an accidental ground circuit. Chapter Six discusses the four voltages that must be considered in the design of a

ground system to prevent electrical shocks: _ _ _ _

Step voltage Touch voltage Mesh voltage Transferred voltage

Chapter Seven discusses the principal design considerations for a grounding system. Chapter Eight discusses grounding requirements for gas-insulated substations. Chapter Nine discusses the requirements for grounding conductor materials and sizes.


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Institute of Electrical and Electronic Engineers Standards (IEEE) (Cont'd)

IEEE 80 Table of Contents Figure 2


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Institute of Electrical and Electronic Engineers Standards (IEEE) (Cont'd) Chapter Ten discusses the soil characteristics that relate to system grounding. Chapter Eleven discusses the different soil structures and how to select a soil model for use in ground system design. Chapter Twelve discusses how to evaluate the ground resistance of a ground system. Chapter Thirteen discusses how to calculate the maximum ground grid current. Chapter Fourteen discusses grounding system design criteria and provides a procedure

for use in ground grid design. Chapter Fifteen reviews the hazards that can result during ground fault conditions that are due to the transfer of potential between the ground-grid area and the points that are outside the ground grid area. Chapter Sixteen discusses the grounding of equipment that requires special attention:

_ _ _ _

Operating handles Fences Cable sheathes Surge arrestors

Chapter Seventeen describes the different methods for construction of a ground grid. Chapter Eighteen discusses methods for performance of field measurements on an

installed grounding system. Chapter Nineteen describes methods for use of scale models in the design of a

grounding system.


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IEEE 81 titled "IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System," discusses present techniques for performance of the measurements. The discussion includes the types of instruments that are available and the possible sources of error. The following specific testing methods are covered in IEEE 81: _

The measurement of the resistance and the impedance to earth of electrodes. The electrodes can be small rods, plates, or large grounding systems.

_

Ground potential surveys that include the measurement of step-andtouch voltages and potential contour surveys.

_

Scale-model tests for a laboratory determination of the ground resistance and the potential gradients for an idealized design.

_

The measurement of earth resistivity.

IEEE 142

IEEE 142 is titled "IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems." Also called the "Green Book," IEEE 142 contains four sections of information, two of which apply to system grounding: _ _

Section One Section Two

Section One discusses the problems that are associated with system grounding and the

advantages and disadvantages of grounded versus ungrounded systems. This section also provides information on how to ground an electrical system, where to ground an electrical system, and how to select equipment for the grounding of neutral circuits. Section Four discusses the problems of obtaining a low-resistance connection to the

earth. The discussion includes the use of ground rods, ground grids, and buried pipes.


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IEEE 367 is titled "IEEE Recommended Practice for Determining the Electric Power Station Ground Potential Rise and Induced Voltage from a Power Fault." IEEE 367 provides guidance on how to calculate the values of ground potential rise and longitudinally induced voltages that interfere with wire-line telecommunications facilities. IEEE 367 also provides guidance on how to reduce the worst case values of ground potential rise and longitudinally induced voltages for use in wire-line telecommunications protection design. National Electrical Code The NEC is published by the National Fire Protection Association. The intent of the NEC is the practical safeguarding of persons and property from the hazards that can arise from the use of electricity. The NEC is updated every three years through proposals that are submitted by the public. The proposals must be reviewed and approved by a series of committee's and councils before the public proposal can become part of the standard. The NEC has two articles that contain information on grounding: _ _

Article 100 Article 250

Article 100

Article 100 contains the definitions of terms that are used in the NEC and that are essential to the proper application of the NEC. Article 100 defines the term "ground" as a conducting connection (whether intentional or accidental) between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth.


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National Electrical Code (Cont'd) Article 250

Article 250 provides information on the requirements for the grounding and bonding of electrical installations. Article 250 is divided into the following 12 parts, A through M, excluding I: _ _ _ _ _ _ _ _ _ _ _ _

Part A, General Requirement Part B, Circuit and System Grounding Part C, Location of System Grounding Connections Part D, Enclosure Grounding Part E, Equipment Grounding Part F, Methods of Grounding Part G, Bonding Part H, Grounding Electrode System Part J, Grounding Conductors Part K, Grounding Conductor Connections Part L, Instrument Transformers, Relays, Etc. Part M, Grounding of Systems and Circuits of 1 kV and Over (High Voltage)


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BASIS FOR INSTALLATION OF SYSTEM GROUNDS IN SAUDI ARAMCO ELECTRICAL SYSTEMS The basis for installation of system grounds is to ensure stable and safe operation of an electrical system. This section discusses the following topics pertinent to stable and safe operation: _ _ _ _ _ _

Electrical Shock Avoidance Fire Protection Ground Fault Protection Electric Noise Control Lightning Protection Static Control

Electrical Shock Avoidance A ground fault in an electrical system can expose a person to a hazardous potential gradient near the fault. Anyone who touches "electrically-live equipment" is exposed to a potential gradient that causes current to flow. The human body operates through the use of low level electrical impulses. A shock current from an electrical power system will overpower these low level electrical impulses. Currents that overpower the impulses that control the voluntary muscles, such as arms and legs, can also affect the involuntary muscle systems of the heart and lungs. The heart and lungs are usually involved with fatalities due to electrical shock. The heart system becomes uncoordinated, going into what is known as fibrillation, in which the pumping action is lost. Once this fibrillation starts, normal operation is rarely resumed even when the shock current is removed. Without blood circulation, human tissues start to die very quickly, particularly in the brain. Even when fibrillation is ended by massage or countershock, irreparable damage has been done if the fibrillation period exceeds one or two minutes. Stoppage of breathing has the same general effect. Even if blood circulates, the lack of oxygen reaching human tissues causes rapid degradation. Resuscitation, when started immediately, does get oxygen into the blood, even before the victim starts breathing naturally. The effects of an electric current passing through the vital parts of a human body depend on the duration, the magnitude, and the frequency of the current. Humans are very vulnerable to the effects of electric current at frequencies of 50 or 60 Hz. Currents of approximately 0.1 A can be lethal. Authorities generally agree that the human body can tolerate more current at low AC frequencies (e.g., 25 Hz or less). Authorities also agree that the body can tolerate five times more DC current than AC current. Even higher currents can be tolerated at frequencies of 3000 - 10,000 Hz.


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Electrical Shock Avoidance (Cont'd) A current of 1 mA is generally recognized as the threshold of perception. The threshold of perception is the current magnitude at which a person is able to detect a slight tingling sensation (in his hands or fingertips) that is caused by the passing current. Currents of 1 to 6 mA (often termed let-go currents) are unpleasant to sustain, but these currents generally do not impair the ability of a person who is holding an energized object to control his muscles and release the object. Currents can be painful in the 9 to 15 mA range. People who receive shocks of this magnitude also experience great difficulty in releasing the energized object that is causing the shock. For still higher currents, muscular contractions can make breathing difficult. These effects are not permanent and disappear when the current is interrupted unless the contraction is very severe and breathing is stopped (not for seconds, but for minutes). Such cases often respond to resuscitation. In the range of 60 to 100 mA, ventricular fibrillation, stoppage of the heart, or inhibition of respiration might occur and cause injury or death. A person who is trained in cardiopulmonary resuscitation should administer CPR until the victim can be treated at a medical facility. Design guides emphasize the importance of the fibrillation threshold. If shock currents can be kept below this value through a carefully designed grounding system, injury or death can be avoided. The probability of electric shock is greatly reduced through fast fault clearing times, in contrast to situations in which the fault currents persist for several minutes, or possibly hours. Both tests and experience show that the chance of severe injury or death is greatly reduced if the duration of current flow through the body is brief. The allowed current value can be based on the clearing time of primary protective devices or that of the back-up protection. Studies have determined that 99.5% of all healthy persons (50 kg or more) can tolerate a current through the heart region that is defined by the following formula: where:

IB = body current in amperes ts = duration of current in seconds Electrical Shock Avoidance (Cont'd) The following publications contain further information on the hazards of electrical shock: _

IEEE Standard, 80 "IEEE Guide for Safety in AC Substation Grounding," Chapters 2, 3, 4, and 5.


