Deck 18: Short-Circuit Calculations and Coordination of Overcurrent Protective Devices

Question 1

What is the importance of selective coordination? What guidelines cover it?

Answer 1

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The importance of selectivity in an electrical system is covered extensively throughout NEC Article 517. This article pertains to health care facilities, where maintaining electrical power is extremely important. The unexpected loss of power in certain areas of hospitals, nursing care centers, and similar health care facilities can be catastrophic. The importance of selectivity is also emphasized in NEC 240.12 (Electrical Systems Coordination); NEC 230.95 (Ground-Fault Protection of Equipment); Informational Note No. 2 , NEC 620.62 (Selective coordination for elevators, escalators, and moving walks); NEC 700.4(B)(Emergency Systems); NEC 700. 32 (coordination for emergency circuits); and NEC 701.4( B )and 701. 32 (Legally Required Standby Systems); NEC 708.22(B)708.52(D), and 708.54 (Critical Operations Power Systems - COPS). These sections refer to installations where additional hazards would be introduced should a nonorderly shutdown occur. The Code defines coordination, in part, as the proper localizing of a fault condition to restrict outages to the equipment affected.

Question 2

The transformer multiplier M₂ is calculated using the formula ____.

A)1 + f
B)1/(1 + f)
C)(f + 1)/f
D)f² + 1

Answer 2

B

Question 3

____ can prevent the occurrence of single phasing.

A)Nothing
B)Motor overload protection
C)Common overload protection
D)Individual overload protection

Answer 3

A

Question 4

To meet the requirements of NEC 110.9 and 110.10, it is absolutely necessary to determine the available fault currents at various points on the electrical system. If a ____ study is not done, the selection of overcurrent devices may be in error, resulting in a hazard to life and property.

A)short-circuit
B)transformer multiplier
C)line-to-line voltage
D)line-to-line wattage

Question 5

The actual impedance value on a UL-listed 25-kilovolt-ampere or larger transformer can vary plus or minus ____ percent from the transformer's marked impedance.

A)5
B)10
C)25
D)50

Question 6

How can you create a selective system using circuit breakers?

Question 7

How can you create a selective system using fuses?

Question 8

​Under what conditions are series-rated systems most often installed?

Question 9

The amount of available short-circuit current decreases as the ____ from the transformer increases.

A)voltage
B)current
C)magnetic field
D)distance

Question 10

If non-series-rated overcurrent devices (standard molded-case circuit breakers)are to be installed in the main switchboard, breakers with adequate ____ must be installed in the panelboards.

A)fault current
B)short-circuit ratings
C)interrupting ratings
D)fuses

Question 11

​The NEC includes a requirement that the ____ short-circuit current available at the service be field marked at or on the service equipment.

A)maximum
B)minimum
C)approximate
D)required

Question 12

The amount of short-circuit current from a motor is equal approximately to the starting (____)current for each motor.

A)accelerating
B)locked-rotor
C)stator
D)final

Question 13

____ power systems are power systems that are generally installed in vital infrastructure facilities that, if destroyed or incapacitated, would disrupt national security, the economy, public health or safety, and where enhanced electrical infrastructure for continuity of operation has been deemed necessary by governmental authority.

A)NSA
B)Emergency
C)Uninterruptible
D)Critical operations

Question 14

A situation known as ____ occurs when a fault on a branch circuit opens not only the branch-circuit overcurrent device but also the feeder overcurrent device.

A)nonselective coordination
B)selective coordination
C)interruption
D)series-rated action

Question 15

A simple method of determining the available short-circuit currents (also referred to as fault current)at various distances from a given location is the ____ method.

A)point-to-fault
B)point-to-ground
C)point-to-point
D)point-to-phase

Question 16

The value of the square root of three (____)is used frequently in 3-phase calculations.

A)1.000
B)1.414
C)1.500
D)1.732

Question 17

To obtain the approximate short-circuit current values when two hot conductors of a 3-phase system are shorted together, use ____ percent of the 3-phase current value.

A)50
B)67
C)83
D)87

Question 18

What is the approximate line-to-ground arcing fault value on a 480/277-volt system where the line-line-line fault current has been calculated to be 30,000 amperes?

A)11,400
B)22,200
C)26,700
D)30,000

Question 19

What is the intent of NEC 110.9 and 110.10 ?

Question 20

When determining the f factor, N represents ____.

A)the length of the circuit to the fault
B)the available fault current in amperes at the beginning of the circuit
C)the number of conductors in parallel
D)the voltage, line-to-line or line-to-neutral