LADWP ELECTRIC DISTRIBUTION MECHANIC
TRAINEE EXAM — CLASS 3879 NEWEST
EVALUATED EXAM PRACTICE QUESTIONS 2025
-2026| ORIGINAL QUESTIONS & ANSWERS |
DETAILED RATIONALES |*HINTED* COMPLETE
EXAM PREP GRADED A+ **INSTANT
DOWNLOAD PDF**
1. A three-phase distribution transformer is supplying a balanced load. If the
primary voltage remains constant while the secondary load increases
substantially, which condition is most likely to occur?
A. Secondary terminal voltage increases
B. Secondary terminal voltage remains exactly unchanged
C. Secondary terminal voltage decreases because of transformer impedance
D. Primary frequency increases
Answer: C. Secondary terminal voltage decreases because of transformer
impedance
As load current increases, voltage drop across the transformer's internal
impedance increases, causing the secondary terminal voltage to decrease.
2. A distribution circuit breaker trips immediately after a feeder is energized.
Insulation resistance testing indicates very low resistance between one
phase conductor and ground. What is the most likely fault?
A. Open circuit
B. Ground fault
C. Balanced three-phase load
D. Overfrequency condition
,Answer: B. Ground fault
A very low insulation resistance between a phase conductor and ground
indicates an unintended conductive path to ground.
3. A mechanic must work on equipment that has been de-energized. After
opening the disconnect, what is the most important next step before
beginning work?
A. Assume the circuit is dead
B. Close the disconnect momentarily
C. Verify absence of voltage using an appropriate test instrument
D. Remove all grounding connections
Answer: C. Verify absence of voltage using an appropriate test instrument
Opening a disconnect does not by itself prove that equipment is de-energized.
Absence of voltage must be verified using an appropriate tester.
4. A transformer has a 4,800-V primary and a 480-V secondary. If the primary
contains 2,400 turns, approximately how many turns are required on the
secondary?
A. 24 turns
B. 48 turns
C. 240 turns
D. 480 turns
Answer: C. 240 turns
The turns ratio equals the voltage ratio. 480/4,800 = 0.10, so 2,400 × 0.10 = 240
turns.
5. A three-phase motor begins rotating in the opposite direction from the
required direction after maintenance. Which correction is normally
appropriate?
A. Increase the frequency
B. Reverse any two phase conductors
,C. Ground the third phase
D. Increase the motor voltage
Answer: B. Reverse any two phase conductors
Interchanging any two phases changes the phase sequence and reverses the
rotational direction of a three-phase motor.
6. A mechanic measures 120 V from line to neutral and 208 V between two
lines on a balanced three-phase system. Which system is most consistent
with these measurements?
A. 120/208-V wye system
B. 240/480-V delta system
C. 277/480-V wye system
D. 120/240-V single-phase system
Answer: A. 120/208-V wye system
In a balanced 120/208-V wye system, line-to-line voltage is approximately 1.732
times the line-to-neutral voltage.
7. A fuse repeatedly opens when a motor starts, even though the motor
operates normally after starting. Which condition should be investigated
first?
A. Excessive starting current
B. Excessive system frequency
C. Reverse polarity of the ground conductor
D. Excessively low conductor resistance
Answer: A. Excessive starting current
Motors can draw several times their normal running current during starting.
Fuse selection and motor starting conditions must account for this inrush
current.
, 8. A distribution transformer becomes noticeably hotter than normal while
supplying a load that has not changed significantly. Which condition is most
important to investigate?
A. Reduced ambient temperature
B. Poor ventilation or abnormal transformer losses
C. Lower conductor resistance
D. Increased power factor
Answer: B. Poor ventilation or abnormal transformer losses
Excessive temperature can result from inadequate cooling, abnormal losses,
internal problems, or other conditions that prevent proper heat dissipation.
9. A circuit has a 20-A load connected through a conductor with excessive
resistance. What is the most direct electrical consequence?
A. Increased voltage at the load
B. Increased voltage drop across the conductor
C. Increased system frequency
D. Elimination of current flow
Answer: B. Increased voltage drop across the conductor
Voltage drop is proportional to current and conductor resistance, so excessive
resistance produces greater voltage loss.
10.A mechanic is troubleshooting a motor that hums but does not accelerate.
The motor shaft turns freely by hand when de-energized. Which problem is
a likely cause?
