LADWP ELECTRIC DISTRIBUTION
MECHANIC EXAM CLASS 3879 WITH
QUESTIONS AND VERIFIED ANSWERS,
PLUS DETAILED RATIONALES/EXPERT
VERIFIED FOR GUARANTEED PASS
2026/LATEST UPDATE/INSTANT
DOWNLOAD PDF
1. A distribution mechanic is troubleshooting a 240-V single-phase
circuit. The measured resistance of the load is 12 Ω. Assuming the
circuit is energized at exactly 240 V and the load behaves as a purely
resistive load, what current should the mechanic expect?
A. 10 A
B. 20 A
C. 28.8 A
D. 2 A
Answer: B. 20 A
Rationale: Ohm's law states that I = V/R. Substituting 240 V and 12 Ω
gives I = 240/12 = 20 A. A mechanic should be able to perform this
calculation quickly because current magnitude is fundamental when
evaluating conductor loading, protective-device operation, voltage
drop, and abnormal circuit conditions.
2. During an inspection of an energized distribution cabinet, a mechanic
notices that a bolted connection is warmer than comparable connections
carrying approximately the same current. Which condition is the most
likely cause?
A. Excessively low system frequency
B. A high-resistance connection
1
,C. An oversized conductor
D. Excessive insulation thickness
Answer: B. A high-resistance connection
Rationale: A loose, corroded, contaminated, or improperly torqued
connection can develop increased contact resistance. Even when
current remains normal, I²R heating can cause a localized
temperature rise. If left uncorrected, the connection can deteriorate
further, potentially causing arcing, equipment damage, or failure.
3. A three-phase distribution load is balanced and connected to a 480-V
system. If one phase is accidentally lost while the motor remains
connected, what is the principal concern?
A. The motor will automatically become more efficient
B. The motor will draw excessive current in the remaining phases
C. The motor will immediately reverse rotation
D. The system voltage will necessarily become zero
Answer: B. The motor will draw excessive current in the remaining
phases
Rationale: Single-phasing can cause severe current imbalance and
overheating in a three-phase motor. The remaining phases may carry
excessive current while the motor develops reduced torque. Continued
operation can rapidly damage windings, so loss-of-phase protection
and proper troubleshooting are important.
4. A transformer has a primary voltage of 2,400 V and a secondary
voltage of 240 V. What is the approximate turns ratio, expressed as
primary-to-secondary?
2
,A. 1:10
B. 10:1
C. 5:1
D. 100:1
Answer: B. 10:1
Rationale: For an ideal transformer, the voltage ratio approximately
equals the turns ratio. Therefore, Np/Ns = Vp/Vs = 2,400/240 = 10.
The transformer therefore has approximately ten times as many
primary turns as secondary turns.
5. Why is a transformer generally unsuitable for directly transforming
steady-state DC voltage through ordinary electromagnetic induction?
A. DC has too much resistance
B. DC cannot flow through copper
C. A transformer requires changing magnetic flux to induce secondary
voltage
D. Transformers only operate below 24 V
Answer: C. A transformer requires changing magnetic flux to
induce secondary voltage
Rationale: Transformer action depends on electromagnetic induction.
Alternating current continually changes the magnetic flux in the core,
inducing voltage in the secondary winding. A steady DC source does
not provide continuous changing flux, so a conventional transformer
cannot continuously transfer DC voltage in the normal manner.
6. A mechanic measures 120 V from an energized conductor to ground
and 0 V from another conductor to ground. Which conclusion is most
appropriate?
3
, A. The second conductor must be energized at 120 V
B. The measurements alone are insufficient to establish the condition of
the circuit
C. The first conductor is necessarily grounded
D. The circuit is automatically safe to touch
Answer: B. The measurements alone are insufficient to establish the
condition of the circuit
Rationale: Voltage measurements must be interpreted in the context of
the circuit configuration, reference point, instrument condition, and
switching arrangement. A zero reading does not automatically
establish that a conductor is safe or de-energized. Proper isolation,
verification, and approved procedures are required before treating
conductors as safe.
7. Which instrument is most appropriate for measuring current in a
conductor without disconnecting the conductor from the circuit?
A. Clamp meter
B. Megohmmeter
C. Phase-sequence indicator
D. Insulation scraper
Answer: A. Clamp meter
Rationale: A clamp meter measures current by sensing the magnetic
field surrounding a conductor. It allows current measurements
without opening the circuit. This is particularly useful in distribution
work where disconnecting an operating circuit solely to insert a
conventional ammeter would be impractical or hazardous.
