UTILITIES (BUILDING SYSTEMS) PRINCIPLES OF ELECTRICITY PRINCIPLES OF ELECTRICITY Definition of Electricity: Below are the most common meanings of the word Electricity. Which one do you think is right? - "Electricity" means electric charge. Examples: CHARGES OF ELECTRICITY. COULOMBS OF ELECTRICITY. - "Electricity" refers to the flowing motion of electric charge. Examples: CURRENT ELECTRICITY. AMPERES OF ELECTRICITY. - "Electricity" means electrical energy. Examples: PRICE OF ELECTRICITY. KILOWATT-HOURS OF ELECTRICITY. - "Electricity" refers to the amount of imbalance between quantities of electrons and protons. Example: STATIC ELECTRICITY. - "Electricity" is a class of phenomena involving electric charges. Examples: BIOELECTRICITY, PIEZOELECTRICITY, TRIBOELECTRICITY, THERMOELECTRICITY, ATMOSPHERIC ELECTRICITY ...ETC. - Other less common definitions: "Electricity" refers to the flowing motion of electric energy (electric power, Watts of electricity) "Electricity" means electric field (Volts of electricity) "Electricity" means the nitrogen/oxygen plasma (sparks of electricity) "Electricity" is a field of science (Basic Electricity, Advanced Electricity) If we wish to agree on a single correct definition of "electricity," below is the "clear" and "simple" description of electricity that results: Electricity is a mysterious incomprehensible entity which is invisible and visible at the same time. It is both matter and energy. It's a type of low-frequency radio wave which is made of protons. It is a mysterious force which looks like blue-white fire and yet cannot be seen. When electricity flows through a light bulb's filament, it gets changed entirely into light.Yet no electricity
is ever used up by the light bulb, and every bit of it flows out of the filament and back down the other wire. College textbooks are full of electricity, yet they have no electric charge. Electricity is a class of phenomena which can be stored in batteries! If you want to measure a quantity of electricity, what units should you use? Why Volts of course. And also Coulombs, Amperes, Watts, and Joules, ALL AT THE SAME TIME. Yet "electricity" is a class of phenomena; it's a type of event. Since we can't have an AMOUNT of an event, we can't really measure the quantity of electricity at all, right? So never ask "WHAT IS ELECTRICITY". Instead, discard the word "electricity" and use the correct names for all the separate phenomena. Here are a few of them: What is electric charge? What is electrical energy? What are electrons? What is electric current? What is an imbalance of charge? What is an electric field? What is voltage? What is electric power? What is a spark? What is electromagnetism? What is electrical science? What is electrodynamics? What is electrostatics? What are electrical phenomena? These questions all have sensible answers. But if you ask WHAT IS ELECTRICITY?, then all answers you find will just confuse you, and you'll never stop asking that question. Electricity. A form of energy produced by the flow of particles of matter and consists of commonly attractive positively (protons [+]) and negatively (electrons [-]) charged atomic particles. A stream of electrons, or an electric current. Electricity \E`lec*tric"i*ty\, n.; pl. Electricities. [Cf. F. ['e]lectricit['e]. See {Electric}.] 1. A power in nature, a manifestation of energy, exhibiting itself when in disturbed equilibrium or in activity by a circuit movement, the fact of direction in which involves polarity, or opposition of properties in opposite directions; also, by attraction for many substances, by a law involving attraction between surfaces of unlike polarity, and repulsion between those of like; by exhibiting accumulated polar tension when the circuit is broken; and by producing heat, light, concussion, and often chemical changes when the circuit passes between the poles or through any imperfectly conducting substance or space. It is generally brought into action by any disturbance of molecular equilibrium, whether from a chemical, physical, or mechanical, cause. Note: Electricity is manifested under following different forms: (a) Statical electricity, called also Frictional or Common electricity, electricity in the condition of a stationary charge, in which the disturbance is produced by friction, as of glass, amber, etc., or by induction.
