LADWP ELECTRIC STATION OPERATOR
SWITCHGEAR & PROTECTION EXAM WITH
QUESTIONS AND VERIFIED ANSWERS,
PLUS DETAILED RATIONALES/EXPERT
VERIFIED FOR GUARANTEED PASS
2026/LATEST UPDATE/INSTANT
DOWNLOAD PDF
1. A station operator is preparing to remove a high-voltage feeder
from service for maintenance. Which device is primarily intended to
interrupt load and fault current before the feeder is isolated?
A. Grounding switch
B. Circuit breaker
C. Disconnect switch
D. Bus tie
Answer: B. Circuit breaker
Rationale: A circuit breaker is specifically designed to make or
interrupt normal load current and, within its interrupting rating,
abnormal fault current. A disconnect switch generally provides visible
isolation but is not intended to interrupt significant load or fault
current. A grounding switch is used to establish a grounded condition
after appropriate isolation, while a bus tie connects sections of a bus.
In a proper switching sequence, the breaker normally interrupts the
current first, followed by isolation with the appropriate disconnects.
2. What is the principal operational purpose of a high-voltage
disconnect switch in station switchgear?
1
,A. Automatically clear short circuits
B. Limit transformer inrush current
C. Provide visible isolation of equipment
D. Detect ground faults
Answer: C. Provide visible isolation of equipment
Rationale: A disconnect switch establishes a physical and normally
visible separation between energized and isolated portions of a power
system. This provides an important safety boundary during
maintenance and switching operations. Unlike a circuit breaker, a
conventional disconnect is generally not intended to interrupt fault
current. Protection relays detect abnormal conditions, while breakers
perform the actual high-current interruption.
3. A protective relay detects a phase-to-ground fault on a feeder.
What is the normal sequence of events?
A. Relay mechanically opens the feeder conductor and then the breaker
closes
B. Relay issues a trip command, causing the associated circuit breaker to
open
C. Disconnect switch opens first and the relay subsequently detects the
fault
D. Transformer tap changer removes the fault
Answer: B. Relay issues a trip command, causing the associated
circuit breaker to open
Rationale: Protective relays continuously monitor electrical quantities
such as current, voltage, frequency, impedance, or differential current.
When programmed criteria indicate a fault, the relay energizes the
appropriate trip circuit. The circuit breaker then opens its contacts and
interrupts the fault current. The relay itself normally does not
2
,physically interrupt the power circuit; it commands the interrupting
device.
4. A station bus has two electrically connected sections. A fault
occurs on one section, and bus differential protection operates. Why
would the protection system trip multiple breakers?
A. To increase load on the healthy bus
B. To isolate the entire station permanently
C. To disconnect all sources feeding the faulted bus section
D. To synchronize the two bus sections
Answer: C. To disconnect all sources feeding the faulted bus section
Rationale: A bus fault can be extremely severe because many sources
may contribute current simultaneously. Bus differential protection
identifies the faulted zone and trips the breakers supplying that zone.
Tripping only one source could leave the fault energized from another
source. The objective is selective but complete isolation of the faulted
bus section while preserving unaffected portions of the system
whenever the scheme permits.
5. What quantity does a conventional overcurrent relay primarily
evaluate?
A. Current magnitude relative to a pickup setting
B. Transformer oil color
C. Circuit-breaker gas pressure only
D. Bus physical temperature only
Answer: A. Current magnitude relative to a pickup setting
Rationale: Overcurrent protection operates when current exceeds a
predetermined pickup level, with the operating time determined by the
3
, particular relay characteristic. Depending on the relay, it may provide
instantaneous, definite-time, or inverse-time protection. The relay
receives a scaled representation of primary current from a current
transformer. Other quantities can be used by specialized protection,
but current magnitude is fundamental to overcurrent protection.
6. A current transformer has a 1200:5 ratio. If the primary current
is 600 A, what secondary current should ideally be available to the
relay?
A. 0.5 A
B. 1.5 A
C. 2.5 A
D. 5.0 A
Answer: C. 2.5 A
Rationale: The CT ratio is 1200 primary amperes to 5 secondary
amperes. At 600 A primary, the ideal secondary current is 600 ×
5/1200 = 2.5 A. The CT allows high primary currents to be represented
by manageable secondary currents for protective relays and metering.
Operators must understand CT ratios because incorrect ratio
assumptions can result in incorrect relay measurements and settings.
7. Why must a current-transformer secondary generally never be
intentionally opened while primary current is flowing?
