Deck 6: Current Transformers

Question 1

The clamp-on ammeter does not have different range settings.

Answer 1

False

Question 2

Current transformers are used to change the full-scale range of ____.

A) wattmeters
B) voltmeters
C) AC ammeters
D) DC ammeters

Answer 2

C

Question 3

CTs are designed to operate with ____ winding shorted.

A) the secondary
B) the primary
C) both the primary and secondary
D) neither

Answer 3

A

Question 4

Transformer formulas can be used to determine the number of secondary turns needed to produce desired ranges.

Question 5

The turns ratio of the transformer can be changed by looping the power wire through the opening in the transformer to produce a primary winding of more than one turn.

Question 6

The number of turns needed for a full-scale range of amperes can be calculated using the formula ____.

A) NS / NP = IS / IP
B) NP x NS = IS / IP
C) NP / NS = IS / IP
D) NP / NS = IP / IS

Question 7

A(n) ____ transformer is a transformer constructed with a hollow core, similar to a donut in that it has a hole in the middle.

A) solenoid
B) eloid
C) toroid
D) winoid

Question 8

____ of a current transformer must be connected in series with the load.

A) The primary winding
B) The secondary winding
C) Both the primary and secondary windings
D) Either the primary winding or the secondary winding

Question 9

Assume that an ammeter provides range values of 5 A, 2.5 A, 1 A, and 0.5 A. Also, assume that the meter movement requires a current flow of 100 mA (0.100) to deflect the meter full scale and that the primary of the current transformer contains 5 turns of wire. The number of turns needed for a full-scale range of 5 amperes is ____ turns.

A) 5
B) 25
C) 125
D) 250

Question 10

When the secondary winding of a CT is opened when there is power applied to the primary, this will cause the transformer to produce a step-down in voltage that could still be high enough to kill anyone who comes in contact with it.

Question 11

The secondary winding of a CT should never be opened when there is power applied to the primary.

Question 12

The clamp-on type of AC ammeter uses a current transformer to operate.

Question 13

A current transformer has a ratio of 600:5. If the primary power wire is inserted through the opening, it will require a current of 600 amps to deflect the meter full scale. If the primary power conductor is looped around and inserted through the window a second time, the primary now contains two turns of wire instead of one. It requires ____ amps of current flow in the primary to deflect the meter full scale.

A) 125
B) 200
C) 300
D) 600

Question 14

To use the clamp-on type of AC ammeter, clamp the jaw of the meter around ____ of the conductors supplying power to the load.

A) only one
B) one or two
C) two
D) all

Question 15

A problem with many clamp-on ammeters is that ____.

A) changing the turns ratio is limited to wrapping two turns of wire around the jaw of the ammeter
B) their mid-range scale values are inaccurate
C) they can have different range settings by changing the turns ratio of the secondary of the transformer
D) their lowest scale value is too high to accurately measure low-current values

Question 16

When current transformers are used, the main power line is inserted through the opening in the transformer. The power line acts as the primary of the transformer and is considered to be two turns.

Question 17

Current transformers differ from voltage transformers in that the primary winding is generally part of the power line.

Question 18

Many electricians use the clamp-on type of AC ammeter.

Question 19

In-line ammeters (ammeters that must be connected directly into the line) that have multiple-range values cannot use a current transformer to provide the different ranges.

Question 20

An ammeter with a full-scale range of 0-6 amperes would not be able to measure accurately a current flow of 0.2 amperes. The answer to this problem is to ____.

A) wrap just one turn of wire around the jaw of the ammeter to change the secondary-scale value
B) simply change the secondary-scale value setting
C) wrap multiple turns of wire around the jaw of the ammeter to change the secondary-scale value
D) none of the above