LADWP Electric Station Operator Exam 2026/2027
Edition | Circuit Breakers, Disconnects, Transformers
Verified Q&A Study Guide (Passed Answers)
Section 1: Circuit Breakers & Protective Interrupting
Q1: What is the primary difference between a circuit breaker and a
disconnect switch in a high-voltage substation?
A) A disconnect switch can interrupt fault current, while a circuit
breaker cannot.
B) A circuit breaker is designed to interrupt load and fault currents,
while a disconnect switch is only designed to isolate equipment when
de-energized.
C) A circuit breaker provides a visible air gap, while a disconnect switch
does not.
D) Disconnect switches operate automatically via protective relays,
while breakers are manually operated.
CORRECT ANSWER>: B) A circuit breaker is designed to interrupt load
and fault currents, while a disconnect switch is only designed to isolate
equipment when de-energized.
Rationale: Circuit breakers have arc-extinguishing mechanisms (like SF6
gas or vacuum) to safely interrupt high currents. Disconnect switches
lack arc-quenching capability and must only be operated after the
circuit breaker has opened the circuit, ensuring no load or fault current
is flowing.
,Q2: In an SF6 (Sulfur Hexafluoride) circuit breaker, what property of the
gas allows it to effectively extinguish an electrical arc?
A) It has a very low boiling point.
B) It is highly flammable, consuming the oxygen around the arc.
C) It is electronegative, meaning it absorbs free electrons, rapidly
cooling and de-ionizing the arc.
D) It creates a physical wind that blows the arc apart.
CORRECT ANSWER>: C) It is electronegative, meaning it absorbs free
electrons, rapidly cooling and de-ionizing the arc.
Rationale: SF6 is a highly effective arc-extinguishing medium because its
molecules capture free electrons in the arc plasma, turning the gas from
a conductor into an insulator almost instantly upon current zero.
Q3: What is the function of the "anti-pump" relay scheme in a circuit
breaker control circuit?
A) It prevents the breaker from automatically reclosing after being
tripped by a protective relay.
B) It ensures that if a close signal is held continuously while the breaker
is tripping, the breaker will not repeatedly open and close (pump).
C) It pumps SF6 gas back into the interrupter tank after an operation.
D) It prevents unauthorized personnel from operating the breaker
controls.
,CORRECT ANSWER>: B) It ensures that if a close signal is held
continuously while the breaker is tripping by a protective relay, the
breaker will not repeatedly open and close (pump).
Rationale: If a fault persists and a close signal is maintained, the anti-
pump circuit cuts off the close power to the breaker mechanism after
one operation, preventing catastrophic mechanical failure and restriking
of the arc.
Q4: When performing a manual close of a 230kV circuit breaker, what is
the standard sequence of operations if the associated disconnect
switches are open?
A) Close the breaker, then close the disconnect switches.
B) Close the disconnect switches, then close the breaker.
C) Close the breaker and disconnects simultaneously.
D) It does not matter as long as no load is connected.
CORRECT ANSWER>: B) Close the disconnect switches, then close the
breaker.
Rationale: Disconnect switches must never be operated with load
current flowing. You must first establish the physical circuit path by
closing the disconnects (which cannot arc because the breaker is open),
and then use the breaker to energize the circuit.
Q5: What does a "52" device number represent in electrical protection
and control schematics?
, A) Time-delay overcurrent relay
B) AC circuit breaker
C) Differential protective relay
C) Undervoltage relay
CORRECT ANSWER>: B) AC circuit breaker.
Rationale: According to IEEE Standard C37.2, device number 52
represents an AC circuit breaker. Device 51 is an AC time overcurrent
relay, and 87 is a differential protective relay.
Q6: Why are 125 VDC battery systems universally used for substation
circuit breaker control and protection power?
A) DC voltage is safer for human contact than AC voltage.
B) DC battery systems are independent of the AC auxiliary power
supply, ensuring breakers can trip during a complete AC station power
failure.
C) DC systems require smaller conductors than AC systems.
D) DC power does not produce arcing when switching contacts.
CORRECT ANSWER>: B) DC battery systems are independent of the AC
auxiliary power supply, ensuring breakers can trip during a complete AC
station power failure.
Rationale: If the station loses AC power (e.g., during a fault that drops
the station voltage), the protective relays and breaker trip coils must
still function to clear the fault. DC battery banks provide reliable,
uninterrupted control power.
