Question 1

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

Accepted Answer

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

Accepted Answer

The transformer multiplier M₂ is calculated using the formula 1/(1 + f), where f is the frequency. This formula is used to adjust the output based on the frequency, ensuring the transformer's output remains stable across different frequencies.

Question 3

____ can prevent the occurrence of single phasing.
A)Nothing
B)Motor overload protection
C)Common overload protection
D)Individual overload protection

Accepted Answer

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

Accepted Answer

A short-circuit study is essential for determining the available fault currents at various points in an electrical system, which is crucial for the correct selection of overcurrent protective devices as per NEC requirements. Without this study, there's a risk of choosing incorrect devices, posing hazards.

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

Accepted Answer

The actual impedance value on a UL-listed 25-kilovolt-ampere or larger transformer can vary plus or minus 10 percent from the transformer's marked impedance, as per industry standards and UL guidelines.

Question 6

How can you create a selective system using circuit breakers?

Accepted Answer

The answer of How can you create a selective system...

Question 7

How can you create a selective system using fuses?

Accepted Answer

The answer of How can you create a selective system...

Question 8

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

Accepted Answer

The answer of ​Under what conditions are series-rated systems most...

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

Accepted Answer

The amount of available short-circuit current decreases as the distance from the transformer increases due to the impedance of the circuit increasing with distance, which limits the flow of fault current.

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

Accepted Answer

Circuit breakers must have adequate interrupting ratings to safely interrupt the highest potential fault current they might have to interrupt. This ensures the safety and reliability of the electrical system by preventing the circuit breaker from failing in the event of a high fault current.

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

Accepted Answer

The NEC requires the maximum short-circuit current available to be field marked at or on the service equipment to ensure safety and proper equipment selection.

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

Accepted Answer

The amount of short-circuit current from a motor is approximately equal to the starting locked-rotor current, which occurs when the motor starts with the rotor at a standstill.

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

Accepted Answer

Critical operations power systems (COPS) are designed for facilities where continuity is deemed essential by authorities, focusing on national security, economy, and public safety.

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

Accepted Answer

Nonselective coordination occurs when a fault in a branch circuit causes both the branch-circuit overcurrent device and the feeder overcurrent device to open, indicating a lack of proper coordination between the devices to isolate the fault effectively.

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

Accepted Answer

The point-to-point method is a simple technique used for determining the available short-circuit currents at various distances from a given location in an electrical system.

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

Accepted Answer

The square root of three is approximately 1.732, which is commonly used in calculations involving three-phase electrical systems.

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

Accepted Answer

The answer of To obtain the approximate short-circuit current values...

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

Accepted Answer

The line-to-ground arcing fault value on a 480/277-volt system can be approximated by taking the line-line-line fault current and multiplying it by the square root of 3 (approximately 1.732). However, this calculation is typically used to go from line-to-ground to line-to-line fault current. For a line-to-ground fault, the current tends to be lower due to the higher impedance path. Given the options and the nature of the question, which seems to imply a reduction from the line-line-line fault current, the closest approximation without performing the exact calculation (which would typically result in a lower value than the line-line-line fault current due to system impedances) is 11,400 amperes, acknowledging that this is a simplification and actual calculations would depend on specific system impedances and configurations.

Question 19

What is the intent of NEC 110.9 and 110.10 ?

Accepted Answer

The answer of What is the intent of NEC 110.9...

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

Accepted Answer

N represents the number of conductors in parallel when determining the f factor, which is used in calculations related to electrical systems, particularly for determining the sizing of equipment grounding conductors.

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

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

The transformer multiplier M₂ is calculated using the formula ____.

____ can prevent the occurrence of single phasing.

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.

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.

How can you create a selective system using circuit breakers?

How can you create a selective system using fuses?

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

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

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.

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

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

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 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.

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

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.

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?

What is the intent of NEC 110.9 and 110.10 ?

When determining the f factor, N represents ____.

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

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

The transformer multiplier M2 is calculated using the formula ____.

____ can prevent the occurrence of single phasing.

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.

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.

How can you create a selective system using circuit breakers?

How can you create a selective system using fuses?

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

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

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.

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

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

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 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.

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

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.

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?

What is the intent of NEC 110.9 and 110.10 ?

When determining the f factor, N represents ____.

The transformer multiplier M₂ is calculated using the formula ____.

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

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