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.

Accepted Answer

Nonselective coordination occurs when a fault on a branch circuit opens not only the branch-circuit overcurrent device but also the feeder overcurrent device, which results in interruption of power to the entire circuit. This type of coordination is not desirable and can result in unnecessary downtime and inconvenience. Selective coordination, on the other hand, ensures that only the overcurrent device nearest to the fault opens, leaving the rest of the system intact and minimizing downtime. Series-rated action refers to a specific type of overcurrent protection where a lower-rated device is used in conjunction with a higher-rated device to provide protection. Interruption simply refers to the loss of power due to a fault or other issue.

Question 2

How can you create a selective system using circuit breakers?

Accepted Answer

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?

Accepted Answer

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?

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 5

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

Accepted Answer

The square root of three is approximately equal to 1.732, therefore the correct choice is D.

Question 6

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 7

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

Accepted Answer

The term "short-circuit current" refers to the current that flows when the supply voltage is directly connected to the motor terminals with a negligible impedance load. This condition occurs if the motor windings are shorted together, which is the definition of a locked-rotor. Therefore, the amount of short-circuit current from a motor is equal approximately to the locked-rotor current for each motor.

Question 8

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

Accepted Answer

The answer of What is the importance of selective coordination?...

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.

Accepted Answer

Circuit breakers must have adequate interrupting ratings to safely interrupt the maximum fault current that can occur at their location in the electrical system.

Question 10

can prevent the occurrence of single phasing.

Accepted Answer

The answer of can prevent the occurrence of single phasing.
...

Question 11

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

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 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.

Accepted Answer

The available fault current refers to the maximum amount of current that could flow through a short circuit in the electrical system. This information is necessary for selecting appropriate overcurrent protection devices. Therefore, option A (short-circuit) is the best choice to determine available fault currents. The other options, transformer multiplier, line-to-line voltage, and line-to-line wattage, are not relevant for calculating available fault currents.

Question 13

The transformer multiplier M2 is calculated by the formula .

Accepted Answer

The formula for transformer multiplier M2 is M2 = 1/(1 + f), as given by the turns ratio formula for the transformer. Therefore, the correct choice is B, which matches the formula for M2.

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.

Accepted Answer

According to the question, the actual impedance value can vary plus or minus a certain percentage from the marked impedance value. The percentage is not given, but the options range from 5% to 50%. The best choice would be the option with the smallest percentage value, which is option B with a variation of plus or minus 10%.

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.

Accepted Answer

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

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.

Accepted Answer

The definition given describes critical operations power systems, which are designed to ensure continuity of operation in facilities that are critical to national security, public health or safety, and the economy. Options A (NSA), B (emergency), and C (uninterruptible) do not fully capture the criteria mentioned in the definition.

Question 17

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

Accepted Answer

The answer of What should you keep in mind when...

Question 18

When determining the f factor, X represents .

Accepted Answer

X represents the fault current in amperes at the transformer terminals.

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.

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 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.

Accepted Answer

The NEC requires that the maximum short-circuit current available at the service be field marked to ensure that the equipment is properly rated and protected.

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