(b) Dynamical electricity, called also Voltaic electricity, electricity in motion, or as a current produced by chemical decomposition, as by means of a voltaic battery, or by mechanical action, as by dynamoelectric machines. (c) Thermoelectricity, in which the disturbing cause is heat (attended possibly with some chemical action). It is developed by uniting two pieces of unlike metals in a bar, and then heating the bar unequally. (d) Atmospheric electricity, any condition of electrical disturbance in the atmosphere or clouds, due to some or all of the above mentioned causes. (e) Magnetic electricity, electricity developed by the action of magnets. (f) Positive electricity, the electricity that appears at the positive pole or anode of a battery, or that is produced by friction of glass; -- called also vitreous electricity. (g) Negative electricity, the electricity that appears at the negative pole or cathode, or is produced by the friction of resinous substance; -- called also resinous electricity. (h) Organic electricity, that which is developed in organic structures, either animal or vegetable, the phrase animal electricity being much more common. 2. The science which unfolds the phenomena and laws of electricity; electrical science. 3. Fig.: Electrifying energy or characteristic. electricity n1: a form of energy associated with moving electrons and protons 2: energy made available by the flow of electric charge through a conductor [syn: {electrical energy}] 3: keen and shared excitement; "the stage crackled with electricity whenever she was on it" Basic Terminologies: Atoms and Molecules Matter is anything that has weight and thus takes up space All matter is made up of molecules All molecules are made up of atoms The atoms is made up of protons and neutrons in a nucleus forming shells around the – much like the solar system Electrons have a negative charge and protons have a positive charge The electrons can be forced to move from one atom to the next and this is accomplished using an electromotive force (EMF)
This movement of electrons is electric current Electric Current: - When electricity flows it is measured in amperes (coulombs per second) - One ampere of current is when one coulomb (6 million million million electrons) move past a point in one second. Thus electrical current is the rate of flow of electricity - To make the electricity flow, an electromotive force is required. Example: Let's imagine that you have a wire, and you somehow observe that 2 coulombs passes through the wire in one second. What is the value of the current? Ans. 2 amperes You observe charge going through a wire for 4 seconds, and you find that 20 coulombs passes. What is the current? Ans. 5 amperes Now, if you reallly understand what current is you can turn this around. In the problems above you were given the charge passing through a wire in a given amount of time. Turning that around we can ask a different question. If we have a constant current, I, flowing through a wire, then we can compute how much charge flows through the wire in some given time interval. Say we have the following situation: I = Current = 3.2 amperes Time interval = 15 seconds. Then we would know that the amount of charge that flowed through the wire in the 15 second time interval would be: Total charge = 3.2 amperes x 15 seconds = (3.2 coul/sec) x 15 sec = 48 couloumbs The flow of electricity has two major forms – DC and AC Direct Current (DC) is the one that flows in the same direction continuously, intermittently, or pulsating. Alternating current (AC) is the one that reverses direction at regular intervals Electromotive Force - Electromotive force is measured in volts - EMF can be generated using batteries that operate from chemical reactions - EMF can be generated by moving a conductor in a magnetic field (generators)
- EMF can be generated by rubbing two dissimilar materials together (car seats and you) - EMF can be generated using many other means including light and pressure to crystals.
