Deck 20: Instrument Transformers

Question 1

Because it is not possible to use standard transformers to change DC levels, ____________________ are used to develop the smaller values needed for DC monitoring equipment.

Answer 1

instrument shunts

Question 2

The secondary of each instrument transformer is ____________________ to provide protection from high-voltage hazards, as provided in Article 250 Part IX of the NEC.

Answer 2

grounded

Question 3

Why must the three-phase instruments be connected correctly in the secondary circuits?

Answer 3

When three-phase instruments are connected in the secondary circuits, these instruments must be connected correctly so that the proper phase relationships are maintained. If this precaution is not observed, the instrument readings will be incorrect. In checking the connections for this three-phase, three-wire metering system, note that the interconnected potential and current secondaries are both grounded to provide protection from high-voltage hazards.

Question 4

What is the main difference between a potential transformer and a power or distribution transformer?

Question 5

Current transformers are used so that ammeters and the current coils of other instruments and relays can be connected directly to high-current lines.

Question 6

The ____________________ of the primary circuit is found by multiplying the primary voltage and current values..

Question 7

The power factor is equal to the power in kilowatts multiplied by the apparent power in kilovolt-amperes.

Question 8

On a three-phase, three-wire system, two potential transformers of the same rating and two current transformers of the same rating are necessary.

Question 9

Potential transformers have highly accurate ratios between the primary and secondary voltage values; generally the error is less than ____________________%.

Question 10

The secondary winding of a current transformer is rated at ____________________ amperes regardless of the current rating of the primary windings.

Question 11

In a three-phase, four-wire system, both the interconnected potential and current secondaries are grounded to protect against possible high-voltage hazards.

Question 12

What caution must be taken when working with the secondary circuit of a current transformer?

Question 13

Describe how the secondary circuits interconnect in three-phase metering.

Question 14

How is the wattmeter multiplier calculated if a voltmeter, an ammeter, and wattmeter are connected in a single-phase, high-voltage line?

Question 15

Potential transformers have ratings from 1,000 to 5,000 volt-amperes (VA).