A. Opening it causes the transformer primary voltage to become zero
B. It can produce dangerously high secondary voltage
C. It automatically closes every associated breaker
D. It eliminates all magnetic flux
Answer: B. It can produce dangerously high secondary voltage
4
SWITCHGEAR & PROTECTION EXAM WITH
QUESTIONS AND VERIFIED ANSWERS,
PLUS DETAILED RATIONALES/EXPERT
VERIFIED FOR GUARANTEED PASS
2026/LATEST UPDATE/INSTANT
DOWNLOAD PDF
1. A station operator is preparing to remove a high-voltage feeder
from service for maintenance. Which device is primarily intended to
interrupt load and fault current before the feeder is isolated?
A. Grounding switch
B. Circuit breaker
C. Disconnect switch
D. Bus tie
Answer: B. Circuit breaker
Rationale: A circuit breaker is specifically designed to make or
interrupt normal load current and, within its interrupting rating,
abnormal fault current. A disconnect switch generally provides visible
isolation but is not intended to interrupt significant load or fault
current. A grounding switch is used to establish a grounded condition
after appropriate isolation, while a bus tie connects sections of a bus.
In a proper switching sequence, the breaker normally interrupts the
current first, followed by isolation with the appropriate disconnects.
2. What is the principal operational purpose of a high-voltage
disconnect switch in station switchgear?
1
,A. Automatically clear short circuits
B. Limit transformer inrush current
C. Provide visible isolation of equipment
D. Detect ground faults
Answer: C. Provide visible isolation of equipment
Rationale: A disconnect switch establishes a physical and normally
visible separation between energized and isolated portions of a power
system. This provides an important safety boundary during
maintenance and switching operations. Unlike a circuit breaker, a
conventional disconnect is generally not intended to interrupt fault
current. Protection relays detect abnormal conditions, while breakers
perform the actual high-current interruption.
3. A protective relay detects a phase-to-ground fault on a feeder.
What is the normal sequence of events?
A. Relay mechanically opens the feeder conductor and then the breaker
closes
B. Relay issues a trip command, causing the associated circuit breaker to
open
C. Disconnect switch opens first and the relay subsequently detects the
fault
D. Transformer tap changer removes the fault
Answer: B. Relay issues a trip command, causing the associated
circuit breaker to open
Rationale: Protective relays continuously monitor electrical quantities
such as current, voltage, frequency, impedance, or differential current.
When programmed criteria indicate a fault, the relay energizes the
appropriate trip circuit. The circuit breaker then opens its contacts and
interrupts the fault current. The relay itself normally does not
2
,physically interrupt the power circuit; it commands the interrupting
device.
4. A station bus has two electrically connected sections. A fault
occurs on one section, and bus differential protection operates. Why
would the protection system trip multiple breakers?
A. To increase load on the healthy bus
B. To isolate the entire station permanently
C. To disconnect all sources feeding the faulted bus section
D. To synchronize the two bus sections
Answer: C. To disconnect all sources feeding the faulted bus section
Rationale: A bus fault can be extremely severe because many sources
may contribute current simultaneously. Bus differential protection
identifies the faulted zone and trips the breakers supplying that zone.
Tripping only one source could leave the fault energized from another
source. The objective is selective but complete isolation of the faulted
bus section while preserving unaffected portions of the system
whenever the scheme permits.
5. What quantity does a conventional overcurrent relay primarily
evaluate?
A. Current magnitude relative to a pickup setting
B. Transformer oil color
C. Circuit-breaker gas pressure only
D. Bus physical temperature only
Answer: A. Current magnitude relative to a pickup setting
Rationale: Overcurrent protection operates when current exceeds a
predetermined pickup level, with the operating time determined by the
3
, particular relay characteristic. Depending on the relay, it may provide
instantaneous, definite-time, or inverse-time protection. The relay
receives a scaled representation of primary current from a current
transformer. Other quantities can be used by specialized protection,
but current magnitude is fundamental to overcurrent protection.
6. A current transformer has a 1200:5 ratio. If the primary current
is 600 A, what secondary current should ideally be available to the
relay?
A. 0.5 A
B. 1.5 A
C. 2.5 A
D. 5.0 A
Answer: C. 2.5 A
Rationale: The CT ratio is 1200 primary amperes to 5 secondary
amperes. At 600 A primary, the ideal secondary current is 600 ×
5/1200 = 2.5 A. The CT allows high primary currents to be represented
by manageable secondary currents for protective relays and metering.
Operators must understand CT ratios because incorrect ratio
assumptions can result in incorrect relay measurements and settings.
7. Why must a current-transformer secondary generally never be
intentionally opened while primary current is flowing?
A. Opening it causes the transformer primary voltage to become zero
B. It can produce dangerously high secondary voltage
C. It automatically closes every associated breaker
D. It eliminates all magnetic flux
Answer: B. It can produce dangerously high secondary voltage
4