Resistance Resistance is the opposition to the flow of electricity. All known matter resist the flow of electricity. The movement of electricity through a resistor causes some of the electrical energy to be converted to heat energy. The unit of resistance is the OHM. Electric Circuit: A circuit is the path electricity takes as it flows from a battery through an object and back to the battery. One end of the connecting wire must touch the positive terminal (+) of a battery (or cell); the other end must touch the negative terminal (-) of a battery. An Electric circuit is made from a power supply, wires and electrical devices. They must be connected without a break for electric current to flow around the circuit. Switches control where the current flows. Circuit diagrams show how the parts of the circuit are connected. The parts of the circuit can be connected in different ways. Ampacity: Ampacity. "The current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating." (Ampacity varies depending on many factors. You must use the appropriate NEC Tables to determine the correct ampacity.) Relation between EMF, Current and Resistance (Ohm’s Law) Ohm’s Law By international standards, one volt is the amount of EMF required to move one ampere of current through a resistance of one ohm. Ohm’s Law is: E = I x R I = E/R R = E/I Example: Given a circuit has a current of 4 amperes and a resistance of 10 ohms. What is the voltage? E = I x R E(volts) = 4 amperes x 10 ohms = 40 volts Given E = 120 volts, and resistance = 60 ohms, what is the current? I = E/R = 120volts/60ohms = 2 amperes
Electric Circuits/Electric Currents Discuss and review the vocabulary you will be using circuit volt voltmeter current dry cell ammeter series resistance terminal parallel conductor ampere ohm watt fuse ELECTRIC CURRENTS AND CIRCUITS Electric current is the flow of electric charge from high potential to low potential. Therefore we say that current is caused by a potential difference. An electric circuit is a closed path through which charge can move (conductor). When one coulomb of charge passes a given point every second, we say that a current of 1 Ampere (Amp or A) exists in the circuit. How can the birds sit on the power lines without getting fried? Why do electricians say, "It's not the volts, it's the amps that can hurt you."? Answers: #1 - The birds can sit on the power lines because there is very little potential difference between their feet and therefore very little current will go through them. #2 Amps are the actual moving of charges, which can cause damage to you as well as interfere with the electrical activity in your body. Volts (potential difference) are what would cause the current to go through you. Volts do not travel through anything and therefore cannot hurt you. The only reason volts are dangerous is the amps they can generate! ResistanceA high voltage (potential difference) does not necessarily lead to a high current, it depends on the resistance of the material that the current has to go through. The resistance of a wire depends on three things: what it is made out of (resistivity), the cross-sectional
area of the wire, and the length of the wire. The resistance can be calculated as follows: r = ρL/A where the ρ resistivity is found using the reference tables. What is the resistance of 1.0 meters of Nichrome wire (diameter = 0.04318 cm)? First we have to find the cross-sectional area (in m2) of the wire. Assuming the wire is round the cross-sectional area is that of a circle, which is: A= πr2 = π (.0002159m) 2 and resistance is =10.2Ω Electric Power and Energy: Power is the rate that energy is transferred. Power is measured in watts which are joules per second. P=VI watts = volts x amperes To calculate the energy (in joules) which is dissipated (lost) in a resistor during a certain period of time just multiply the power in watts times the time in seconds. Another unit of energy is the kilowatt hour (kWhr). This is found by multiplying the power in kilowatts (1000 watts) times the time in hours. Joules. The modern definition of the calorie is based one the meter-Kg-sec units and is defined as 4.184 joules. The joule is the unit of energy in meter-Kg-sec units. Electric Circuits: Electric circuits are complete paths including resistors made between the positive and negative terminals of a battery. Multiple resistors can either be connected to make one path, called a series circuit or multiple paths, called a parallel circuit. Kirchhoff's Current Law (KCL). KCL states that the algebraic sum of the currents in all the branches which converge in a common node is equal to zero sum(Iin )=sum(Iout ) Kirchhoff's Voltage Law. Kirchhoff's Voltage Law states that the algebraic sum of the voltages between successive nodes in a closed path in the network is equal to zero.
Sum(E) = sum(IR) KIRCHHOFF'S RULES. The following two rules are known as Kirchhoff's laws. 1. Junction or Point rule: Sum of all currents entering a junction must equal sum of currents leaving the junction. 2. Loop or Circuit rule: For a closed loop in a circuit, the algebraic sum of all potential changes encountered while completing a cycle around the loop must be zero. In this Loop rule, we may consider a rise in the potential to be positive and a drop in the potential to be negative.
