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66 Cards in this Set
- Front
- Back
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How is electric service usually made available to a building ?
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1. overhead service drop
2. underground service lateral to building 3. connection to underground utility line with service lateral to building. (1167) |
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What 10 factors determine if service to a building will be run overhead or underground?
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1. Length of service run
2. Type of terrain 3. Customer participation in the cost of service installation 4. Service voltage 5. Size and nature of electric load 6. Importance of appearance 7. Local practices and ordinances 8. Maintenance and service reliability 9. Weather conditions 10. Type of interbuilding distribution, if applicable. (1167) |
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What are the advantages (to the utility) of overhead electric lines?
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First and foremost: Low cost.
Additionally, easy to maintain and repair. (1168) |
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In what 3 other conditions would it be better to have overhead lines?
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"length of service run is several hundred feet or more...
voltages exceeding 5000V. ...terrain is rocky and electrical load is heavy." (1168) |
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What are the three types of overhead cables?
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1. bare
2. weatherproof 3. preassembled aerial cable. (1168) |
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What are the 3 advantages of underground electric service?
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'...attractiveness (lack of overhead physical and visual clutter),
service reliability, and long life." (1168) |
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What is the principal disadvantage to underground electric service?
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High cost, both for installation and servicing. (1168)
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What are the three methods for underground wiring?
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1. Direct burial
2. concrete encased duct 3. direct burial duct (1168) |
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What is a TRANSFORMER?
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"...a device that changes or TRANSFORMS alternating current (ac) of one voltage to alternating current of another voltage." (1171)
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In what units is transformer capacity rated?
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kVA (1171)
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What is the formula for calculating current and voltage in a transformer?
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"primary current x primary voltage
= secondary current x secondary voltage (1171) |
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What are the 6 advantages of an outdoor transformer installation?
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"No building space required
Reduced noise problem within the building Lower first cost Ease of maintenance and replacement No interior heat problem Opportunity to use low-cost, long-life, oil-filled units" (1174) |
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What percentage of a transformer's rating is converted to heat?
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"between 1% and 1 1/2% of a transformer's rating, depending on the type, is converted to heat at full load." (1175)
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What are the two indoors disadvantages of oil-insulated transformers?
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fire hazard
must be installed in a fire-resistant vault (expensive). (1175) |
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What are the 7 advantages of oil-insulated transformers?
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"small size,
low first cost, low losses, long life, excellent electrical characteristics, low noise level and high overload capacity." (1175) |
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What is the advantage of the "Less-Flammable" Liquid-Insulated transformer?
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"Use of these units achieves the electrical and physical advantages of oil-insulated units without the cost penalty of vault construction and installation." (1175)
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What are the advantages of a Nonflammable Fluid-Filled Transformer?
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"Such units have most of the advantages of oil-filled units and do NOT require a vault unless the voltage is very high." (1176)
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What are the disadvantages of a Nonflammable Fluid-Filled Transformer?
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1. "require a sump or catch basin of sufficient capacity for all of the contained liquid.
2. ...high first cost." (1176) |
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What is the transformer unit of choice for the majority of indoor installations?
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Dry-Type Transformers
(1176) |
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What are the 2 principal advantages of a dry-type transformer?
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"...ease of installation and
almost unrestricted choice of location." (1176) |
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What are the 5 disadvantages of dry-type transformers?
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"shorter life,
higher losses, high noise level, greater weight, and larger size than liquid filled units." (1176) |
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What is a TRANSFORMER VAULT?
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"basically a fire-rated enclosure provided because of the possibility of fire due to rupture of an oil-filled transformer." (1176)
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Where is metering provided in the sequence of a service line into a building?
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"Metering must be provided ahead (electrically) of the building's service entrance switch(es)." (1176)
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Who furnishes transformers/equipment for electrical service to a building?
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"If high-voltage service is purchases,then the transformers and all equipment beyond the service connection must be furnished by the owner.
Conversely, if low-voltage service is purchased, all equipment necessary to provide low voltage is furnished by the utility." (1177) |
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What is the purpose of the electric service switch(es) in a building?
