PHEP 30 Flashcards
Whenever a current carrying circuit is opened
Current continues to flow across contacts as they separate
What type of switch does TPDT indicate/identify?
Knife switch (3 Pole Double Throw)
What kind of knife switch can interrupt field current to generators and synchronous machines?
Field Switch
Why are disconnect switches used on systems greater than 440V
To isolate or ground electrical equipment/systems not in service
Mercury Switches are
Silent Operating
May be single or double throw
“Maintenance Free”
Mercury Switches are not generally used for <120V circuits
What is the main difference between a circuit breaker and other switches?
Circuit breakers opens automatically when current exceeds limit
Ways to extinguish Arcs
Increasing Arc Path
Restricting Arc cross section
De-ionizing/cooling gas/surrounding path
Ways not to Extinguish Arcs
Reducing reluctance across contacts
What kinds of Circuit breakers use blowout coils?
Air Magnetic Circuit Breakers
What is a trip free operation?
Precedence that takes place over any closing operation
i.e. Trip that disables any closing before a manual reset
When does a trip free action take place?
When circuit breaker closes while protective relays is still energized
Circuit breakers are classified by what?
Continuous current rating
Interrupt rating
Amount of current that can be carried for 4-5 seconds
Why should trip speed be a rating?
Less difficult to extinguish arc
Reduces damage caused by a fault
Protects equipment not involved w/ fault’s area
What energy opens contacts in circuit breakers?
Springs and gravity
Oil circuit breaker Characteristics
Dielectric strength of arc gap is improved by forcing oil into arc path
Oil insulates tank from live wires & extinguishes arcs
OCBs have a vent at top of oil tank
OCB misconceptions
AC arc is always extinguished @ first AC current zero
Why is oil circuit breaker’s opening time important?
Synchronizing generators
