Deck 10: Transformer Installation

Question 1

If the primary current is less than 2 amperes, the short circuit protective device can be set at not more than 250% of that value.

Answer 1

False

Question 2

If the primary current is ____ amperes or more, it is permissible to increase the fuse size to the next highest standard rating.

A) 2
B) 4
C) 7
D) 9

Answer 2

D

Question 3

The actual impedance of a transformer is determined by its physical construction, such as the gage of the wire in the winding, the number of turns, the type of core material, and the magnetic efficiency of the core construction.

Answer 3

True

Question 4

According to the NEC, fuses must be plainly marked with the interrupting current or rating if it is other than 8,000 amperes.

Question 5

Assume that a single-phase transformer is a 2400/480 volt 15 kVA transformer. The full load current rating of the secondary winding is ____A.

A) 3
B) 5
C) 31.25
D) 160

Question 6

If the primary current is 9 amperes or more, the fuse or circuit breaker size is set at ____% of the primary current.

A) 110
B) 125
C) 153
D) 167

Question 7

There are several steps that should be followed when selecting a conductor. The first step is to determine the temperature rating of the devices and terminals.

Question 8

The temperature ratings of devices such as circuit breakers, fuses, and terminals are found in the ____ product directories.

A) Underwriters Laboratories'
B) National Electrical Code's
C) National Electrical Code Library's
D) Underwriters Library's

Question 9

____ impedance is an empirical value that can be used to predict transformer performance.

A) Decimal
B) Percent
C) Numeric
D) Summary

Question 10

The percent impedance is measured by connecting a(n) ____ across the low-voltage terminals and a variable voltage source across the high-voltage terminals.

A) ammeter
B) voltmeter
C) conductor
D) wattmeter

Question 11

The ability of a fuse or circuit breaker to open under the ____ fault current is expressed as the interrupting rating of the device.

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

Question 12

In general, fuses are rarely selected for protection of high voltage circuits because they are more expensive than other types of protection.

Question 13

____ can be used as the branch circuit protective devices in circuits supplying power to transformers.

A) Only fuses
B) Only circuit breakers
C) Fuses or circuit breakers
D) Neither fuses nor circuit breakers

Question 14

The minimum interrupting rating permitted for a fuse or circuit breaker in a specific installation is the ____ fault current available at the location of the protective device.

A) maximum linear
B) minimum symmetrical
C) maximum symmetrical
D) minimum surface

Question 15

When working with any transformer, the primary is the winding that is connected to a voltage source, and the secondary is the winding that is connected to an electrical load.

Question 16

NEC 215.2(A) states that the rating of a branch-circuit shall not be ____.

A) less than noncontinuous load plus 110% of the continuous load
B) more than continuous load plus 125% of the noncontinuous load
C) less than noncontinuous load plus 125% of the continuous load
D) more than continuous load plus 150% of the noncontinuous load

Question 17

The NEC states that an autotransformer rated at 1000 volts or less must be protected by an overcurrent device in each of the input ungrounded conductors.

Question 18

Occasionally, the temperature rating of devices such as circuit breakers, fuses, and terminals may be found on the piece of equipment, but this is the exception and not the rule.

Question 19

Percentages must be converted to a decimal form before they can be used in a mathematical formula. When this conversion has been made to represent the percent impedance in decimal form, this is called the decimal impedance.

Question 20

To determine the impedance of a transformer, first compute the full load current rating of the secondary winding using the formula I = E / VA.