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"...to disconnect the normal service to the building." (1177)
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Where should service switches be located?
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"...disconnecting apparatus must be located at a readily accessible spot near the point at which the service conductors enter to building." (1177)
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What is the function of a SWITCH?
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to open or close an electrical circuit by bringing them into physical contact to close the circuit (flow) or separating them to open the circuit (no flow) (1179)
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What are the 5 ways that electrical switches are rated?
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"current and voltage,
duty, poles and throw, fusibility, and enclosure." (1179) |
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What is the current rating of a switch?
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"The current rating is the amount of current that the switch can carry continuously and interrupt safely." (1179)
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What is the voltage rating of a switch?
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"...by voltage class...
250 V, 600V or 5 kV as required." (1180) |
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What is a GENERAL DUTY SAFETY SWITCH?
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"Switches intended for normal use in lighting and power circuits..." (1180)
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What is a HEAVY DUTY switch?
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"Switches intended for frequent interruting, high-fault currents, and ease of maintenance..." (1180)
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What is a CONTACTOR?
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"A contactor is a switch.
Instead of a handle-operated, movable blade, a contactor uses CONTACT blocks of silver-coated copper, which are forced together to MAKE (close) or are separated to BREAK (open) the circuit." (1181) |
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What types of switches are used extensively in building work?
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Remote control (RC) Switches
Automatic Transfer Switches Time-Controlled Switches (1182) |
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What is a solid-state switch?
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"A solid-state switch is an electronic device with a conducting state and a non-conducting state, corresponding to a conventional switch in its closed position and its open position." (1183)
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What is NEMA?
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National Electronic Manufacturers Association"
a standards-setting body for many industries including the data industry." (internet) |
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Why are circuit-protective devices needed?
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"To protect insulation, wiring, switches, and other apparatus from the destructive effects of overload and short-circuit currents..." (1185)
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What are the two most common circuit-protective devices?
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fuses
circuit breakers (1186) |
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What is a fuse?
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"The fuse is a simple device consisting of a FUSIBLE link or wire of low melting temperature...encased in an insulating fiber tube (cartridge fuse)" or "enclosed in a porcelain cup" (plug fuse). (1186)
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What is a circuit breaker?
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"A circuit breaker is an electromechanical device that performs the same protective function as a fuse and , in addition, acts as a switch." (1186)
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What are the 6 advantages of fuses (switch-and-fuse combination)?
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"Simple and foolproof
Constant characteristics (no aging) Initial economy Very high interrupting capacity (IC) No maintenance Instantaneous; energy limiting (1187) |
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What are the 7 disadvantages of fuses (switch-and-fuse combination)?
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"Fuses are single pole only
Necessity for storage of replacement fuses Nonrenewable (one-time operation) Nonadjustable Nonindicating No electric or remote control Not trip-free" (1187) |
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What are the 8 advantages of circuit breakers?
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"Usable as switches
Multipole No replacement storage Resettable Indicates trip Trip-free Remote control Adjustable" (1187) |
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What are the 4 disadvantages of circuit breakers?
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"Low to medium interrupting capacity...
Periodic maintenance required High initial cost Complex construction changes with age" (1187) |
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What is a switchboard?
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"A switchboard is a large, free-standing assembly of switches, fuses, and/or circuit breakers, which normally provide switching and overcurrent protection to a number of circuits connected to a single source." (1188)
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What is a unit substation aka transformer load center aka load-center substation?
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"An assembly, comprising a primary voltage switch-and-fuse or circuit breaker, a step-down transformer, meters, controls, buswork, and secondary (low-voltage) switchgear..." (1191)
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What is a panelboard aka electrical panel?
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"An electrical panel or PANELBOARD, serves essentially the same function as a switchboard except on a smaller scale; that is, it accepts a relatively large block of power and distributes it in smaller blocks." (1192)
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What is the key to energy conservation and electric demand limitation?
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load control (1193)
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What is the most common motor used in building equipment?