UTILITIES (ELECTRICAL SYSTEMS) WIRING MATERIALS Fuse & Circuit Breaker: Fuses and circuit breakers are designed to interrupt the power to a circuit when the current flow exceeds safe levels. For example, if your toaster shorts out, a fuse or breaker should "trip", protecting the wiring in the walls from melting. As such, fuses and breakers are primarily intended to protect the wiring -- UL or CSA approval supposedly indicates that the equipment itself won't cause a fire. Fuses contain a narrow strip of metal which is designed to melt (safely) when the current exceeds the rated value, thereby interrupting the power to the circuit. Fuses trip relatively fast which can sometimes be a problem with motors which have large startup current surges. For motor circuits, you can use a "time-delay" fuse (one brand is "fusetron") which will avoid tripping on momentary overloads. A fusetron looks like a spring-loaded fuse. A fuse can only trip once, then it must be replaced. Breakers are fairly complicated mechanical devices. They usually consist of one spring loaded contact which is latched into position against another contact. When the current flow through the device exceeds the rated value, a bimetallic strip heats up and bends. By bending it "trips" the latch, and the spring pulls the contacts apart. Circuit breakers behave similarly to fusetrons - that is, they tend to take longer to trip at moderate overloads than ordinary fuses. With high overloads, they trip quickly. Breakers can be reset a finite number of times - each time they trip, or are thrown when the circuit is in use, some arcing takes place, which damages the contacts. Thus, breakers should not be used in place of switches unless they are specially listed for the purpose. Neither fuses nor breakers "limit" the current per se. A dead short on a circuit can cause hundreds or sometimes even thousands of amperes to flow for a short period of time, which can often cause severe damage.
WIRING MATERIALS Insulators and Conductors:
- Some materials such as wood and glass have high resistance and are called insulators. - Other materials have low resistance and are called conductors such as copper and aluminum To keep electricity from flowing where it is not supposed to go, conductors are covered with insulators (insulated wire) Insulators: Amount and type of insulation is determined by the voltage that will exist between them. Type of insulation is decided under which the conductors must operate with regard to heat, moisture, or other conditions that might have a deleterious effect on insulation. NEC has established a system of letters that indicate their characteristics. It may be noted that the letters R, RU and T refer to material (rubber, latex rubber, or thermoplastic) Letters H refer to high temperature or heat resistant and W is moisture resistant. Conductors: Any metallic substance conducts an electric current. The relative ability of a material to conduct is determined by its resistivity, expressed in ohms-circular mil per foot. Best conductor is silver but very expensive. Choice is narrowed to copper and aluminum. Copper is the most common and is stronger than aluminum. For larger conductors, aluminum is preferred because of its lower cost and weight. Heat on Conductors: Greatest hazard conductors must endure is heat. Continued exposure to excessive heat causes insulation to become soft, melt and in extreme cases to burn. This heat comes from two sources: ambient air surrounding the conductors or from the current the conductors must carry. NEC has a table for each specific insulation that determine the current-carrying capacity (or ampacity) of each size of conductor. Wire Sizes:
Wires are usually round and the unit for measuring the cross-sectional area of wires is the circular mil (abbreviated cmil). A circular mil is a circle 0.001 inch in diameter. A circular mil-foot is a circular wire 1 foot in length and I mil in diameter. This is used to express resistance of a wire. American Wire Gauge (AWG): AWG was developed to assure the manufacturers of conductors in sizes that will be suitable for all applications. It assigns a number to a particular size of wire. It starts with #40 as the smallest with a diameter of 3.145 mils. The gauge numbers then descend in order to #000, the largest with a dia of 460 mils. NEC is used to select a wire size for a given insulation as it establishes the allowable current capacity for each insulated wires. Stranded Wires: Consists of a group of wires which are usually twisted to form a metallic string. Stranding improves the flexibility of a wire. Multiply the circular-mil area of each strand by the number of strands to find the total circular-mil cross section. An insulated stranded wire is called a cord. Wire Size and Ampere of Circuit: What size wire should I use? Here's a quick table for normal situations. Gauge - Amps 14 - 15 12 - 20 10 - 30 8 - 40
6 - 65
RACEWAYS A channel for holding wires, cables, or bus bars May be in the form of a pipe called conduit; a thinner wall conduit called electrical metallic tubing; or a square sheet metal duct of which one side has a removable cover All raceways are mechanically installed as a complete system with all necessary outlet boxes and fittings. Afterwards the conductors are installed (pulled through) the raceway. A cable is a complete assembly consisting of conductors and raceways as a unit. The raceway is actually a covering that may be either metallic or non-metallic. Wiring Methods and Type of Raceway: a. Metal-Clad Cable - Code permits two types of metal-clad cable: AC and ACL - The metal covering for these cables is a steel spiral wrapping that forms a flexible raceway - Manufactured as a complete assembly with the conductors installed. - AC is commonly known as BX cable and cannot be buried in concrete or damp or wet locations. - ACL is a cable with lead-covered conductors available for wet locations. b. Non-Metallic Sheathed Cable - Has a nonmetallic covering of fabric or plastic. - Available in size nos. 14 to 1 AWG in copper and nos. 12 to 2 AWG in aluminum conductors. - Used extensively for wiring of buildings - Nonmetallic sheathed cable is called Romex. c. Electrical Metallic Tubing (EMT) - Has a thin wall that does not permit threading. - Connectors and couplings are secured either by compression or set screws. - An excellent raceway for conductors, can be buried in concrete but must not be subject to continuous moisture. d. Rigid Conduit - Has all the outward appearances of plumber’s pipe - Have a smooth enameled interior to facilitate the pulling and installation of wires. - Connections to boxes are made with locknuts and bushings after conduit is threaded.