4
MECHANIC EXAM CLASS 3879 WITH
QUESTIONS AND VERIFIED ANSWERS,
PLUS DETAILED RATIONALES/EXPERT
VERIFIED FOR GUARANTEED PASS
2026/LATEST UPDATE/INSTANT
DOWNLOAD PDF
1. A distribution mechanic is troubleshooting a 240-V single-phase
circuit. The measured resistance of the load is 12 Ω. Assuming the
circuit is energized at exactly 240 V and the load behaves as a purely
resistive load, what current should the mechanic expect?
A. 10 A
B. 20 A
C. 28.8 A
D. 2 A
Answer: B. 20 A
Rationale: Ohm's law states that I = V/R. Substituting 240 V and 12 Ω
gives I = 240/12 = 20 A. A mechanic should be able to perform this
calculation quickly because current magnitude is fundamental when
evaluating conductor loading, protective-device operation, voltage
drop, and abnormal circuit conditions.
2. During an inspection of an energized distribution cabinet, a mechanic
notices that a bolted connection is warmer than comparable connections
carrying approximately the same current. Which condition is the most
likely cause?
A. Excessively low system frequency
B. A high-resistance connection
1
,C. An oversized conductor
D. Excessive insulation thickness
Answer: B. A high-resistance connection
Rationale: A loose, corroded, contaminated, or improperly torqued
connection can develop increased contact resistance. Even when
current remains normal, I²R heating can cause a localized
temperature rise. If left uncorrected, the connection can deteriorate
further, potentially causing arcing, equipment damage, or failure.
3. A three-phase distribution load is balanced and connected to a 480-V
system. If one phase is accidentally lost while the motor remains
connected, what is the principal concern?
A. The motor will automatically become more efficient
B. The motor will draw excessive current in the remaining phases
C. The motor will immediately reverse rotation
D. The system voltage will necessarily become zero
Answer: B. The motor will draw excessive current in the remaining
phases
Rationale: Single-phasing can cause severe current imbalance and
overheating in a three-phase motor. The remaining phases may carry
excessive current while the motor develops reduced torque. Continued
operation can rapidly damage windings, so loss-of-phase protection
and proper troubleshooting are important.
4. A transformer has a primary voltage of 2,400 V and a secondary
voltage of 240 V. What is the approximate turns ratio, expressed as
primary-to-secondary?
2
,A. 1:10
B. 10:1
C. 5:1
D. 100:1
Answer: B. 10:1
Rationale: For an ideal transformer, the voltage ratio approximately
equals the turns ratio. Therefore, Np/Ns = Vp/Vs = 2,400/240 = 10.
The transformer therefore has approximately ten times as many
primary turns as secondary turns.
5. Why is a transformer generally unsuitable for directly transforming
steady-state DC voltage through ordinary electromagnetic induction?
A. DC has too much resistance
B. DC cannot flow through copper
C. A transformer requires changing magnetic flux to induce secondary
voltage
D. Transformers only operate below 24 V
Answer: C. A transformer requires changing magnetic flux to
induce secondary voltage
Rationale: Transformer action depends on electromagnetic induction.
Alternating current continually changes the magnetic flux in the core,
inducing voltage in the secondary winding. A steady DC source does
not provide continuous changing flux, so a conventional transformer
cannot continuously transfer DC voltage in the normal manner.
6. A mechanic measures 120 V from an energized conductor to ground
and 0 V from another conductor to ground. Which conclusion is most
appropriate?
3
, A. The second conductor must be energized at 120 V
B. The measurements alone are insufficient to establish the condition of
the circuit
C. The first conductor is necessarily grounded
D. The circuit is automatically safe to touch
Answer: B. The measurements alone are insufficient to establish the
condition of the circuit
Rationale: Voltage measurements must be interpreted in the context of
the circuit configuration, reference point, instrument condition, and
switching arrangement. A zero reading does not automatically
establish that a conductor is safe or de-energized. Proper isolation,
verification, and approved procedures are required before treating
conductors as safe.
7. Which instrument is most appropriate for measuring current in a
conductor without disconnecting the conductor from the circuit?
A. Clamp meter
B. Megohmmeter
C. Phase-sequence indicator
D. Insulation scraper
Answer: A. Clamp meter
Rationale: A clamp meter measures current by sensing the magnetic
field surrounding a conductor. It allows current measurements
without opening the circuit. This is particularly useful in distribution
work where disconnecting an operating circuit solely to insert a
conventional ammeter would be impractical or hazardous.
4