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_

IEEE Standard 399, "IEEE Recommended Practice for Power System Analysis," Chapter 12.


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_

ISA Monograph 110, "Electrical Safety Practices," Chapter on "Human Electrical Safety."

Pages 28 and 29 of the publication "Grounding and Shielding in Facilities," by Morrison and Lewis, discuss how shock hazards are developed in electrical equipment and show the importance of grounding conductors in preventing electrical accidents. Fire Protection When a fault occurs in an electrical system, heat is generated at the fault point. If the system is properly grounded, a conductor provides a low-impedance return path for the fault current. This low-impedance path results in a high-fault current that trips the circuit protective device. Improper system grounding or poor ground connections can result in reduced fault currents that might not trip the circuit breaker or fuse. Heating is a function of time. More heat is generated for long-duration faults than for short-duration faults. In addition, electrical faults tend to get worse until the faults burn themselves out. As the two previous statements indicate, when faults are removed in a timely manner, the chances of fire are greatly reduced. The hazard of an electrical fire can be eliminated or reduced through adequate grounding, in accordance with relevant codes and standards, such as those issued by the IEEE and NEC. The IEEE Standards generally deal with the grounding of large electrical substations or distribution systems. The NEC grounding regulations are primarily concerned with lower voltage equipment that are installed within buildings and plants and that are accessible to untrained personnel. Read grounding for fire protection in the supplemental text "Grounding and Shielding in Facilities," page 28. This section provides information on the generation of heat due to faults. Ground Fault Protection Good system grounding, coupled with a low impedance ground return path, will result in a current flow during fault conditions that will activate a ground fault protection device and that will isolate the damaged circuit. There are two forms of ground fault protection: one designed to protect people and the other designed to protect equipment. Devices that protect people operate on currents of 5 mA. The rating of 5 mA is far too sensitive to be applied to normal industrial systems. In industrial systems, the protection (safety) for people is provided through use of the ground grid and ground system. Ground fault protection for many power systems of 480V and less is provided by circuit breakers and fuses. Low voltage circuit breakers and fuses trip on current values that exceed their ampere ratings. However, the NEC defines ground-fault protection as a system that is intended to protect equipment from damaging line-to-ground fault currents by causing a disconnecting means to open all ungrounded conductors of the faulted circuit. This protection is provided at current levels that are less than the levels that are required to protect conductors from damage Saudi Aramco DeskTop Standards

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through the operation of a supply circuit overcurrent device. The NEC has specific rules where ground fault protection is required.


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For voltage levels above 480V, circuit breakers and fuses are used to isolate circuits for ground faults. Circuit breakers can be controlled through ground fault or overcurrent relays. Ground fault relays operate on low ground fault currents. The type of relay that is used depends on how the system is grounded and on the location of the circuit breaker in the system. Fuses are devices in which tripping time depends on the magnitude of the ground fault current. If the system has a high grounding resistance or is otherwise poorly grounded, a fuse might not operate. For more information on ground fault protection, the Engineer should consult the following standards: _

IEEE Std. 242 - 1986 Protection and Coordination of Industrial and Commercial Power Systems, Chapter 7, Ground-Fault Protection.

_

National Electrical Code - 1990- Article 250.


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Electrical Noise Control Electrical noise is a disturbance in an electrical system or in an instrumentation and control system that interferes with the transmission of the signal containing the information. Noise adversely affects both analog and digital electronic equipment systems that are below 30V. Analog systems that operate in the mA ranges and digital signals are most affected by electrical noise. The ability of a control system to perform its designed function is directly dependent upon the quality of the signal of the measured variables. This quality is dependant upon the elimination or attenuation of noise that can deteriorate the actual transducer signal. Electrical noise can be a severe problem in industries such as steel, power, and petroleum, where power consumption is high and complex electrical networks exist. Electrical noise can be reduced in a number of ways. The proper selection of cables and installation is important. Careful evaluation of grounding methods help to ensure that noise generation is eliminated. Conformance with the requirements of the National Electrical Code will also help to eliminate noise generation. Read the supplemental text "Grounding and Shielding," chapters 6 and 7. These chapters provide detailed information on electromagnetic interference (EMI) in different situations and present solutions to noise problems. Lightning Protection Lightning is the discharge of high-potential cells (usually negative) within clouds to the earth. The discharge current increases from zero to a maximum in 1 to 10 _s and then declines to half the peak value in 20 to 1000 _s. This discharge current can be repeated one or more times, over the same path, in rapid succession. The average peak stroke is about 20,000A -although some peak strokes are as great as 270,000A. Lightning can strike a facility in two ways. One way is a direct strike on a building or on another elevated structure; such a strike causes fires or physical damage. The other way is through a lightning strike on an overhead power line. The overvoltage surge can be transmitted through the power line to the facility substation and through the transformers to all the electrical equipment in the facility. It would be necessary to completely enclose a building in metal to provide the building with 100% protection from lightning. This extent of protection is not practical. Design techniques for lightning protection are based on the building size, occupancy, location, and other such factors to provide a practical level of safety at a reasonable cost.


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Lightning Protection (Cont'd) Read the supplemental text "Grounding and Shielding in Facilities," pages 46 and 85. These pages provide general information on lightning protection for buildings, including the following: _ _ _

Where lightning terminals are required. Installation of lightning terminals. Installation of ground cables.

The electrical path for lightning should be straight to ground. Preferably, this path should not include bends. If bends in the path are required, the bend should have a large radius. When possible, multiple air terminals should be interconnected vertically and horizontally. High voltage lightning surges on transmission lines are eliminated or greatly reduced through the use of devices that are known as lightning or surge arresters. Surge arresters act as insulators during normal system operation. During a high voltage surge, however, these devices directly shunt the current to ground without developing dangerous voltages. These devices should be connected directly to the system ground. An overhead grounding wire that runs above the phase wires and is grounded at frequent intervals also is used to protect equipment from lighting strokes. For more information on lightning protection, the Engineer should refer to the following standards: _

IEEE Std. 142-1982-IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, Chapter 3, Static and Lightning Protection Grounding.

_

NFPA 78 - 1989 - Lightning Protection Code.

The following Saudi Aramco Engineering Standards and Saudi Aramco Design Practices apply to lightning protection: _ _ _

SAES-P-111 Chapter 9, Lightning Protection SAES-P-119 Chapter 6, Substation Yard SADP-P-111 Chapter 9, Lightning Protection of Building and Structures


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Static Control Static electricity is the accumulation of electrostatic charges on the surfaces of nonconductors or on conducting bodies that are insulated from their surroundings. The following are the common ways that static electricity is generated in industry: _

Belts that are made of rubber, leather, or other insulating materials that are running at moderate or high speeds generate considerable quantities of static electricity. The generation occurs where the belt separates from the pulley. The charges will occur on the pulley (regardless of whether it is conducting or nonconducting) and the belt.

_

When a tank truck that is insulated from the ground by dry rubber tires is being filled with liquid, a charge develops on the surface. This surface charge will attract a charge of the opposite polarity on the interior of the metal tank wall. The exterior of the tank will have a free charge of the same polarity as the surface charge of the liquid. This charge is capable of producing a spark to ground.

_

The human body in a low-humidity area can accumulate static charges of several thousand volts in different ways. Contact of shoes with floor coverings can develop a charge. Also, proximity to machinery that generates static electricity can also result in a charge being developed.

Static electricity can be controlled and eliminated in industrial processes. A common method of control is to allow the static charge to bleed off through bonding or grounding. Unlike general system grounding, a low resistance to ground is not necessary to dissipate static charges. For information on static control, the Engineer should consult the following Saudi Aramco and industry codes and standards: _

SADP-P-111 Chapter 14 - Safeguard Against Static Electricity, Lightning and Stray Currents.

_

IEEE Std. 142 - 1982 IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, Chapter 3 - Static and Lighting Protection Grounding.

_

NFPA 77 - 1988 Static Electricity.

_

NFPA - Electrical Installations in Hazardous Locations.