- Used in most severe cases where the possibility of mechanical injury or the presence of moisture presents a problem. e. Wireways - Sheet-metal troughs with removable covers. - Cannot be concealed, but are very useful in maintaining a complete raceway system when many devices must be interconnected in a limited area. Selection of Raceways: - A conduit or tubing system must be installed completely before conductors are inserted. - In anticipation of larger loads, conduits larger that necessary may be installed. - A maximum number of conductors are permitted in the standard sizes of conduit or tubing for new applications. (refer to tables)
UTILITIES (ELECTRICAL SYSTEMS): WIRING SYSTEM WIRING DEVICES - The term includes all devices that are normally installed in wall outlet boxes, including receptacles, switches, dimmers, and pilot lights. Receptacles: - Identified by number of poles and wires. - Special types such as explosion proof and specific usage types such as range receptacles are available. Switch devices: - Switches up to 30A that can be outlet-box mounted fall into this category. - Normal constructions are single-pole, 2-pole, 3-way and 4-way. - Operating handles are toggle type, key, push, touch, rotary and tap-plate types. - Programmable switch is available which can readily program to switch the controlled circuit or device at preset times. Outlet and Device Boxes: - Generally of galvanized stamped sheet metal. PVC now available. - Most common sizes are the 4” square and 4” octagonal boxes for fixtures and junctions. - 4” x 2-1/8” box used for single devices where no splicing is required. EMERGENCY/STANDBY POWER EQUIPMENT: - emergency systems are used to supply electric power to equipment essential for safety to human life, upon interruption of normal supply - included here are illumination of areas of assembly to permit safe exit.
Standby systems are divided into two categories: required and optional. - The former are intended to power systems whose stoppage might create hazards or hamper firefighting operations. - Optional systems are at the discretion of the owner intended to protect property and prevent financial loss. An engine-generator set comprises three components: the fuel system, the set itself, and the space housing the unit. Battery equipment used only to supply limited amounts of emergency power, primarily for lighting. Grounding Electrode System: The grounding electrode system is a method by which the neutral and grounding conductors are connected to the common "earth" reference. The connection from the electrical system to the grounding system is made in only one place to avoid ground loops. The grounding electrode system is _not_ intended to carry much current. Ground faults (Ie: hot to grounded case short) are conducted down the ground wire to where it is interconnected with the neutral and hopefully the breaker/fuse trips. The grounding electrode does not participate in such a situation. While the conductors involved in this are relatively large, they're sized for lightning strikes and other extremely short duration events. The grounding electrode system is specifically _not_ expected to have enough conductivity to trip a 15A breaker. The grounding electrode often has a moderately high resistance. For example, according to the NEC, an acceptable ground electrode system may have 25 ohms of resistance – only 5A at 120V, not enough to trip a 15A breaker. A grounding electrode system usually consists of a primary grounding electrode, plus possibly a secondary electrode. A primary electrode can be (if in direct contact with the earth): 10' of ground rod. 10' of well casing or metallic water pipe (must be connected within 5' of pipe entrance to house). 20' of copper wire buried in the bottom of the footings. A secondary electrode will be required if the primary is a water pipe or (NEC) if the primary electrode is >25 ohms to the dirt. Surges, spikes, zaps, grounding and your electronics: Theoretically, the power coming into your house is a perfect AC sine wave. It is usually quite close. But occasionally, it won't be. Lightning strikes and other events will affect the power. These usually fall into two general categories: very high voltage spikes (often into 1000s of volts, but usually only a few microseconds in length) or surges (longer duration, but usually much lower voltage). Most of your electrical equipment, motors, transformer-operated electronics, lights, etc., won't even
notice these one-shot events. However, certain types of solid-state electronics, particularly computers with switching power supplies and MOS semiconductors, can be damaged by these occurances. For example, a spike can "punch a hole" through an insulating layer in a MOS device (such as that several hundred dollar 386 CPU), thereby destroying it. The traditional approach to protecting your electronics is to use "surge suppressors" or "line filters". These are usually devices that you plug in between the outlet and your electronics. Roughly speaking, surge suppressors work by detecting overvoltages, and shorting them out. Think of them as voltage limiters. Line filters usually use frequency-dependent circuits (inductors, capacitors etc.) to "tune out" undesirable spikes - preventing them from reaching your electronics.
