LADWP DISTRIBUTION TRANSFORMERS
PRACTICE EXAM WITH QUESTIONS AND
VERIFIED ANSWERS, PLUS DETAILED
RATIONALES/EXPERT VERIFIED FOR
GUARANTEED PASS 2026/LATEST
UPDATE/INSTANT DOWNLOAD PDF
1.
A distribution transformer is rated 25 kVA, 4,160 V primary, and
240/120 V secondary. Ignoring losses, which statement best describes
the relationship between primary and secondary current?
A. Primary current is approximately 6.0 A and secondary current is
approximately 104.2 A
B. Primary current is approximately 104.2 A and secondary current is
approximately 6.0 A
C. Both primary and secondary currents are approximately 52 A
D. Primary current is approximately 12 A and secondary current is
approximately 208 A
Answer: A. Primary current is approximately 6.0 A and secondary
current is approximately 104.2 A
Rationale: For a single-phase transformer, apparent power is
approximately equal on both sides: S = VI. The primary current is
25,000/4,160 ≈ 6.01 A. At 240 V, the secondary current is 25,000/240 ≈
104.17 A. The dramatic increase in secondary current occurs because
the voltage is stepped down while approximately maintaining the same
apparent power.
2.
1
,What is the fundamental operating principle of a conventional
distribution transformer?
A. Electrostatic induction between two insulated plates
B. Electromagnetic induction between coupled windings
C. Chemical conversion of electrical energy into magnetic energy
D. Mechanical conversion of alternating current into direct current
Answer: B. Electromagnetic induction between coupled windings
Rationale: A transformer transfers AC electrical energy from one
circuit to another through electromagnetic induction. Alternating
current in the primary winding establishes a changing magnetic flux
in the core, and that changing flux induces voltage in the secondary
winding. A conventional transformer does not require a mechanical
connection between the windings.
3.
Why are distribution transformers generally unsuitable for direct
connection to a DC source?
A. DC produces excessive frequency in the transformer core
B. DC prevents the secondary winding from becoming grounded
C. A steady DC current does not continuously produce the changing
magnetic flux required for transformer induction
D. DC automatically changes the turns ratio
Answer: C. A steady DC current does not continuously produce the
changing magnetic flux required for transformer induction
Rationale: Transformer action depends on changing magnetic flux.
With a steady DC source, the primary flux initially changes during
energization but then becomes substantially steady. The resulting lack
of continuous induced secondary voltage, combined with excessive
2
,primary current and core saturation, can cause severe overheating and
damage.
4.
A transformer has a primary-to-secondary turns ratio of 10:1. If 4,800 V
is applied to the primary under ideal conditions, what secondary voltage
should be expected?
A. 48 V
B. 480 V
C. 4,800 V
D. 48,000 V
Answer: B. 480 V
Rationale: The ideal transformer voltage relationship is V₁/V₂ = N₁/N₂.
With a 10:1 ratio, the secondary voltage is 4,800/10 = 480 V. Actual
measured voltage can differ slightly because of impedance, regulation,
tap position, and loading.
5.
Which component provides the primary electrical connection to many
pole-mounted distribution transformers?
A. Primary bushing
B. Secondary neutral strap
C. Radiator drain plug
D. Core clamp
Answer: A. Primary bushing
Rationale: The primary bushing provides an insulated passage for the
high-voltage conductor into the transformer tank. It must maintain
3
, adequate insulation and mechanical integrity because failure can
result in flashover, arcing, or transformer damage. Secondary
connections use appropriately rated secondary bushings or terminals.
6.
What is the primary purpose of transformer insulating oil in an oil-filled
distribution transformer?
A. Increase the secondary frequency
B. Reduce the transformer turns ratio
C. Provide insulation and assist with heat transfer
D. Convert AC into DC
Answer: C. Provide insulation and assist with heat transfer
Rationale: Transformer oil performs two major functions: electrical
insulation between energized components and heat transfer away from
windings and the core. Degraded, contaminated, or moisture-laden oil
can reduce dielectric strength and impair transformer reliability.