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DETERMINING THE APPROPRIATE METHOD OF SYSTEM GROUNDING FOR SAUDI ARAMCO ELECTRICAL SYSTEMS Electrical Engineers must be able to determine the appropriate method of system grounding before ground grids can be designed for Saudi Aramco electrical systems. Work Aid 2 provides a table showing the grounding methods that should be used for different combinations of voltages, phases, and loads. The sections that follow provide information on the following topics: _ _ _ _ _ _

Solid Grounding Grounding Transformers Impedance/Resistance Grounding Reactance Grounding Ungrounded Systems Comparison of Methods (Advantages/Disadvantages)

Solid Grounding A solidly-grounded system is a system of conductors in which one conductor or point is grounded. Figure 3 shows a 115 kV transmission system, a 69 kV transmission system, a 480V bus and a 240/120 V bus that are all solidly grounded. Solid grounding indicates that no impedance is intentionally inserted between the electrical system and the earth ground point. The connection point is normally the middle wire or the neutral point of a transformer or generator winding. Solid grounding provides the highest level of ground fault current and the lowest level of transient overvoltages. All Saudi Aramco systems that are rated 600V and below should be solidly grounded. Saudi Aramco transmission and distribution systems of 34.5 kV, 69 kV, 115 kV, and the receiving point of 230 kV systems should also be solidly grounded. The reasons for solid grounding at higher voltages are as follows: _

Rotating equipment is seldom rated above 15 kV.

_

It is not necessary to limit ground fault current to protect motors at higher voltages.

_

Voltages above 15 kV are usually outdoors.

_

Hazards to buildings and personnel are reduced at high voltages.


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Solid Grounding (Cont'd)

Solidly Grounded Systems Figure 3 Saudi Aramco DeskTop Standards

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Grounding Transformers Saudi Aramco 2400V distribution systems receive power from transformers with ungrounded delta secondary windings. These ungrounded systems are subject to overvoltage conditions and unstable phase voltage conditions due to lack of a grounded neutral. Saudi Aramco Engineering Standards and current engineering practices require that all electrical systems be grounded for reasons of safety and reliability. The existing ungrounded delta secondary winding could be corner grounded, but corner grounding places the other two phases at line-to-line voltage, which is undesirable. The preferred method for provision of a neutral point for grounding in an existing ungrounded delta system is to use a grounding transformer. A grounding transformer is a transformer that is installed in a system for the sole purpose of providing a neutral point for grounding the system. Saudi Aramco uses two types of grounding transformers: _ _

Distribution Transformers Zig Zag Transformers

Both types of grounding transformers provide a suitable system ground connection, although the zig zag transformer is more economical and should be selected over the distribution transformer for most installations. The only time a distribution transformer is normally used is when a distribution transformer is readily available and a zig zag transformer is not readily available. Distribution Transformers

A three-phase distribution transformer with wye-delta connections or three single-phase distribution transformers that are connected in a wye-delta configuration can be used to provide a system ground on an existing ungrounded system. Figure 4 shows a three-phase distribution transformer that is connected to provide a system ground. The distribution bus that is shown in Figure 4 receives power from the ungrounded deltaconnected secondary of the system power supply transformer. The system ground for the distribution bus is obtained through connection of the distribution type grounding transformer. When a phase-to-ground fault occurs on the distribution bus, a complete path for the groundfault current (shown by the arrows) exists: _

Ground-fault current will flow through the phase-to-ground fault from the power supply end and the load end of the distribution bus.


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Grounding Transformers (Cont'd)

Three-Phase Distribution Transformer Connected to Provide a System Ground Figure 4


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Grounding Transformers (Cont'd) _

The ground-fault current will then flow through the earth and into the system ground connection.

_

The ground-fault current will flow through the grounding resistor and into the neutral of the wye connected primary of the distribution type grounding transformer.

_

The ground-fault current will flow out of the wye-connected primary of the distribution type grounding transformer and back to the distribution bus to complete the path for the ground-fault current.

_

When the ground-fault current flows through the wye-connected primary of the distribution type grounding transformer, current will also flow in the delta-connected secondary of the distribution type grounding transformer.

When a distribution transformer with wye-delta connections is used to provide a system ground, the secondary winding must be a closed-delta connection to allow zero sequence currents to flow. An open-delta secondary connection would also reflect an extremely high impedance into the primary winding, and the resulting high primary winding impedance would severely limit the amount of ground-fault current that could flow. Zig Zag Transformers

Zig Zag is defined as a line or course that turns sharply in one direction and later turns sharply in another direction. A zig zag transformer has two phase windings on each leg of the transformer core. The internal connection of the transformer is shown in Figure 5. The impedance of the zig zag transformer to a balanced three-phase voltage is high. The zig zag transformer has a neutral lead that is connected to ground and three other leads (line leads) that are connected to the bus. When there is no fault on the system, only a small magnetizing current flows in the transformer winding. However, the transformer impedance to zerosequence voltages is low so that the transformer allows high ground fault currents to flow. The transformer divides the ground fault current into three equal current components. These equal currents are in phase with each other and flow in the three windings of the zig zag transformer. The method of winding, shown in Figure 5, is such that when these three equal currents flow, the current in one section of the winding on each leg of the core is in a direction that is opposite to the current flow in the other section of the winding on that leg of the core. The result is that the ground-fault current is equally divided in the three lines. This division accounts for the low impedance of the transformer to ground currents.


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Zig Zag Transformer Figure 5


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The distribution bus in Figure 6 receives power from the system power supply transformer. The system ground is obtained through connection of a zig zag transformer, as shown in Figure 6. When a phase-to-ground fault occurs, zero sequence currents will flow. Zero sequence currents are in phase and have same magnitude. The zero sequence currents will flow through the grounding resistor and into the zig zag transformer, where the currents divide equally in the three legs of the transformer. Each leg has two windings that are wound in the reverse direction. The two windings cancel each other's magnetic flux. This cancellation results in a low impedance for zero sequence currents. Under normal operating conditions, the windings of each leg are 120o out of phase. Because this phase relationship results in a large transformer impedance, only a small magnetizing current flows through the zig zag transformer when there is no ground fault in the system.


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Zig Zag Transformer Connected to Provide a System Ground Figure 6


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Impedance/Resistance Grounding The terms "impedance grounding" and "resistance grounding" have similar meaning. The electrical characteristic that is known as resistance is actually a component of the electrical characteristic that is known as impedance. SADP-P-111 defines both impedance grounding and resistance grounding. In an impedance grounded system, the neutral line is connected to ground through an impedance. SAES-P-111 lists grounding impedances as a resistor, a reactor, or a distribution transformer. In a resistance grounded system, the neutral line is connected to ground through an impedance, and the principal element of that impedance is resistance. IEEE-Std. 142-1982 and SADP-P-111 give the "resistance grounded system" the same definition. IEEE Std. 142-1982 does not define the term "impedance grounded." IEEE Std. 142-1982 uses the terms "resistance grounded" and "reactance grounded" to refer to the two methods of grounding an electrical system through an impedance. When the Saudi Aramco Standards refer to impedance grounding, these standards are actually referring to resistance grounding because resistors are the preferred type of grounding impedance for Saudi Aramco electrical systems. In cases where Saudi Aramco does not use resistors as the grounding impedance, the type of grounding impedance that is required will be specified by a term other than "impedance grounded" (e.g., reactance grounded). In an ideal electrical system, impedance grounding would be used for all voltages above 600V because the short circuit capability of the system increases as the system voltage increases. The short circuit capability of a system refers to the ability of the system to damage itself under fault conditions (grounds) due to the excessive current that flows through the system under ground-fault conditions. When a ground-fault occurs in a system, the ground fault current must flow from the power line through ground and must return through the grounded neutral line. The magnitude of the ground-fault current can be significantly reduced through placement of an impedance in series with the neutral line. The specific reasons for limiting the amount of ground-fault current that can flow in a system can include one or more of the following: _

To reduce the burning and melting effects in failed electrical equipment such as transformers, cables, and rotating equipment

_

To reduce the mechanical stresses in circuits and apparatus that are carrying fault currents


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Impedance/Resistance Grounding (Cont'd) _

To reduce the electrical shock hazards to personnel that are caused by ground-fault currents in the ground return path

_

To reduce the arc blast or flash hazard to personnel who may have caused or who are close to the ground-fault

_

To reduce the momentary line voltage drop caused when ground-faults occur and when ground-faults are subsequently cleared

_

With high resistance grounding to gain control of transient overvoltages and to avoid the shutdown of a faulty circuit on the occurrence of the first ground fault.