Edition | Circuit Breakers, Disconnects, Transformers
Verified Q&A Study Guide (Passed Answers)
Section 1: Circuit Breakers & Protective Interrupting
Q1: What is the primary difference between a circuit breaker and a
disconnect switch in a high-voltage substation?
A) A disconnect switch can interrupt fault current, while a circuit
breaker cannot.
B) A circuit breaker is designed to interrupt load and fault currents,
while a disconnect switch is only designed to isolate equipment when
de-energized.
C) A circuit breaker provides a visible air gap, while a disconnect switch
does not.
D) Disconnect switches operate automatically via protective relays,
while breakers are manually operated.
CORRECT ANSWER>: B) A circuit breaker is designed to interrupt load
and fault currents, while a disconnect switch is only designed to isolate
equipment when de-energized.
Rationale: Circuit breakers have arc-extinguishing mechanisms (like SF6
gas or vacuum) to safely interrupt high currents. Disconnect switches
lack arc-quenching capability and must only be operated after the
circuit breaker has opened the circuit, ensuring no load or fault current
is flowing.
,Q2: In an SF6 (Sulfur Hexafluoride) circuit breaker, what property of the
gas allows it to effectively extinguish an electrical arc?
A) It has a very low boiling point.
B) It is highly flammable, consuming the oxygen around the arc.
C) It is electronegative, meaning it absorbs free electrons, rapidly
cooling and de-ionizing the arc.
D) It creates a physical wind that blows the arc apart.
CORRECT ANSWER>: C) It is electronegative, meaning it absorbs free
electrons, rapidly cooling and de-ionizing the arc.
Rationale: SF6 is a highly effective arc-extinguishing medium because its
molecules capture free electrons in the arc plasma, turning the gas from
a conductor into an insulator almost instantly upon current zero.
Q3: What is the function of the "anti-pump" relay scheme in a circuit
breaker control circuit?
A) It prevents the breaker from automatically reclosing after being
tripped by a protective relay.
B) It ensures that if a close signal is held continuously while the breaker
is tripping, the breaker will not repeatedly open and close (pump).
C) It pumps SF6 gas back into the interrupter tank after an operation.
D) It prevents unauthorized personnel from operating the breaker
controls.
,CORRECT ANSWER>: B) It ensures that if a close signal is held
continuously while the breaker is tripping by a protective relay, the
breaker will not repeatedly open and close (pump).
Rationale: If a fault persists and a close signal is maintained, the anti-
pump circuit cuts off the close power to the breaker mechanism after
one operation, preventing catastrophic mechanical failure and restriking
of the arc.
Q4: When performing a manual close of a 230kV circuit breaker, what is
the standard sequence of operations if the associated disconnect
switches are open?
A) Close the breaker, then close the disconnect switches.
B) Close the disconnect switches, then close the breaker.
C) Close the breaker and disconnects simultaneously.
D) It does not matter as long as no load is connected.
CORRECT ANSWER>: B) Close the disconnect switches, then close the
breaker.
Rationale: Disconnect switches must never be operated with load
current flowing. You must first establish the physical circuit path by
closing the disconnects (which cannot arc because the breaker is open),
and then use the breaker to energize the circuit.
Q5: What does a "52" device number represent in electrical protection
and control schematics?
, A) Time-delay overcurrent relay
B) AC circuit breaker
C) Differential protective relay
C) Undervoltage relay
CORRECT ANSWER>: B) AC circuit breaker.
Rationale: According to IEEE Standard C37.2, device number 52
represents an AC circuit breaker. Device 51 is an AC time overcurrent
relay, and 87 is a differential protective relay.
Q6: Why are 125 VDC battery systems universally used for substation
circuit breaker control and protection power?
A) DC voltage is safer for human contact than AC voltage.
B) DC battery systems are independent of the AC auxiliary power
supply, ensuring breakers can trip during a complete AC station power
failure.
C) DC systems require smaller conductors than AC systems.
D) DC power does not produce arcing when switching contacts.
CORRECT ANSWER>: B) DC battery systems are independent of the AC
auxiliary power supply, ensuring breakers can trip during a complete AC
station power failure.
Rationale: If the station loses AC power (e.g., during a fault that drops
the station voltage), the protective relays and breaker trip coils must
still function to clear the fault. DC battery banks provide reliable,
uninterrupted control power.
