7 30 Flashcards
Whenever a circuit which is carrying current is opened:
Current tends to continue to flow across the contacts as they separate.
The initials DPDT would normally be used to identify a type of:
Knife switch
The type of knife switch usually used to interrupt field current to generators and synchronous machines is a:
Field switch
Disconnect switches are commonly used on 440v or higher voltage circuits to:
Isolate or to ground electrical equipment or circuits not in service.
Of the following statements concerning mercury control switches, the one which is most nearly not correct is:
They are generally used only where the circuit voltage is 120 volts or less.
The main difference between a circuit breaker and other kinds of electrical switches is that circuit breakers:
Open automatically when the current exceeds a predetermined limit.
Which of the below statements does not correctly describe a means of method for quenching an arc?
Reducing the reluctance across the contacts
A blow coil would most likely be found in:
An air magnetic circuit breaker
Of the following statements, the one which most nearly describes what is meant by trip-free operation is:
The priority which takes place over any closing operation.
A trip-free operation takes place if:
A circuit breaker is closed while the protective relay is energized.
Circuit breakers are generally classified by:
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
Speed is a desirable quality during the tripping operation of a circuit breaker to:
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.
The energy needed to rapidly open the contacts of most circuit breaker designs is usually provided:
Springs and gravity
Of the following statements concerning ocb’s the one which is not correct:
The AC arc is always extinguished during the first current zero.
The closing time of an ocb becomes more important when it is considered from the standpoint of:
Synchronizing generators
A fast realistic circuit breaker opening operation (arc-out time) would be:
3 to 4 cycles
A three phase 138kV circuit breaker can interrupt 12,000 amps. What is it’s interrupting capacity in MVA?
1656 MVA
(138,000 X 12,000) / 1,000,000 =
1,656 MVA
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?
25kA
(25k) (34.5/20) = 43,125 amps
Therefore the capacity is 25 kA
A 4.8kV circuit breaker has an interrupting rating of 30,000 amps. What is it’s interrupting capacity when used at 5.1kV?
28,235amps
(30k) (4.8/5.1) = 28,235 amps
The high dielectric strength of gases, such as SF6, under static conditions is maintained by:
Pressurizing