Saudi Aramco follows the current engineering practice of using impedance grounding only in medium voltage electrical systems (1001V - 15,000V). The main reason impedance grounding is not used above 15,000V is that the required resistors are so large that the costs are prohibitive. Two classes of impedance grounding are available: _ _

high resistance low resistance

The two classes of impedance grounding differ in the magnitude of ground-fault current that is permitted to flow. The sections that follow describe each class in more detail. High Resistance Impedance Grounding

Saudi Aramco does not have any normal applications for high resistance impedance grounding. High resistance impedance grounding uses grounding resistors that limit the ground-fault current to 10A or less. High resistance impedance grounding has very few practical applications because of this low level (10A) of ground-fault current. 10A of groundfault current is not enough to reliably operate protective devices. High resistance impedance grounding can only be used in applications in which power supply continuity is critical and in which the system can tolerate a ground-fault for the anticipated period of time that is necessary to locate and clear the ground-fault.


30



Impedance/Resistance Grounding (Cont'd) Low Resistance Impedance Grounding

Low resistance impedance grounding uses grounding resistors that limit the ground-fault current to 25A or more. Low resistance impedance grounding is the preferred method of impedance grounding because low resistance impedance grounding limits the ground-fault current to safe levels and provide sufficient ground-fault current to reliably operate protective devices. In the following systems, Saudi Aramco uses low resistance impedance grounding with resistors that are sized to limit the ground-fault current to 400 amps for ten seconds: _ _

Distribution systems serving 4160 volt, three-phase loads Distribution systems serving 13,800 volt, three-phase industrial loads

In 13,800 volt residential distribution systems, Saudi Aramco uses low resistance impedance grounding with resistors that are sized to limit the ground-fault current to 1000 amps for ten seconds. Sizing Impedance Grounding Resistors

Three electrical ratings are required to select the correct size of grounding resistor: _ _ _

Grounding Resistor Voltage Raging Grounding Resistor Current Rating Grounding Resistor Time Rating

Grounding Resistor Voltage Rating - The grounding resistor voltage rating is equal to the phase-to-neutral voltage of the system. The phase-to-neutral voltage is also called the phase-to-ground voltage. The phase-to-neutral voltage of the system is calculated through division of the phase-to-phase voltage by the . The phase-to-phase voltage is also called the line-to-line or system voltage. For example, the phase-to-neutral voltage of a 13,800 volts distribution system would be calculated as follows:

This example shows that the required grounding resistor voltage rating for a 13,800 volt distribution system is 7967 + 10% voltage variation = 8764 volts.


31



Impedance/Resistance Grounding (Cont'd) Grounding Resistor Current Rating - The grounding resistor current rating is equal to the

value of ground-fault current that will flow in the system. The designer of the system must choose the grounding resistor current rating on the basis of a compromise of the following two requirements: _

The ground-fault current must be low enough to minimize the damage resulting from a ground-fault.

_

The ground-result current must be high enough to reliably operate protective relays.

The generally accepted engineering standard is that ground relays should operate on 10% of the maximum current allowed by the grounding resistor. Most distribution systems use grounding resistors with current ratings of 50 amps and higher because of this 10% standard. With 50 amps or higher ground-fault currents, there are many readily available relays and CT's that will reliably operate on 5 amps of current (10% of 50 amps). Saudi Aramco uses grounding resistor current ratings of 400 amps. Grounding Resistor Time Rating - The standard grounding resistor time ratings are as

follows: _ _ _ _

Ten seconds Nine minute Ten minutes Extended time

The grounding resistor time rating indicates the amount of time that a grounding resistor can operate under ground-fault conditions without exceeding the allowable temperature rise above 50oC. The allowable temperature rises above 50oC are as follows: _ _ _ _ _

60oC temperature rise for ten second time ratings. 60oC temperature rise for one minute time ratings. 10oC temperature rise for ten minute ratings. 10oC temperature rise for extended time ratings. 85oC temperature rise for steady state conditions.


32



Impedance/Resistance Grounding (Cont'd) Grounding resistors with ten second time ratings should be specified for systems that have protective relays to isolate the circuit under ground-fault conditions. The ten second time rating is standard for Saudi Aramco electrical systems. Some Engineers may specify a one minute or ten minute time rating for an extra margin of safety; however, grounding resistors with higher time ratings are more expensive. The extended time rating is normally only specified for distribution systems that must continue to operate with a ground-fault for more than ten minutes. An example would be a distribution system that supplies power to a facility that cannot be shutdown in mid-process without a significant cost. Reactance Grounding A reactance grounded system is a system in which the neutral line is connected to ground through an impedance and in which the principal element of that impedance is reactance. The reactance is provided through the use of a grounding reactor. Grounding reactors are less expensive than grounding resistors for low impedance, high current applications. Saudi Aramco uses reactance grounding in place of solid grounding for systems in which the ground-fault current could exceed the three-phase fault current by 25%. A possible application might arise on a system when a solid ground is indicated, but the ground fault currents could exceed the three-phase fault levels and the circuit breaker short circuit capacities. Ungrounded Systems In an ungrounded system, the generator or transformer neutral does not have a physical connection to ground. In reality, all systems are grounded to some degree because of the natural capacitance between the system elements and ground through the insulation system. Ungrounded electrical systems were originally designed on the basis of the assumption that an ungrounded system would provide greater service continuity than a grounded system. Greater service continuity is achieved because grounding of any one phase of an ungrounded system will not cause the protective devices to operate; a complete path for the ground fault current will not exist. The only change in the system will be an increase in the voltage of the ungrounded phases. Recent experience has shown that in many systems greater service continuity is obtained with grounded-neutral systems than with ungrounded neutral systems. Because of this recent experience, current engineering practice is to install grounded rather than ungrounded electrical systems.


33



Ungrounded Systems (Cont'd) Saudi Aramco still has some 2400V ungrounded electrical systems; however, Saudi Aramco does not permit the installation of new ungrounded systems. Saudi Aramco also does not permit the extension of existing ungrounded electrical systems without the approval of the Consulting Services Department, Dhahran. Comparison of Methods (Advantages/Disadvantages) The advantages and disadvantages of each method of system grounding change with the voltage level of the system in which the grounding method is used. The following sections list the advantages and disadvantages of the different methods of system grounding for the following three ranges of system voltages: _ _ _

80 Volts and Below 1.4 kV to 13.8 kV above 13.8 kV

480 Volts and Below

Solidly grounded systems in this voltage range provide the following advantages: _

Transient overvoltages will not be excessive.

_

The faulted zone of the system can be automatically segregated.

_

Initial costs for installation are lowest.

_

Sufficient ground-fault currents are provided at remote locations to operate protective devices when the ground network is installed properly.

Solidly grounded systems in this voltage range have the following disadvantage: _

The grounding network for the system must provide a very low impedance return path for the ground-fault currents in order for the protective devices to operate properly.

There are no stated advantages and disadvantages for low resistance grounding in 480 volt systems. Low resistance grounding is not specifically covered by the National Electrical Code and should not be considered as an adequate grounding system.


34



Comparison of Methods (Advantages/Disadvantages) (Cont'd) High resistance grounding in this voltage range is permitted when all of the following conditions are met: _

The conditions of maintenance and supervision ensure that only qualified people will service the installation.

_

Continuity of power is required.

_

Ground detectors are installed on the system.

_

Fine-to-neutral loads are not served.

High resistance grounding in this voltage range provides the following advantages: _

Because one ground fault will not isolate power to electrical equipment, operation will not be interrupted.

_

Transient overvoltages will not be excessive.

