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

Question 1

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

Answer 1

A

Question 2

How can you create a selective system using circuit breakers?

Answer 2

For normal overload situations, a circuit breaker having an ampere rating lower than the ampere rating of an upstream circuit breaker will trip.The upstream breaker will not trip.The system is "selective." For low­level faults less than the instantaneous­trip setting of an upstream circuit breaker, a circuit breaker having an ampere rating lower than the ampere rating of the upstream breaker will trip, and the upstream breaker will not trip.The system is selective.For fault-current levels above the instantaneous-trip setting of the upstream circuit breaker, both the branch-circuit breaker and the upstream circuit breaker will trip off.The system is nonselective.

Question 3

How can you create a selective system using fuses?

Answer 3

The proper choice of the various classes and types of fuses is necessary if selectivity is to be achieved, Figure 18-7.Indiscriminate mixing of fuses of different classes, time-current characteristics, and even manufacturers may cause a system to become nonselective.
To ensure selective operation under low overload conditions, it is necessary only to check and compare the time-current characteristic curves of fuses.Selectivity occurs in the overload range when the curves do not cross one another.
Fuse manufacturers publish selectivity guides to be used for short-circuit conditions.When using these guides, selectivity is achieved by maintaining a specific amperage ratio between the various classes and types of fuses.A selectivity chart is based on any fault current up to the maximum interrupting ratings of the fuses listed in the chart.

Question 4

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 5

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 6

What is the intent of NEC 110.9 and 110.10?

Question 7

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 8

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

Question 9

If noncurrent-limiting overcurrent devices (standard molded-case circuit breakers) are to be installed in the main switchboard, breakers having adequate must be installed in the panelboards.

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

Question 10

can prevent the occurrence of single phasing.

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

Question 11

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

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

Question 12

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 13

The transformer multiplier M2 is calculated by the formula .

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

Question 14

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 15

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 16

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 17

What should you keep in mind when installing a series-rated system?

Question 18

When determining the f factor, X 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 fault current in amperes at the transformer terminals
D)the voltage, line-to-line or line-to-neutral

Question 19

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 20

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

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