ELECTRICAL SYSTEM AND MATERIAL: SERVICE AND UTILIZATION Junction Box A junction box is a box used only for connecting wires together. Junction boxes must be located in such a way that they're accessible later. Ie: not buried under plaster. Utilities (Electrical Systems): Service & Utilization
ELECTRICAL SYSTEM AND MATERIAL: SERVICE & UTILIZATION ELECTRIC SERVICE: Service is tapped onto the utility lines at a mutually agreeable point at or beyond the property line. Overhead Service: - low cost, easily maintained and repaired, and faults easily located Types of overhead service: a. bare copper cable – supported on porcelain or glass insulators on crossarms normally used for high voltage (2.4 kV and higher) lines. b. Weatherproof – secondary circuits at 600V and below run on porcelain spool secondary racks with 1/c weatherproof cable as the conductor. c. preassembled aerial cable – consists of three or four insulated cables wrapped together with a metallic tape and suspended by hooks from the poles. Underground Service: - Preferred in areas where there are extreme weather conditions, where combination of snow, wind and ice increase the possibility of outages.
- Attractiveness (lack of overhead visual clutter); service reliability, and long life. - Disadvantage is high cost. Types of underground wiring: a. Direct burial – low cost and ease of installation b. Direct burial duct – medium but little strength c. Concrete encased duct – offers highest strength but expensive - Cable used is the basic service entrance cable or type SE. - When provided with moisture proofing, designation is SE type U or USE. - Underground cable other than service runs is classified type UF (underground feeder). SERVICE EQUIPMENT: TRANSFORMERS - A device that changes or transforms alternating current of one voltage to alternating current of another voltage. - Transformer is used to step down an incoming 4160V service to 480V for distribution within a building. (from primary voltages-2400V and up, to secondary voltages-480V and below) - Another transformer would be used in a local electric closet to step down the 480V to 240V or 120V. - Specified by type, phase, kVA rating, sound level and insulation class. - Transformers are available in single phase or three-phase construction and rated in kVA or kilovoltamperes. Types of transformers: a) Dry (air-cooled) – units in the 600V class usually installed indoors intended for general puspose light and power circuits. b) Liquid filled – units rated above 5000V installed in substations mounted on concrete pad. Cheapest cooling medium used is mineral oil but application is limited because of its flammability. New coolants are being developed but more expensive. - Insulation used is either organic, inorganic, asbestos or silicone. Transformer Outdoors: - Service transformer bank is necessary when the facility utilization voltage is different from the utility voltage. - Advantages are: no building space required; reduce noise problem within building; lower cost; ease of maintenance and replacement; no interior heat problem. Transformers Indoors: - Subject to stringent NEC regulations for safety.