High resistance grounding in this voltage range has the following disadvantages: _

A dangerous potential will exist on the faulted equipment until the ground fault is cleared.

_

Most circuit breakers do not sense a sufficient amount of ground current to function.

_

Ground fault can be difficult to locate.

_

ine-to-neutral loads cannot be used.

_

lectrical coordination is lost.

_

Additional expense of ground fault detection is required.

_

Ground fault on one phase increases the voltage of the other phases to line voltage-to-ground.


35



Comparison of Methods (Advantages/Disadvantages) (Cont'd) 2.4 kV to 13.8 kV

Solidly grounded systems in this voltage range provide the following advantages: _

Transient voltages are limited.

_

Solidly grounded systems are less expensive than are resistance or reactance grounded systems.

Solidly grounded systems in this voltage range have the following disadvantages: _ _

Ground fault currents may exceed three-phase fault currents. Ground fault currents at this level are dangerous to equipment and people.

Low resistance grounded systems in this voltage range provide the following advantages: _ _ _

Phase-to-ground fault currents are greatly reduced. Haulted zones are automatically tripped. Transient overvoltages are not excessive.

Low resistance grounded systems in this voltage range have the following disadvantages: _ _

These systems are more expensive than solid grounding. Relaying is required.

High resistance grounded systems in this voltage range provide the following advantages: _ _ _

Phase-to-ground fault current is reduced to a low level. Power is not disconnected for ground faults. Transient overvoltages are not excessive.

High resistance grounded systems in this voltage range have the following disadvantages: _ _ _ _

These systems are more expensive than solidly grounded systems. Ground detection equipment is required. Fault levels are too low for normal relaying. Ground faults on the system are dangerous to people and equipment.


36



Comparison of Methods (Advantages/Disadvantages) (Cont'd) Above 13.8 kV

Solidly grounded systems in this voltage range provide the following advantage: _ _

Transient voltages are limited. These systems are the least expensive.

Solidly grounded systems in this voltage range have the following disadvantages: _

High fault currents can be produced and these fault currents represent a hazard to personnel if these voltages are carried inside buildings.

Resistance grounded systems in this voltage range provide the following advantage: _ _

Tansient voltages are limited Phase-to-ground fault currents are reduced

Resistance grounded systems in this voltage range have the following disadvantages: _

Resistors are too expensive at this level

Low reactance grounded systems in this voltage range provide the following advantages: _

These systems reduce ground fault currents to produce 26-100% of the three-phase fault value.

_

Transient overvoltages are limited.

_

Faults can be automatically isolated.

Low reactance grounded systems in this voltage range have the following disadvantage: _

These systems are more expensive than solid grounding.


37



DESIGNING A SUBSTATION/PLANT GROUND GRID FOR SAUDI ARAMCO ELECTRICAL INSTALLATIONS An Engineer must be familiar with the following topics in order to design substation/plant ground grids: _ _ _ _ _ _ _ _ _

Ground Grid Concepts Surface Soil Resistivity Ground Potential Rise None of Influence Transfer Potential Step-and-Touch Potential Grid Depth and Number of Ground Rods Wire Sizing Fault Times

Ground Grid Concepts As explained in SAES-P-111, grounding systems perform the following main functions: _

To safeguard a person from electric shock by ensuring that, under fault conditions, all surfaces with which a person is in simultaneous contact, including those of metallic equipment and the ground, remain at safe relative potentials.

_

To safeguard electrical equipment by grounding power systems to ensure that, under fault conditions, both voltages and currents are within predictable limits and that the protective devices will operate reliably and with appropriate discrimination.

_

To provide a path to ground from lightning arrestors that might operate due to direct lightning strikes, to lightning induced surges, or to switching surges.

_

To reduce the possibility of static electricity discharge that would present a fire risk in hazardous areas.

A grounding system consists of the grounding conductors that connect all items to be grounded and of a grounding electrode or grounding electrodes. The use of multiple grounding electrodes is known as a ground grid, and the ground grid forms the medium of contact with the earth. The ground grid can consist of buried conductors in a cross, of a grid, or of another formation.


38



Ground Grid Concepts (Cont'd) The purposes of the ground grid are to provide a low resistance path to earth for fault currents and to limit the rise of ground potentials that could generate surface gradients that are unsafe for human contact. The following factors influence the design of a grounding system: _

The maximum prospective ground-fault current that can pass between the fault location and the system neutral point or points and the duration of the ground-fault current flow. The size of the ground fault current governs the grounding conductor size.

_

The proportion of the ground fault current that will pass between the grounding system and the body of earth and the duration of the current flow. This factors govern the electrode design.

_

Site soil resistivity.

_

The degree of exposure to mechanical damage and corrosion. This factor will influence the choice of materials and the manner of installation.

Surface Soil Resistivity The grid resistance and the voltage gradient within a substation are directly dependent on the soil resistivity. The surface soil resistivity is the resistivity of the upper layer of the soil. This resistivity is important because it helps limit the body current through addition of resistance to the equivalent body resistance. That is, if the upper layer of the soil is high in resistivity, the amount of current through the body of a person in contact with an energized component is reduced. A thin layer of crushed rock on the surface can raise the surface soil resistivity and can result in higher permissible step and touch voltages. Because the effective length of the grid conductor is inversely proportional to the permissible body contact voltages, an increase in surface soil resistance allows a greater body contact voltage; consequently, a shorter grid length can be used for the same area. The result is greater grid spacing.


39



Ground Potential Rise (GPR) Ground potential rise (GPR) is an AC potential difference between remote earth (reference of zero potential) and local ground. The magnitude of the maximum expected GPR determines the type of protection that is required for communication equipment as follows: _

Locations at which the maximum expected GPR (or voltage stress magnitude) is less than 300V are classified as low risk sites. The amount of protection that is required for circuits at these sites is minimal and depends upon the reliability needs.

_

Locations at which the maximum expected GPR (or voltage stress magnitude) is between 300V and 1500V are classified as moderate hazards. Protection must be applied to all circuits. The acceptable protection methods should be determined by SAES-T-887.

_

Locations at which the maximum expected GPR is above 1500V are considered severe hazard sites. Protection methods will include isolating or neutralizing transformers as determined by SAES-T-887.

Ground potential rise (GPR) is essentially the product of the following: _

The total ground grid resistance to a remote earthing point (outside the zone of influence of the GPR at the power system fundamental frequency).

_

The total net fault current that flows through the ground grid.

GPR can be expressed by the following equation: GPR where:

=

IG x RG

IG grid

=

portion of the total fault current that flows through the to remote earth.

RG

=

ground grid resistance to remote ground.


40



Ground Potential Rise (GPR) (Cont'd) The total prospective ground-fault current at a location should be determined through a system fault current study. The sub-transient symmetrical R.M.S. value of the ground-fault current "Ig" should be used. Frequently, a fault level analysis or relaying study will have been performed for the location and will give the required information. For all power plants, it should be assumed that the maximum ground-fault current is equal to the breaker's symmetrical interrupting capability. The Electrical Engineer should determine the proportion of this current that is passing between the electrode and earth; he should then apply a factor for future system growth. This factor is a matter of judgement and is based on the following indicators: _

Nearness of the calculated three-phase symmetrical fault level for the location to the circuit breaker interrupting capacity. If these values are close, the location is at a point of high fault level and the chance of future increases are limited.

_

The probability of development in the area (especially power generation). A remote location on the fringe of an established oil field can increase little in fault level. A location in an area of future development can increase greatly.

The computer program MALT, developed for Saudi Aramco installations, can analyze the effects of buried grounding electrodes. MALT should be utilized to review the grounding electrode design for all major industrial facilities (e.g., desalting facilities, seawater injection plants, gas plants, etc.). MALT can be used to determine the following: _

The resistance to remote earth of grounding electrodes (grid) in a one layer soil model (regardless of shape, depth of burial, and size of conductors).

_

The resistance to remote earth of electrodes (grid) in a two-layer soil model (regardless of shape, depth of burial and size of conductors).

_

The GPR of a given site.

