Chapter 9 Unit 25-26

What is the approximate available short circuit current on the secondary of a three-phase, 112.50 kVA, 480V to 208Y/120V transformer having an impedance rating of 2.40 percent? (a) 10,500A (b) 11,000A (c) 12,500A (d) 13,000A

1. Transformer Secondary Amps = VA/(Volts x 1.732) 112,500/(208 x 1.732) = 313 2. Short-Circuit Current = Transformer Secondary Amps/Transformer Impedance 313/2.40 = 130.41 130.41 x 100 = 13,000A

A special purpose GFCI device is used in circuits where the voltage-to-ground is greater than ________. (a) 30V (b) 50V (c) 120V (d) 150V

150V

The current in an arcing fault is limited by the system impedance of the arcing fault itself. Typically, at a receptacle, fault current will be above 75A, but not likely above _______. (a) 100A (b) 220A (c) 330A (d) 450A

450A

If the difference between the current leaving and returning through the current transformer of the GFCI protective device exceeds ______(+/- 1mA), the solid-state circuitry de-energizes (a) 1 mA (b) 3mA (c) 5 mA (d) 10 mA

5 mA

When a sustained overload occurs through the element(s) of a fuse, the heat melts _____, stopping the flow of current. (a) several elements (b) all of the elements (c) a portion of the element (d) none of these

a portion of the element

________ circuit breakers permit the thermal and electromagnetic trip current and/or time settings to be adjusted to provide the use with flexibility for specific applications. (a) inverse time (b) adjustable trip (c) instantaneous (d) all of these

adjustable trip

The factor(s) that impact the available short-circuit current on the load side of a transformer is(are) _________. (a) system voltage (b) kVA rating (c) impedance (d) all of these

all of these

Over-current is current in excess of the equipment ampere rating or ampacity of the wires. It may result from a(an) _____ (a) overload (b) short circuit (c) ground fault (d) any of these

any of these

A arc-fault circuit interrupter is a device intended to de-energize a circuit when it detects the current waveform characteristics unique to a(an)__________ fault. (a) ground (b) neutral (c) arcing (d) any of these

arcing

The available short circuit current is highest at the utility transformer and lowest at the ______ load because of the impedance of the circuit. (a) branch-circuit (b) feeder (c) service (d) utility transformer

branch-circuit

A ______ is. capable of being opened and closed manually and automatically opens during an over-current condition. (a) fuse (b) circuit breaker (c) motor starter (d) disconnect

circuit breaker

Fuses consist of a ______ element connected to end blades or cps, surrounded by silica sand filler material enclosed in a tube. (a) conductive (b) foam (c) light (d) nonconductive

conductive

To quickly clear a short circuit, the short-circuit _________ must rise to a level that is between ten and twenty times the rating of the circuit over-current protective device. (a) voltage (b) current (c) resistance (d) power

current

A _______ fuse is designed to clear a short circuit or ground fault in less than one-half a cycle. (a) one-time fuses (b) dual-element fuses (c)special trip fuses (d) current-limiting

current-limiting

A______ fuse is often called a "time-delay fuse." (a) single-element (b) dual-element (c) current-limiting (d) all of these

dual-element

Inverse time circuit breakers have a(an) ______ trip unit that responds to short circuit and ground-fault currents. (a) adjustable (b) instantaneous (c) electromagnetic (d) none

electromagnetic

A hazard can exist if the electronics within a GFCI fail because the circuit remains ______ without GFCI protection. (a) de-energized (b) energized (c) off (d) any of these

energized

A(an) _________ wire is not necessary for the proper function of a GFCI device. (a) phase (b) grounded (c) neutral (d) equipment grounding

equipment grounding

_____ sources of transient voltage include the switching of utility power factor correction capacitors or lightning strikes. (a) special (b) interior (c) exterior (d) any of these

exterior

Over current protection is typically provided by ________. (a) fuses (b) circuit breakers (c) fuses or circuit breakers (d) none of these

fuses or circuit breakers

A(an) ______ occurs when an unintentional electrical connection occurs between a phase wire and metal parts of enclosures, raceways, and/or equipment. (a) overload (b) short-circuit (c) ground fault (d) all of these

ground fault

To quickly clear a(an) ________, the ground-fault current needs to rise to level between ten and twenty times the rating of the circuit over-current protective device. (a) overload (b) arcing fault (c) ground fault (d) none

ground fault

GFCI protective devices detect the _____ current between circuit waves. (a) grounded (b) neutral (c) imbalance of (d) phase

imbalance of

As the overload current ________, the time it takes for the thermal trip element to open decreases. (a) increases (b) decreases (c) remains the same (d) none of these

increases

______ circuit breakers operate on the principle of electromagnetism only and are commonly known as motor circuit protectors. (a) inverse time (b) adjustable trip (c) instantaneous (d) all of these

instantaneous

If the protective device is not rated to ________ the available fault current at its listed voltage rating, it can explode while attempting to clear the fault. (a) interrupt (b) withstand (c) hold (d) break

interrupt

________ circuit breakers operate on the principle that as the current increases, the time it takes for the device to open decreases. (a) inverse time (b) adjustable time (c) instantaneous (d) all of these

