PHEP 30

Question 1

Whenever a circuit which is carrying current is opened:

Answer 1

Current tends to continue to flow across the contacts as they separate.

Question 2

The initials DPDT would normally be used to identify a type of:

Answer 2

Knife switch

Question 3

The type of knife switch usually used to interrupt field current to generators and synchronous machines is a:

Answer 3

Field switch

Question 4

Disconnect switches are commonly used on 440 V or higher voltage circuits to:

Answer 4

Isolate or to ground electrical equipment or circuits not in service

Question 5

Of the following statements concerning mercury control switches, which one is most nearly not correct:

Answer 5

They are generally used only where the circuit voltage is 120 Volts or less

Question 6

Mercury switches that are commonly used in control circuits where circuit voltage is 230 V or less have what properties?

Answer 6

They are single or double throw action, and they are silent operating.

Question 7

The main difference between a circuit breaker and other kinds of electrical switches is that circuit breakers:

Answer 7

Open automatically when the current exerts a predetermined limit.

Question 8

The energy needed to rapidly open the contacts of most circuit breaker designs is usually provided by…

Answer 8

Springs and gravity

Question 9

What is meant by trip free-operation?

Answer 9

The precedence which takes place over any closing operation

Question 10

Speed is desirable and opening a circuit breaker for what three reasons?

Answer 10

1. Extinguish the arc.
2. Minimize damage caused by a fault.
3. Reduce the effect of a fault on lines and equipment not directly involved with the fault.

Question 11

A trip-free operation takes place if…

Answer 11

A circuit breaker is closed while the protective relay is energized.

Question 12

What are the three means/methods of quenching an arc?

Answer 12

1. Increase the length of the arc path.
2. Restrict the cross sectional area of the arc path.
3. De-ionizing or cooking the gas.

Question 13

A three phase 138KV circuit breaker can interrupt 12,000A. What is the interrupting capacity in MVA?

Answer 13

138,000 X 12,000 = 1656 MVA

PHEP 30 pg.21

Question 14

4.8 KV circuit breaker has an interrupting rating of 30,000 A. What is its interrupting capacity when used at 5.1 KV?

Answer 14

Operating P = Rated P
4800V X 30,000A = 5100 V X I
I(rated) = 28,235A

PHEP 30 pg. 21

Question 15

The closing time of an OCB, becomes more important when it is considered from the standpoint of…

Answer 15

Synchronizing generators.

Question 16

Of the following statements concerning OCBs, which one is not correct?

a. dielectric strength of the arc gap is improved by forcing oil into the arc path.

b. The oil serves both to insulate the tank from the live parts, and to extinguish the arc.

c. The AC arc is always extinguished during the first current zero.

d. All OCBs are equipped with a vent which is usually located at the top of the tank.

Answer 16

Not true
C. The arc is always extinguished during the first current zero.

Question 17

A blowout coil would most likely be found in what type of circuit breaker?

Answer 17

An air magnetic circuit breaker

Question 18

The high dielectric strength of gases, such as SF6 under static conditions is maintained by

Answer 18

Pressurizing

Question 19

A fast realistic circuit breaker opening operation (arc out time) would be _______ cycles

Answer 19

3–4 cycles

Question 20

Circuit breakers are generally classified by…

Answer 20

• Their continuous current rating
  -Their interrupting rating
  -The amount of current that they will be carrying for a period of 4-5 seconds