Zone of Influence The elevated potential of the industrial site grounding electrode during a ground fault results in a rise in potential of the earth inside and outside of the plant boundaries. The potential gradient that is outside of the site decreases as the distance outward increases. This decreasing potential gradient is known as the zone of influence.


41



Zone of Influence (Cont'd) A potential contour survey can locate the hazardous potential gradients near grounded electrical structures for each fault type and location. The voltage drop to points surrounding the structure are measured from a known reference point and is plotted on a map of the location. A potential contour map then can be drawn through connection of the points of equal potential with continuous lines. If the contour lines have equal voltage differences between them, greater hazards are indicated by closer lines. Actual gradients that are due to ground fault currents are obtained through multiplication of test current gradients by the ratio of the fault current to the test current. A typical contour map of a substation grid is shown in Figure 7. The area that is contained by the perimeter (B) in Figure 7 is termed the zone of influence of the GPR. The permissible magnitude of the voltage that is along the perimeter (B) is by choice or design and is often limited by an agreement among the authorities concerned to a maximum of 300V. The most accurate measurements of potential gradients are made through use of the voltammeter or current injection method. A known current, usually between 1 and 100 A and between 55 and 70 Hz, is injected into a remote ground test electrode through use of an insulated conductor. A current that is greater than 50 A (personnel and equipment safety considerations are to be observed) is preferred where the ground impedance is less than 1_. Where electronic measuring instruments are used (for example, a digital frequency selective voltmeter), a test current much less than 50 A is satisfactory. This procedure would not apply where insulated overhead ground wires are employed and where calculations would be required. A remotely located ground test electrode is necessary to prevent gradient distortion from the mutual impedance of inadequately spaced ground electrodes. The distance between the ground under test and the remote current electrode can vary from less than 300 meters for a small ground grid or an isolated station to a kilometer or more for larger installations and for installations in densely populated areas. Measurements of the potential should be made with a very high impedance meter that is connected between the ground grid and a test probe, which is driven into the earth along the profile lines radial to the power station. Unless suitable means are employed to mask out the residual ground current and the other interference, the test current must be of sufficient magnitude to do the masking. External power frequency and harmonic components are removed through use of filtering. At the same time, in order to avoid variations in voltage gradients during a series of measurements, care must be taken to prevent heating and drying of the soil that is in contact with the ground grid or the test electrode. Low-current test methods will produce approximate results. Economics and the necessary or the desired accuracy that is required will dictate the use of these methods or other methods and the number of measurements to be made.


42



Zone of Influence (Cont'd)

Boundary of the GPR Zone of Influence Figure 7


43



Zone of Influence (Cont'd) When more than one overhead or underground cable is connected to a substation, potential gradients in and around the substation can be quite different for faults that are on different lines or cables. Similarly, faults at different locations in large substations also can result in differences in potential gradients in and around the power station. Potential gradients in and around a large substation should be determined for two or more fault conditions. Underground metallic structures, metallic structures on the surface of the earth, metallic fences, and overhead ground wires that are near a substation, whether connected to the ground grid or not, will usually have a significant effect on the potential gradients and should be considered in potential gradient measurements. These structures include neutral conductors, metallic cable sheaths, metallic water and gas lines, and railroad rails. When a potential gradient study cannot be economically justified, potential gradients can be calculated from ground resistance and soil resistivity measurements. The accuracy of such calculations will depend on the accuracy of the measurements and on the unknown abnormalities of the earth around and below the ground grid. The adequacy of such calculations then can be verified with relatively few potential gradient measurements. Depending upon the magnitude of a GPR, the following effects can arise outside a substation or adjacent to a power line grounding electrode or transmission line tower (within the zone of influence of the GPR): _

The potential can be transferred through a metal part, bonded with or coupled resistively to the plant grounding electrode(s), to remote locations.

_

The touch voltage between a part that is grounded to the plant grounding electrode and a local ground (for example, a high-voltage interface ground) can be excessive.

_

Reversed touch voltage (or voltage stress) between the local ground and a part having a lower or even zero potential (for example, a telephone cable protection interface) can become excessive.


44



Zone of Influence (Cont'd) Paragraph 4.1.6 in SAES-P-111 describes a situation in which a new grounding system is connected to an existing grounding system. The resultant zone of influence is that of the composite system. SAES-P-111 paragraph 4.1.6 reads as follows: "Where a new grounding system is connected to or located within the zone of influence of an existing grounding system, the two grounding systems shall be interconnected by a minimum of two conductors per grid. The designer of the new grounding system shall be responsible to review the overall grounding system. Recommendations for upgrading the existing system(s), if required, shall be made to the Operations Department of the respective area." Transfer Potential Transfer is defined as the relocation of a hazardous potential from a ground-grid area to points that are outside of the ground-grid area. A serious hazard may result during a fault from the transfer of potentials between the ground-grid areas and outside points. This transfer of potentials is done by conductors (such as communication and signal circuits, low-voltage neutral wires, conduit, pipes, rails, and metallic fences). The danger usually is from contacts of the touch type. The importance of the problem results from the very high magnitude of potential difference that is often possible. Induced voltages on unshielded communication circuits, static wires, and pipes, can result in transferred potentials exceeding the sum of the GPR's of both the faulted substation and the source substation. Rails entering the station, when connected either intentionally or otherwise to the ground grid, can theoretically create a hazard at a remote point by transferring the grid potential rise during a fault. Similarly, if the rails are grounded remotely, a hazard can be introduced into the station area. Hazards are possible where the neutrals of low-voltage feeders or secondary circuits that serve points that are outside of the station area are connected to the station ground. When the potential of the station ground grid rises as the result of ground-fault current flow, all or a large part of this potential rise can appear at remote points as a dangerous voltage between this grounded neutral wire and the adjacent earth. Where other connections to earth are also provided, the flow of fault current through these connections can, under unfavorable conditions, create gradient hazards at points that are remote from the station. Each installation should be reviewed for transfer potential hazards, and corrective action should be taken as required.


45



Transfer Potential (Cont'd) For communication circuits, schemes have been developed that involve protective devices and insulating and neutralizing transformers to safeguard personnel and terminal communication equipment. The introduction of fiber optics to isolate the substation communications terminal from the remote terminal can eliminate the transfer of high potentials. Fiber optics should be considered when potentials cannot be easily controlled by more conventional means. Rail hazards can be removed by removable track sections where the rails leave the groundgrid area, or through installation of several insulating joints in the rails that are leaving the grid area. A second set of insulating joints that are beyond the first set would protect against the shunting of a single set by a metal car or the soil itself and would also reduce the remote hazard of potential differences across a joint itself. The insulating joints must be capable of withstanding the potential difference between remote earth and the potential transferred to the joint. Adequate creepage distance should be ensured to offset any pollution or contamination problems. Step and Touch Potential Step potential (voltage) is the difference in surface potential that is experienced by a person that is bridging a distance of one meter with his feet, without contacting any other grounded object. Touch potential (voltage) is the potential difference between the ground potential rise and the surface potential at the point where a person is standing with his hands in contact with a grounded structure. The maximum touch voltage is to be found within a mesh of a ground grid. Death can occur from step and touch potentials, depending on the magnitude and the duration of the fault. Body conditions that can reduce resistance, such as wet hands or shoes, also can increase the probability of death or injury.


46



Step and Touch Potential (Cont'd) As given by IEEE Std. 80, the following formulas are used to determine the allowable stepand-touch potentials: _

The maximum driving voltage of any accidental circuit should not exceed the following limits:

-

For step voltage the limit is for a man weighing 50 kilograms is:

Similarly, the touch voltage limit is: where:Cs (hs1 K)

=

1 for no protective surface layer.

_s

=

the resistivity of the surface material in ohm - M = 150 (assumed measured value) or 3000 ohm-m for crushed rock.

ts

= for

duration of shock current in seconds - (1 second Saudi Aramco installations).

Grid Spacing

If either the step voltage limit or the touch voltage limit are exceeded, a revision of the grid design is required. The revision may include smaller conductor spacings, and adding additional ground rods. Details of the potential revision can be found in IEEE Std. 80, Section 14.7. Even if the step voltage limit or touch voltage limits are met, additional grid conductors and ground rods can be required if the grid design does not include conductors that are near the equipment to be grounded (such as surge arrestors and transformers).


