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Flashcards in Test Review Deck (46)
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1

Why is speed a desirable quality to have in opening circuit breakers?

-facilitates extinguishing an arc
-minimizes damage caused by a fault
-reduces effects of a fault on lines and equipment not directly involved with the fault

2

What is an isolation transformer?

A transformer that is magnetically coupled, but electrically and mechanically isolated.

3

What are harmonics? When do they occur?

Spikes of frequency that occur at multiples of 60 Hz

4

What must be known about a transformer to determine the different values of voltage and current?

Turn ratio

5

What part of the transformer is connected to the load and what part is connected to the power supply?

The secondary winding is connected to the load and the primary winding is connected to the power supply

6

True or False:

In a step down transformer, the power in does not equal the power out.

False

7

What is a transformer?

A magnetically operated machine that can change voltage, current, and impedance without changing frequency

8

How are transformers rated?

kVA

9

Name the three basic contacts of a PCB?

Butt, wedge, and bayonet

10

What is the meaning of electrically trip free regarding a PCB?

The PCB will not allow the closing coil to close under a fault condition.

11

What is the most common gas used in circuit breakers to quench an arc?

Sulfur hexafluoride (SF6)

12

What is continuous current?

The amount of current that a circuit breaker can carry continuously

13

What is interrupting time?

The time it takes a circuit breaker to open after the tripping coil is energized

14

How many transformers make an open delta connection?

Two

15

What is the frequency of the second harmonic?

120 Hz

16

What is a triplen harmonic?

Odd multiple of the third harmonic

17

What is the primary function of a circuit breaker?

Protection

18

A transformer has a primary voltage of 240 V, a secondary voltage of 180 V, and the windings are connected subtractively. What voltage will appear across the entire connection?

240 V - 180 V = 60 V

19

What is another name for an arc?

Plasma

20

Two transformers rated at 100 kVA each are connected in an open delta connection. What is the total output power supplied?

.866(100kVA + 100kVA) = 173.2 kVA

Or

1.732(100 kVA) = 173.2 kVA

21

What is a knife switch?

The simplest form of a circuit breaker

22

How many seconds does it take for a circuit breaker to open?

1/20 of a second

23

A transformer has a primary voltage of 240 V and a secondary voltage of 80 V. What is the turn ratio?

Np/Ns = 240V/80V

Np/Ns = 3/1

3:1

24

How are circuit breakers rated?

-interrupting current capacity
-continuous current capacity
-voltage class

25

What are the three mediums that extinguish the arc in circuit breakers?

Air, oil, and SF6

26

A transformer has a turn ratio of 1:6 and a primary current of 18 A. What is the secondary current?

Np/Ns = Is/Ip
1/6 = Is/18
Is= 3 A

27

True or False:

Phase voltage and line voltage are 90 degrees out of phase on a three phase system.

False

They are 120 degrees out of phase

28

What two things are important about a wye connected three phase system?

Line current equals phase current

E(Line) = E(Phase) x 1.732

29

What two things are important about a delta connected three phase system?

Line voltage equals phase voltage

I(Line) = I(Phase) x 1.732

30

True or False:

Voltage, current, frequency, and impedance are things that can be changed by a transformer.

False

Frequency can’t be changed