LADWP
Electric Station Operator Exam
Study Guide: Questions & Answers with Rationales
Covering:
Circuit Breakers | Disconnect Switches | Transformers and
More
LADWP Electric Station Operator Exam Prep • Page 1
,Question 1
What is the primary function of a circuit breaker in a substation?
Answer: To interrupt fault or load current and open a circuit
Rationale:
A circuit breaker is designed to make, carry, and break current under both
normal load conditions and abnormal fault conditions. Unlike a disconnect
switch, it is built to safely interrupt current, including high fault currents,
protecting equipment and personnel.
Question 2
Which medium is used in an SF6 circuit breaker to extinguish the arc?
Answer: Sulfur hexafluoride gas
Rationale:
SF6 (sulfur hexafluoride) is a heavy, electronegative gas with excellent
dielectric and arc-quenching properties. It absorbs free electrons in the arc,
rapidly increasing resistance and extinguishing the arc when contacts
separate.
Question 3
What is the main advantage of a vacuum circuit breaker over an oil circuit
breaker?
Answer: No flammable insulating medium and minimal maintenance
Rationale:
Vacuum interrupters extinguish the arc in a sealed vacuum bottle with no oil
or gas to degrade, leak, or catch fire. This gives vacuum breakers longer
contact life, less maintenance, and no risk of oil fire or contamination.
LADWP Electric Station Operator Exam Prep • Page 2
, Question 4
In an oil circuit breaker, how is the arc extinguished?
Answer: The arc vaporizes oil, creating hydrogen gas that cools and
de-ionizes the arc
Rationale:
When contacts part under oil, the arc heat decomposes the oil into hydrogen
and other gases. Hydrogen has high thermal conductivity and dielectric
strength, which cools the arc column and helps interrupt current at current
zero.
Question 5
What does 'interrupting rating' of a circuit breaker specify?
Answer: The maximum fault current the breaker can safely interrupt
Rationale:
The interrupting (or breaking) rating is the maximum rms symmetrical (or
asymmetrical) fault current the breaker is designed to clear without damage.
Applying a breaker beyond this rating can cause catastrophic failure.
Question 6
What is meant by the 'continuous current rating' of a breaker?
Answer: The maximum load current the breaker can carry continuously
without overheating
Rationale:
This rating defines the steady-state current the breaker's contacts and
conductors can carry indefinitely under rated conditions without exceeding
thermal limits, distinct from its momentary and interrupting ratings.
Question 7
Why must a circuit breaker be racked out before maintenance?
Answer: To physically disconnect it from the bus and load for safe
isolation
LADWP Electric Station Operator Exam Prep • Page 3
Electric Station Operator Exam
Study Guide: Questions & Answers with Rationales
Covering:
Circuit Breakers | Disconnect Switches | Transformers and
More
LADWP Electric Station Operator Exam Prep • Page 1
,Question 1
What is the primary function of a circuit breaker in a substation?
Answer: To interrupt fault or load current and open a circuit
Rationale:
A circuit breaker is designed to make, carry, and break current under both
normal load conditions and abnormal fault conditions. Unlike a disconnect
switch, it is built to safely interrupt current, including high fault currents,
protecting equipment and personnel.
Question 2
Which medium is used in an SF6 circuit breaker to extinguish the arc?
Answer: Sulfur hexafluoride gas
Rationale:
SF6 (sulfur hexafluoride) is a heavy, electronegative gas with excellent
dielectric and arc-quenching properties. It absorbs free electrons in the arc,
rapidly increasing resistance and extinguishing the arc when contacts
separate.
Question 3
What is the main advantage of a vacuum circuit breaker over an oil circuit
breaker?
Answer: No flammable insulating medium and minimal maintenance
Rationale:
Vacuum interrupters extinguish the arc in a sealed vacuum bottle with no oil
or gas to degrade, leak, or catch fire. This gives vacuum breakers longer
contact life, less maintenance, and no risk of oil fire or contamination.
LADWP Electric Station Operator Exam Prep • Page 2
, Question 4
In an oil circuit breaker, how is the arc extinguished?
Answer: The arc vaporizes oil, creating hydrogen gas that cools and
de-ionizes the arc
Rationale:
When contacts part under oil, the arc heat decomposes the oil into hydrogen
and other gases. Hydrogen has high thermal conductivity and dielectric
strength, which cools the arc column and helps interrupt current at current
zero.
Question 5
What does 'interrupting rating' of a circuit breaker specify?
Answer: The maximum fault current the breaker can safely interrupt
Rationale:
The interrupting (or breaking) rating is the maximum rms symmetrical (or
asymmetrical) fault current the breaker is designed to clear without damage.
Applying a breaker beyond this rating can cause catastrophic failure.
Question 6
What is meant by the 'continuous current rating' of a breaker?
Answer: The maximum load current the breaker can carry continuously
without overheating
Rationale:
This rating defines the steady-state current the breaker's contacts and
conductors can carry indefinitely under rated conditions without exceeding
thermal limits, distinct from its momentary and interrupting ratings.
Question 7
Why must a circuit breaker be racked out before maintenance?
Answer: To physically disconnect it from the bus and load for safe
isolation
LADWP Electric Station Operator Exam Prep • Page 3