47



Step and Touch Potential (Cont'd) Another method to improve step and touch potentials is addition of crushed rock on the surface of the soil. Crushed rock will change the K factor in the CS (hS1K) portion of the step voltage limit and touch voltage limit formula as follows: where:

_s

=

crushed rock resistivity in ohm-m

_

=

earth resistivity in ohm-m

The reduction factor Cs is also changed as a function of the change in K and the change in the thickness of the layer of crushed rock (hs), as shown in Figure 8.

Reduction Factor Cs as a Function of Reflection Factor K and Crushed Rock Layer Thickness hs Figure 8


48



Step and Touch Potential (Cont'd) Example:

Find the ETouch 50 for the following conditions: _s ts hs _

= = = =

2000 (using crushed rock) 1 0.1 150

for this example, K is determined as follows: From Figure 8, Cs = 0.58 Therefore: = 318V Grid Depth and Number of Ground Rods Saudi Aramco Engineering Standard SAES-P-111 states that ground grids are to be buried to a minimum depth of 460 mm (18 in.). This method effectively reduces step and touch voltages on the earth's surface. The total length of the grid reduces the step and touch voltages and the grid resistance. The grid length includes both conductor length and ground rod length. The physical conditions at a substation dictate the number and the length of the ground rods vs. the conductor grid length. The ground rods are normally installed at the perimeter of the grid to moderate the increase of the surface gradient that is near the peripheral meshes. Ground rods should also be installed at major equipment and especially at lightning arresters. Rods that penetrate the lower resistivity soil are far more effective in dissipating fault currents when a two-or-multilayer soil is encountered and when the upper soil layer has higher resistivity than the lower soil layers. Ground rods that are in proximity are far less effective at dissipating fault currents than individual ground rods that are well spaced. Wire Sizing In AWG, the numbers are regressive: that is, a larger number denotes a smaller wire. Each wire size (in AWG) often is represented in circular mils. One circular mil (cm) is the area of a wire with a diameter of 0.001 inches. The cm measure is simply the diameter in mils squared.


49



Under fault conditions, all of the heat is assumed to be retained in the conductor, because little time is available to dissipate the heat. The fusing temperature of the conductor, the temperature limit of the connections, and the physical strength of conductors are evaluated to determine a conductor size. The conductor size must relate to the current/time rating of the neutral grounding device or the devices, subject to a minimum size of 780 sq. mm. (No. 2/0 AWG) for mechanical robustness. Figures 9 and 10 show the minimum size for conductors to be used to ground Saudi Aramco systems.

Solidly Grounded Systems over 600V Saudi Aramco DeskTop Standards

50



Figure 9


51



Wire Sizing (Cont'd)

Impedance Grounded Systems over 600V Figure 10 Fault Times Fault time is the duration of time that the fault current flows before being interrupted. The required wire size (Figures 9 and 10) may not be adequate for assumed long fault times. If long fault times are assumed, larger wire sizes may have to be used. The required wire size, based on fault times, can be calculated to determine whether the required wire size is within the suggested size in Figures 9 and 10.


52



Fault Times (Cont'd) The following formula is used to relate the conductor's size to the assumed fault time/temperature limits: where:

_r

I = = = = Ta =

Ko tc TCAP

= = =

A TM Ta ar

= RMS current in kA. conductor cross-section in cmils. maximum allowable temperature in oC. Ambient temperature in oC. thermal coefficient of resistivity at reference temperature in oC. the resistivity of the ground conductor at reference temperature Ta in __-cm. I/ao, or (I/dr) - Tr. time of current flow in sec. thermal capacity factor, in J/cm3/oC.

For standard annealed soft copper wire: ar @ 20oC

=

0.00393

_r @ 20oC

=

1.7241

TCAP (J/cm3/0o)

=

3.422

Tm(0oC)

=

1083

Ko (1/ao @ 0oC)

=

234

I = KA


53



Fault Times (Cont'd) As an example, assume a ground fault of 22,000A that lasts for a duration of five seconds. By calculation, the required wire size for a 22 kA fault that is assumed to last five seconds is 530 cm. Because the calculated value exceeds the values that are given in Figure 9, the calculated value must be used.


54



WORK AID 1: SAUDI ARAMCO AND INDUSTRY STANDARDS FOR LOCATING SYSTEM GROUNDING INFORMATION This Work Aid is designed to help the Participants in performing Exercise 1. Saudi Aramco Design Practices _

SADP-P-111 : Grounding

Saudi Aramco Engineering Standards _ _ _

SAES-P-100 : Basic Power System Design Criteria SAES-P-111 : Grounding SAES-P-119 : Substations

IEEE Standards _

IEEE 80 : IEEE Guide for Safety in AC Substation Grounding

_

IEEE 81 : IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System

_

IEEE 142 : IEEE Grounding of Industrial and Commercial Power Systems

_

IEEE 367 : IEEE Recommended Practice for Determining the Electric Power Station Ground Potential Rise and Induced Voltage from a Power Fault

National Electrical Code


55



WORK AID 2:

TABLE OF SAUDI ARAMCO SYSTEM GROUNDING METHODS

This Work Aid is designed to assist the Participants in performing Exercise 2. This Work Aid shows the grounding method that should be used for different combinations of voltage, phases, and loads. System Voltage 120/240V 208/120V 480V 4160V

Phase 1 3 3 3

13,800V

3

13,800V

3

69,000V 115,000V 230,000V

3 3 3


Grounding Method Solidly Grounded Solidly Grounded Solidly Grounded Low Resistance Grounded (400A, 10 Sec, Resistor) Low Resistance Grounded (400A, 10 Sec, Resistor) Low Resistance Grounded (1000A, 10 Sec. Resistor) Solidly Grounded Solidly Grounded Solidly Grounded

Comments

Industrial Load Residential Distr. System

56



WORK AID 3:

PROCEDURES AND REFERENCES FOR DESIGNING SUBSTATION/PLANT GROUND GRIDS

This Work Aid is designed to help the Participants in performing Exercise 3. Exercise 3 requires the Participants to design a substation/plant ground grid for a hypothetical installation. The Participant must complete the following steps that are covered in the designated Work Aid to design a substation/plant ground grid. _

Determine need for Ground Grid Protection - Work Aid 3A

_

Determine Step Potential - Work Aid 3B

_

Determine Touch Potential - Work Aid 3B

_

Determine Required Grid Spacing - Work Aid 3B

_

Determine Number of Grounding Rods - Work Aid 3C

_

Determine Ground Wire Sizes - Work Aid 3D

_

Adjust Ground Wire Size for Fault Time - Work Aid 3E


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WORK AID 3 (Cont'd) Work Aid 3A:

Checklist for Evaluating the Need for Ground Potential Rise (GPR) Protection

This Work Aid is designed to help the Participant in performing Exercise 3A. To evaluate the need for ground potential rise protection, perform the following steps: _

Calculate Rg; the impedance to remote earth of the grounding electrode.

where:

_ A h L _

Earth resistivity of substation in ohms-m. Area occupied by ground grid in m2. Depth of the ground grid in m. Buried length of conductors in m not including ground rods.

Calculate Ig; fault current flowing to the grounding electrode. Ig

_

= = = =

=

Ground fault current current division factor

Calculate GPR; Ground potential rise GPR

=

Rg

Ig

_

If the GPRis less than 300V, the area is classified as a low risk site. No protection is required for communication equipment.

_

If the GPR is between 300V to 1500V, the area is classified as a moderate hazard site. Protection must be applied to all communication equipment circuits.

_

If the GPR is above 1500V, the area is classified as severe hazard site. Protection must be applied to all communication equipment circuits.


