7 30 Flashcards

1
Q

Whenever a circuit which is carrying current is opened:

A

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

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2
Q

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

A

Knife switch

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3
Q

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

A

Field switch

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4
Q

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

A

Isolate or to ground electrical equipment or circuits not in service.

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5
Q

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

A

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

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6
Q

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

A

Open automatically when the current exceeds a predetermined limit.

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7
Q

Which of the below statements does not correctly describe a means of method for quenching an arc?

A

Reducing the reluctance across the contacts

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8
Q

A blow coil would most likely be found in:

A

An air magnetic circuit breaker

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9
Q

Of the following statements, the one which most nearly describes what is meant by trip-free operation is:

A

The priority which takes place over any closing operation.

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10
Q

A trip-free operation takes place if:

A

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

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11
Q

Circuit breakers are generally classified by:

A

All of the above

continuous current rating,

  • interrupting rating,
  • the amount of current that they will carry for a period of four to five seconds
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12
Q

Speed is a desirable quality during the tripping operation of a circuit breaker to:

A

All three

Make it less difficult to extinguish the arc,

  • reduce the damage that which could be caused by a fault,
  • and protect equipment not directly involved in the fault area.
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13
Q

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

A

Springs and gravity

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14
Q

Of the following statements concerning ocb’s the one which is not correct:

A

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

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15
Q

The closing time of an ocb becomes more important when it is considered from the standpoint of:

A

Synchronizing generators

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16
Q

A fast realistic circuit breaker opening operation (arc-out time) would be:

A

3 to 4 cycles

17
Q

A three phase 138kV circuit breaker can interrupt 12,000 amps. What is it’s interrupting capacity in MVA?

A

1656 MVA

(138,000 X 12,000) / 1,000,000 =
1,656 MVA

18
Q

A 34.5kV circuit breaker has a rated interrupting capacity of 25,000 amps. What will be its approximate interrupting capacity when used on a 20kV circuit?

A

25kA

(25k) (34.5/20) = 43,125 amps
Therefore the capacity is 25 kA

19
Q

A 4.8kV circuit breaker has an interrupting rating of 30,000 amps. What is it’s interrupting capacity when used at 5.1kV?

A

28,235amps

(30k) (4.8/5.1) = 28,235 amps

20
Q

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

A

Pressurizing