PHEP 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 440 V or higher voltage circuits to:
Isolate or to ground electrical equipment or circuits not in service
Of the following statements concerning mercury control switches, which one is most nearly not correct:
They are generally used only where the circuit voltage is 120 Volts or less
Mercury switches that are commonly used in control circuits where circuit voltage is 230 V or less have what properties?
They are single or double throw action, and they are silent operating.
The main difference between a circuit breaker and other kinds of electrical switches is that circuit breakers:
Open automatically when the current exerts a predetermined limit.
The energy needed to rapidly open the contacts of most circuit breaker designs is usually provided by…
Springs and gravity
What is meant by trip free-operation?
The precedence which takes place over any closing operation
Speed is desirable and opening a circuit breaker for what three reasons?
- Extinguish the arc.
- Minimize damage caused by a fault.
- Reduce the effect of a fault on lines and equipment not directly involved with the fault.
A trip-free operation takes place if…
A circuit breaker is closed while the protective relay is energized.
What are the three means/methods of quenching an arc?
- Increase the length of the arc path.
- Restrict the cross sectional area of the arc path.
- De-ionizing or cooking the gas.
A three phase 138KV circuit breaker can interrupt 12,000A. What is the interrupting capacity in MVA?
138,000 X 12,000 = 1656 MVA
PHEP 30 pg.21
4.8 KV circuit breaker has an interrupting rating of 30,000 A. What is its interrupting capacity when used at 5.1 KV?
Operating P = Rated P
4800V X 30,000A = 5100 V X I
I(rated) = 28,235A
PHEP 30 pg. 21
The closing time of an OCB, becomes more important when it is considered from the standpoint of…
Synchronizing generators.
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.
Not true
C. The arc is always extinguished during the first current zero.
A blowout coil would most likely be found in what type of circuit breaker?
An air magnetic circuit breaker
The high dielectric strength of gases, such as SF6 under static conditions is maintained by
Pressurizing
A fast realistic circuit breaker opening operation (arc out time) would be _______ cycles
3–4 cycles
Circuit breakers are generally classified by…
- Their continuous current rating
-Their interrupting rating
-The amount of current that they will be carrying for a period of 4-5 seconds