inverse time

Available short-circuit current is the _______ current in amperes that is available at a given point in the electrical system. (a) phase (b) line (c) largest (d) smallest

largest

To remove dangerous touch voltage on metal parts produced by a ground fault, the fault-current path must have ________ to allow the fault current to quickly rise to facilitate the opening of the protection device. (a) high impedance (b) low impedance (c) high voltage (d) low voltage

low impedance

When a short circuit or ground fault occurs through the element of a fuse, the heat in the element rises to a point where _________ in the fuse melt to stop electrical current flow. (a) multiple conductive segments (b) a small portion of the segment (c) all of the segments (d) none of these

multiple conductive segments

GFCIs detect connections between the neutral wire and the metal parts of the electrical system (case). If a neutral connection to metal parts occurs, the GFCI protective device will _______. (a) open (b) explode (c) overload (d) none of these

open

A(an) ______ is the operation of equipment or wires in excess of their rated ampacity. (a) overload (b) short circuit (c) ground fault (d) all of these

overload

A _______ arcing fault can occur when the insulation between two wires degrades to a level where arcing can take place across the insulation between the two wires. (a) series (b) parallel (c) ground (d)

parallel

Surge protective devices are connected in _____ with the load. (a) parallel (b) series (c) series-parallel (d) none of these

parallel

Ground-fault protection of equipment is not intended to protect _____ because its opening ground-fault trip setting is 30 mA—not 6 mA as with a GFCI. Answer 1: property Answer 2: persons Answer 3: equipment Answer 4: any of these

persons

Severe electric shock or death can occur if a person touches the ______ and neutral wires at the same time, even if the circuit is GFCI protected. (a) phase (b) service (c) equipment grounding (d) none of these

phase

Lack of selective coordination of over-current protective devices can result in unintended ______ to loads not part of the faulted circuit. (a) surges (b) power losses (c) blackouts (d) shutdowns

power losses

"_______" of over-current protective devices is when the devices are designed to clear a short circuit or ground fault in a manner that localizes the short circuit or ground fault in a manner that localizes the short circuit or ground fault to that given circuit. (a) plotting (b) sequencing (c) selective coordination (d) none of these

selective coordination

A ______ arcing fault occurs when the wire within a cord is unintentionally broken, causing the current to arc across the gap in the wire. (a) series (b) parallel (c) ground (d) neutral

series

A(an) ________ is an electrical connection between any two phase wires, or a phase wire and neutral wire. (a) overload (b) short circuit (c) ground fault (d) all of these

short-circuit

Equipment must have a(an)__________ current rating that permits the protection device to clear a short circuit or ground fault without extensive damage to the equipment components. (a) overload (b) short-circuit (c) ground-fault (d) none of these

short-circuit

What is the approximate available short-circuit current on the secondary of a 50 kVA, 480V to 240V transformer having an impedance rating of 1.20 percent? (a) 10,000A (b) 12,000A (c) 14,000A (d) 17,000A

short-circuit current = transformer secondary amps/transformer 1. Transformer secondary amps = VA/Volts 50,0000/240 = 208A 2. Short-Circuit Current = Transformer Secondary Amps/ Transformer Impedance % 208/1.2 = 173.33A 173.33 x 100 = 17,000A

A _____ fuse is referred to as a nontime-delay fuse and is the least expensive type. (a) single-element (b) dual-element (c) current-limiting (d) all of these

sing-element day

Circuit breakers and fuses are intended to interrupt the circuit, and they must have an ampere interrupting rating (AIR) _______ for the available short-circuit current. (a) inadequate (b) sufficient (c) sustainable (d) none of these

sufficient

________ protective devices protect equipment by preventing damaging transient voltage from reaching the equipment. (a) short-circuit (b) overload (c) overcurrent (d) surge

surge

GCFIs can only be properly tested by pressing the GFCI ______ button. (a) disconnect (b) reset (c) test (d) any of these

test

The _______ trip element of a circuit breaker operates on the time-current principle of being inversely proportional to the magnitude of the current, which means that as the overload current increases, the time it takes for the trip element to open decreases. (a) magnetic (b) electronic (c) thermal (d) none

thermal

The time it takes for an over-current protective device to open is plotted on a _______ curve chart. (a) time current characteristics (b) bar graph (c) pie chart (d) none

time-current char

_______ voltages inside a building can originate from high current loads that are switches on and off, such as those for copiers, laser printers, motors, and air-conditioning. (a) actual (b) sag (c) transient (d) nominal

transient

Selective coordination of over-current protective devices prevents ______ power losses to loads not part of the faulted circuit. (a) intended (b) unintended (c) excessive (d) none

unintended

The available short-circuit current at the __________ transformer is provided by the electric utility; no calculation required. (a) occupancy (b) building (c) premises (d) utility

utility

The available short-circuit current is different at each point of the electrical system; it is highest at the ________. (a) branch circuit (b) feeder (c) service (d) utility transformer

utility transformer

If the equipment is not rated to _______ the available short-circuit current, it can explode while waiting for the circuit protective device to clear the fault. (a) interrupt (b) withstand (c) hold (d) break

withstand