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WORK AID 3 (Cont'd) Work Aid 3B:

Formulas and Procedures for Determining Step and Touch Potentials and Grid Spacing

This Work Aid is designed to help the Participant in performing Exercise 3B. _

To calculate the step voltage limit for a man weighing 50 kg, use the formula:

where:

Cs (hsiK) _s ts seconds

= = =

factor for surface soil resistivity of surface soil material duration of shock current in

_

To calculate the touch voltage limit for a man weighing 50 kg use the formula:

_

Compare the resultant ESTEP 50 limit and the ETOUCH 50 limit to the calculated GPR. If both the ESTEP 50 limit and the ETOUCH 50 limit are above the GPR, no further actions are required, the ground grid design is complete. If either (or both) of the ESTEP 50 limit or the ETOUCH 50 limits are below the GPR, further design improvements of the ground grid are required.

_

If the ground grid design uses the normal soil as the surface soil, consider addition of a layer of crushed rock to the surface, which will increase the resistivity of the surface.

_

If crushed rock is added recalculate ESTEP 50 and ETOUCH 50.

_

A further reduction in ESTEP 50 and ETOUCH 50 can be accomplished through an increase in the length of the ground grid conductor and through an increase in the number of ground rods. (Note: An increase in the length of the ground grid conductor and the number of ground grids can only be accomplished through change to the grid spacing because the overall dimensions of the grid have already been established). The following equation will give an estimation of the required length of the ground grid conductor to obtain the maximum voltage below the ESTEP 50 and ETOUCH 50 limits:


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WORK AID 3 (Cont'd) here:

L

=

Total length of grounding grid including length of grid conductor and ground rods in m.

Ki

=

0.656 + 0.172n.

n

=

Number of parallel conductors in one direction; also equals for equally spaced rectangular grids where NA is the number of conductors running in one direction, and NB is the number of conductors running in the other direction. _ = Soil resistivity in ohm-m. IG

= grid

Maximum grid current that flows between the ground and the surrounding earth.

ts

=

Duration of shock current in seconds.

=

Factor for surface soil.

_s

=

Resistivity of the surface soil.

Km

=

Mesh factor determined by formula below:

D

=

spacing between parallel conductors in m.

h

=

depth of ground grid in m.

d

=

diameter of ground grid conductor.

Kh

=

Cs(hs1K)

where:

WORK AID 3 (Cont'd) Kii

=

1 if ground grid has rods along perimeter.

or Kii


=

for grids with no ground rods or only a few ground rods and not on perimeter.

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_

When the required length of ground grid (L) is known, determine how much of L will be Lc (total grid conductor length) and how much will be LR (total ground rod length). L can be expressed by the equation: L = Lc - LR

_

For the new value of LC and LR, determine the maximum mesh voltage. The maximum mesh voltage should be less than (or equal to) the ETOUCH 50 limit. The maximum mesh voltage (Em) is calculated through use of the following equation:

_

Use the new value of L to calculate the maximum step voltage. The maximum step voltage should be less than (or equal to) the ESTEP 50 limit. The maximum step voltage (Es) is calculated through use of the following equation:

_

If Em and Es are equal to or less than the ETOUCH 50 and ESTEP 50 limits, the length of LC and LR are acceptable.


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WORK AID 3 (Cont'd) Work Aid 3C:

Formulas, Procedures, and References for Determining Number of Ground Rods

This Work Aid is designed to help the Participant perform Exercise 3C. _

To calculate the total resistance for a group of ground rods, the resistance of one rod must be calculated first.

_

Through use of SADP-P-111, the resistance of one rod can be determined from Figure 13.

_

Ratio the value taken off of Figure 13 to the actual soil resistivity through use of the following equation:

_

Find the group ratio for the spacing of the ground rods for Figure 14.

_

To find the resistance for the group of ground rods, take the value calculated as the actual value for a single rod, and the group ratio; use the following equation to calculate:


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WORK AID 3 (Cont'd)

Resistance of a Single Rod Figure 13


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WORK AID 3 (Cont'd)

Ground Ratio Figure 14


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WORK AID 3 (Cont'd) Work Aid 3D:

Table of Wire Sizes and Ampacity

Use Work Aid 3D to complete Exercise 3D. The size of ground grid conductors is determined by the magnitude of the fault current and the time of flow, based on the maximum allowable temperature rise. Figures 15 and 16 show wire sizes vs. short-time ampacity for Saudi Aramco Systems.

Solidly Grounded Systems over 600V Figure 15


65



WORK AID 3 (Cont'd)

Impedance Grounded Systems over 600V Figure 16


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WORK AID 3 (Cont'd) Work Aid 3E:

Procedures and References for Adjusting Ground Wire Sizes for Fault Times

Use Work Aid 3E to complete Exercise 3E. _

After the size of the ground wire has been determined, the ability of the ground wire to handle the expected short-time current without exceeding the temperature limit must be verified. The temperature limit of 450oC is for use when the ground grid has brazed connections. The 250oC limit is for use when the connections are bolted.

_

Use Figure 17 to find the minimum allowed circular mils for the expected fault current, given the time duration.

_

Multiply the value from Figure 17 by the expected ground fault current.


67



WORK AID 3 (Cont'd)

Nomogram for Conductor Sizing Figure 17


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GLOSSARY

circuit breaker

A device that is designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating.

electric potential

The potential difference between the point and some equipotential surface, usually the surface of the earth, which is arbitrarily chosen as having zero potential (remote earth).

electrical noise

The disturbance in an electrical system that interferes with the normal transmission of signals carrying information.

equipment ground

A ground connection to non-current carrying metal parts of a wiring installation or of electric equipment, or both.

fault time

The duration for which a fault current flows prior to being interrupted.

ground

A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth, or to some conducting body, of relatively large extent and that serves in place of the earth.

ground bus

A bus to which the grounds from individual pieces of equipment are connected and that, in turn, is connected to ground at one or more points.

ground circuit

A circuit in which one conductor or point (usually the neutral conductor or neutral point of transformer or generator windings) is intentionally grounded, either solidly or through a grounding device.

ground conductor

A conductor or system that is intentionally grounded, either solidly or through a current-limiting device.

ground current

Current that is flowing in the earth or in a grounding connection.


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ground grid

A system of grounding electrodes consisting of interconnected bare cables that are buried in the earth to provide a common ground for electric devices and metallic structures.

ground potential

An AC potential difference between remote earth and local rise (GPR) ground.

grounded

Connected to earth or to some extended conducting body that serves in place of the earth, whether the connection is intentional or accidental.

grounding conductor

The conductor that is used to establish a ground and that (ground conductor) connects an equipment, device, wiring system, or another conductor (usually the neutral conductor) with the grounding electrode or electrodes.

grounding electrode

A conductor that is used to establish a ground (for instance, (ground electrode) ground grids, ground rods, or groundwells).

grounding transformer

A transformer that is primarily intended to provide a neutral point for grounding purposes.

impedance grounded

Grounded through impedance.

neutral ground

An intentional ground applied to the neutral conductor or neutral point of a circuit, transformer, machine, apparatus, or system.

reactance grounded

Grounded through impedance, the principle element of which is reactance.

resistance grounded

Grounded through impedance, the principle element of which is resistance.

resistivity (material)

A factor such that the conduction-current density is equal to the electric field in the material divided by the resistivity.

service ground

A ground connection to a service equipment or a service conductor or both.


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solidly grounded

Grounded through an adequate grounded connection in which (directly grounded) no impedance has been inserted intentionally.

static electricity

The accumulation of electrostatic charges on the surfaces of conducting and non-conducting bodies that are insulated from their surroundings.

step potential

The potential difference between two points on the earth's surface, separated by a distance of one pace, that will be assumed to be one meter, in the direction of maximum potential gradient.


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surface soil resistivity

The resistance of the upper layer of soil in a ground-grid area.

transfer potential

The relocation of a hazardous potential from a ground-grid area to outside points.

touch potential

The potential difference between a grounded metallic structure and a point on the earth's surface separated by a distance equal to the normal maximum horizontal reach (approximately one meter).

ungrounded

A system, circuit, or apparatus without an intentional connection to ground except through potential indicating or measuring devices or other very high impedance devices.

voltage to ground

The voltage between any live conductor of a circuit and the earth.

zig zag transformers

A special grounding transformer that has two-phase windings on each core leg.

zone of influence

The potential gradient outside of the site.


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Grounding

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