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squirrel-cage induction machines (1197)
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What are the two most common types of squirrel-cage induction motors?
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"TYPE B: standard design, high efficiency and power factor, normal torque; applicable to fans, blowers and pumps
TYPE C: high starting torque, fair efficiency and power factor; applicable to compressors, conveyors, and other devices that start under load" (1197) |
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What are the two functions of a motor controller?
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"to start and stop the motor and
to protect it from overload" (1198) |
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What are the two modes of operation for a motor contoller?
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1. manual starter - "when the contacts are operated by hand..."
2. magnetic controller/starter - "when contacts are operated by a magnetic coil controlled by push buttons, thermostats, or other devices..." (1198) |
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What are the advantages of a solid-state starter?
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"provide continuous (stepless), controlled motor starting.
limit inrush current... by adjusting acceleration time, the starting torque of the motor can be maintained. ...provide SOFT starts (i.e., starts that minimize the mechanical stresses caused by rapid application of accelerating torque). reduced in size and weight, long life, more sophisticated motor protection, and additional operating functions such as jogging and reversal." (1199) |
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What are the disadvantages of a solid-state starter?
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"higher cost and
possible radio frequency noise problems." (1199) |
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What is VVVF (VFD)?
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"variable-voltage, variable-frequency (VVVF) controller that gives smooth, continuous speed control over a range exceeding 30 to 1 while maintaining motor torque...also known as VARIABLE FREQUENCY DRIVES (VFD)." (1199)
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What devices are included in the general term "WIRING DEVICES"?
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"The general term WIRING DEVICES includes all devices that are normally installed in wall outlet boxes, including receptacles, switches, dimmers, fan controls, and so on. Attachment plugs, also called CAPS, and wall plates are also included in wiring devices." (1201)
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What are the three NEMA and UL grades for electrical device quality?
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hospital grade
federal specification grade UL general-purpose grade (1201) |
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What are the characteristics of hospital-grade devices?
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"Hospital-grade devices are identified by a green dot on the device face. These devices are built to withstand severe abuse while maintaining reliable operation and must meet UL requirements for this grade." (1201)
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How do manufacturers usally grade their devices?
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Hospital grade
Premium or industrial specification grade Commercial specification grade Residential grade (1201) |
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In applications, where are the various grades of electrical devices used?
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industrial specification grade - industrial and high-grade commercial construction
commercial specification grade - most educational and good residential buildings standard or residential grade - low cost construction of all types (1201) |
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How is a RECEPTACLE defined by NEC?
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"a contact device installed at the outlet for the connection of a single attachment plug." (1201)
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What is POWER CONDITIONING?
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"..the process of converting utility-supplied electrical power, which is frequently characterized by transient surges, spikes, radio frequency noise, and voltage fluctuation, to a pure (noiseless), accurately voltage-regulated sinusoidal waveform, normally referred to as COMPUTER-GRADE POWER. (1212)
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How does TVSS (transient voltage surge suppression) work?
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" 1. By placing one or more of these devices that present a high impedance in series with the incoming voltage transient, thereby limiting the amount of let-through current.
2. By placing ...device(s) across the incoming power line, in parallel with the protected load, that present a low impedance to the high transient voltage and thereby bypass (shunt away) the incoming wave's current." (1215) or |
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What are 6 important guidelines in selection and application of TVSS devices?
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1. Separate TVSS for every power level.
2. "In the absence of accurate data on the surge tolerance of electronic equipment, use protection for the most sensitive level." 3. Use Mode 1 aka Mode A for stand-alone items. 4. "The primary criteria for selection and comparison are the ACTUAL clamping voltage and maximum surge current capacity." 5. install as close as possible to the protected load 6. Also protect telephone and data lines. (1219) |
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What are the two primary criteria for selection and comparison of TVSS devices?
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1. ACTUAL clamping voltage
2. maximum surge current capacity. (1219) |
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What is an uninterruptible power supply (UPS)?
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"an arrangement of normal and backup power supplies that transfer a facility's critical load from the normal to the backup supply in so short a time that no computer malfunction results..."
(1220) |