Types: a. oil-filled transformers – small size, low weight, low first cost, long life, excellent electrical characteristics but flammable and must be installed in a fire-resistant vault which involves heavy cost b. non-flammable liquid-filled units – have most of the advantages of the above and do not require a vault unless voltage is very high. Requires a sump or catch basin for all of the contained liquid. Relatively high first cost. c. dry-type units – shorter life, high noise level, greater weight and large size but is the majority choice. Advantage is ease of installation and almost unrestricted choice of location. Transformer Vaults: - basically a fire-rated enclosure, provided because of the possibility of transformer case rupture and an oil fire. - Should be located where they can be ventilated to the outside air without use of ducts. SERVICE EQUIPMENT ARRANGEMENTS AND METERING: Metering: - Provided at either the utility or facility voltage, and at either the service point or inside the building. - Must be available for inspection and service - Furnished and installed by the utility company. Service Switch: - The purpose is to disconnect all of the electric service in the building except emergency equipment. - Located at a readily accessible spot near the point at which the service conductors enter the building. Switches: - Traditional electrical switching devices which close and open an electric current by physically moving tow electrical conductors into contact with each other to close the circuit, and physically separating them to open the circuit. - Rated by current and voltage, duty, poles and throw, fusibility, and enclosure. - Current rating of the switch is the amount of current that the switch can carry continuously and interrupt safely. - General duty safety switches are intended for normal use in lighting and power circuits. - HD or heavy duty switches are intended for frequent interrupting, high fault currents and ease of maintenance. - Switch may be constructed with or without provision for fusing. Fusible switch if provided and nonfusible if otherwise. Contactors: - A switch that uses contact blocks of silver-coated copper, which are forced together to close the circuit or are separated to break the circuit. - The common wall light switch is a small mechanically operated contactor
- A relay is a small electrically operated contactor. - Most contactors are operated by means of an electromagnet that causes the contacts to close. - They open by spring action or by gravity. - Advantage of contactor over switches is their facility for remote control; switches are manually thrown. - The magnetic contactor is inherently a remote-controlled device.
Special Switches: - Remote-control switches – mechanically held, electrically operated contactor - Automatic transfer switch – a double throw switch arranged so that on failure of normal service it automatically transfer to the emergency service. Control devices are voltage sensors that sense the condition of the service and operate the switch accordingly. - Time-controlled switches – operation is time based. - Solid state switches; programmable switches CIRCUIT-PROTECTIVE DEVICES: - To protect insulation, wiring, switches, and other apparatus from overload and short-circuit. Fuses: - Fuses contain a narrow strip of fusible link or metal which is designed to melt (safely) when the current exceeds the rated value, thereby interrupting the power to the circuit. - Cartridge fuse when enclosed in an insulating fiber tube. Made up to 600A. - Plug fuse when in a porcelain cup used normally in homes; rated 5A to 30A - Fuses trip relatively fast which can sometimes be a problem with motors which have large startup current surges. For motor circuits, you can use a "time-delay" fuse (one brand is "fusetron") which will avoid tripping on momentary overloads. A fusetron looks like a spring-loaded fuse. - A fuse can only trip once, then it must be replaced. Circuit Breakers: - An electromechanical device that performs the same protective function as a fuse and in addition, acts as a switch and is equipped with both thermal and magnetic trips. - When the current flow through the device exceeds the rated value, a bimetallic strip heats up and bends, "trips" the latch, and the spring pulls the contacts apart - The heavier the overload, the faster the trip action. - Breakers can be reset a finite number of times - each time they trip, or are thrown when the circuit is in use, some arcing takes place, which damages the contacts. Thus, breakers should not be used in place of switches unless they are specially listed for the purpose. Switchboards and Switchgear: - Freestanding assemblies of switches, fuses, and/or circuit breakers, which normally provide switching
and feeder protection to a number of circuits connected to a main source. - Modern switchboards are enclosed in a metal structure. - High-voltage equipment (above 600V) is referred to as switchgear. - Main metal-clad switchgear is located in basements or housed in separate, well-ventilated, electrical switchgear rooms.
Unit Substations: - An assembly of primary switch and fuse or breaker, step down transformer, meters, controls, buswork, and secondary switchgear is called a unit substation. Panelboards: - Same function as switchboard but in a smaller scale. - Accepts a relatively large block of power and distributes it in smaller blocks. - Comprises main buses to which are connected circuit-protective devices (breaker or fuses), which feed smaller circuits.