A. Loss of a phase in a three-phase supply
B. Excessively high power factor
C. Correct phase sequence
D. Proper overload protection
Answer: A. Loss of a phase in a three-phase supply
TRAINEE EXAM — CLASS 3879 NEWEST
EVALUATED EXAM PRACTICE QUESTIONS 2025
-2026| ORIGINAL QUESTIONS & ANSWERS |
DETAILED RATIONALES |*HINTED* COMPLETE
EXAM PREP GRADED A+ **INSTANT
DOWNLOAD PDF**
1. A three-phase distribution transformer is supplying a balanced load. If the
primary voltage remains constant while the secondary load increases
substantially, which condition is most likely to occur?
A. Secondary terminal voltage increases
B. Secondary terminal voltage remains exactly unchanged
C. Secondary terminal voltage decreases because of transformer impedance
D. Primary frequency increases
Answer: C. Secondary terminal voltage decreases because of transformer
impedance
As load current increases, voltage drop across the transformer's internal
impedance increases, causing the secondary terminal voltage to decrease.
2. A distribution circuit breaker trips immediately after a feeder is energized.
Insulation resistance testing indicates very low resistance between one
phase conductor and ground. What is the most likely fault?
A. Open circuit
B. Ground fault
C. Balanced three-phase load
D. Overfrequency condition
,Answer: B. Ground fault
A very low insulation resistance between a phase conductor and ground
indicates an unintended conductive path to ground.
3. A mechanic must work on equipment that has been de-energized. After
opening the disconnect, what is the most important next step before
beginning work?
A. Assume the circuit is dead
B. Close the disconnect momentarily
C. Verify absence of voltage using an appropriate test instrument
D. Remove all grounding connections
Answer: C. Verify absence of voltage using an appropriate test instrument
Opening a disconnect does not by itself prove that equipment is de-energized.
Absence of voltage must be verified using an appropriate tester.
4. A transformer has a 4,800-V primary and a 480-V secondary. If the primary
contains 2,400 turns, approximately how many turns are required on the
secondary?
A. 24 turns
B. 48 turns
C. 240 turns
D. 480 turns
Answer: C. 240 turns
The turns ratio equals the voltage ratio. 480/4,800 = 0.10, so 2,400 × 0.10 = 240
turns.
5. A three-phase motor begins rotating in the opposite direction from the
required direction after maintenance. Which correction is normally
appropriate?
A. Increase the frequency
B. Reverse any two phase conductors
,C. Ground the third phase
D. Increase the motor voltage
Answer: B. Reverse any two phase conductors
Interchanging any two phases changes the phase sequence and reverses the
rotational direction of a three-phase motor.
6. A mechanic measures 120 V from line to neutral and 208 V between two
lines on a balanced three-phase system. Which system is most consistent
with these measurements?
A. 120/208-V wye system
B. 240/480-V delta system
C. 277/480-V wye system
D. 120/240-V single-phase system
Answer: A. 120/208-V wye system
In a balanced 120/208-V wye system, line-to-line voltage is approximately 1.732
times the line-to-neutral voltage.
7. A fuse repeatedly opens when a motor starts, even though the motor
operates normally after starting. Which condition should be investigated
first?
A. Excessive starting current
B. Excessive system frequency
C. Reverse polarity of the ground conductor
D. Excessively low conductor resistance
Answer: A. Excessive starting current
Motors can draw several times their normal running current during starting.
Fuse selection and motor starting conditions must account for this inrush
current.
, 8. A distribution transformer becomes noticeably hotter than normal while
supplying a load that has not changed significantly. Which condition is most
important to investigate?
A. Reduced ambient temperature
B. Poor ventilation or abnormal transformer losses
C. Lower conductor resistance
D. Increased power factor
Answer: B. Poor ventilation or abnormal transformer losses
Excessive temperature can result from inadequate cooling, abnormal losses,
internal problems, or other conditions that prevent proper heat dissipation.
9. A circuit has a 20-A load connected through a conductor with excessive
resistance. What is the most direct electrical consequence?
A. Increased voltage at the load
B. Increased voltage drop across the conductor
C. Increased system frequency
D. Elimination of current flow
Answer: B. Increased voltage drop across the conductor
Voltage drop is proportional to current and conductor resistance, so excessive
resistance produces greater voltage loss.
10.A mechanic is troubleshooting a motor that hums but does not accelerate.
The motor shaft turns freely by hand when de-energized. Which problem is
a likely cause?
A. Loss of a phase in a three-phase supply
B. Excessively high power factor
C. Correct phase sequence
D. Proper overload protection
Answer: A. Loss of a phase in a three-phase supply