7.
A transformer begins operating at a substantially higher temperature
than normal under a load that has historically been acceptable. Which
condition should be considered first?
A. Possible overload or abnormal cooling condition
B. A reduction in magnetic flux caused by low frequency
C. An increase in secondary frequency
D. An automatic increase in insulation resistance
Answer: A. Possible overload or abnormal cooling condition
4
PRACTICE EXAM WITH QUESTIONS AND
VERIFIED ANSWERS, PLUS DETAILED
RATIONALES/EXPERT VERIFIED FOR
GUARANTEED PASS 2026/LATEST
UPDATE/INSTANT DOWNLOAD PDF
1.
A distribution transformer is rated 25 kVA, 4,160 V primary, and
240/120 V secondary. Ignoring losses, which statement best describes
the relationship between primary and secondary current?
A. Primary current is approximately 6.0 A and secondary current is
approximately 104.2 A
B. Primary current is approximately 104.2 A and secondary current is
approximately 6.0 A
C. Both primary and secondary currents are approximately 52 A
D. Primary current is approximately 12 A and secondary current is
approximately 208 A
Answer: A. Primary current is approximately 6.0 A and secondary
current is approximately 104.2 A
Rationale: For a single-phase transformer, apparent power is
approximately equal on both sides: S = VI. The primary current is
25,000/4,160 ≈ 6.01 A. At 240 V, the secondary current is 25,000/240 ≈
104.17 A. The dramatic increase in secondary current occurs because
the voltage is stepped down while approximately maintaining the same
apparent power.
2.
1
,What is the fundamental operating principle of a conventional
distribution transformer?
A. Electrostatic induction between two insulated plates
B. Electromagnetic induction between coupled windings
C. Chemical conversion of electrical energy into magnetic energy
D. Mechanical conversion of alternating current into direct current
Answer: B. Electromagnetic induction between coupled windings
Rationale: A transformer transfers AC electrical energy from one
circuit to another through electromagnetic induction. Alternating
current in the primary winding establishes a changing magnetic flux
in the core, and that changing flux induces voltage in the secondary
winding. A conventional transformer does not require a mechanical
connection between the windings.
3.
Why are distribution transformers generally unsuitable for direct
connection to a DC source?
A. DC produces excessive frequency in the transformer core
B. DC prevents the secondary winding from becoming grounded
C. A steady DC current does not continuously produce the changing
magnetic flux required for transformer induction
D. DC automatically changes the turns ratio
Answer: C. A steady DC current does not continuously produce the
changing magnetic flux required for transformer induction
Rationale: Transformer action depends on changing magnetic flux.
With a steady DC source, the primary flux initially changes during
energization but then becomes substantially steady. The resulting lack
of continuous induced secondary voltage, combined with excessive
2
,primary current and core saturation, can cause severe overheating and
damage.
4.
A transformer has a primary-to-secondary turns ratio of 10:1. If 4,800 V
is applied to the primary under ideal conditions, what secondary voltage
should be expected?
A. 48 V
B. 480 V
C. 4,800 V
D. 48,000 V
Answer: B. 480 V
Rationale: The ideal transformer voltage relationship is V₁/V₂ = N₁/N₂.
With a 10:1 ratio, the secondary voltage is 4,800/10 = 480 V. Actual
measured voltage can differ slightly because of impedance, regulation,
tap position, and loading.
5.
Which component provides the primary electrical connection to many
pole-mounted distribution transformers?
A. Primary bushing
B. Secondary neutral strap
C. Radiator drain plug
D. Core clamp
Answer: A. Primary bushing
Rationale: The primary bushing provides an insulated passage for the
high-voltage conductor into the transformer tank. It must maintain
3
, adequate insulation and mechanical integrity because failure can
result in flashover, arcing, or transformer damage. Secondary
connections use appropriately rated secondary bushings or terminals.
6.
What is the primary purpose of transformer insulating oil in an oil-filled
distribution transformer?
A. Increase the secondary frequency
B. Reduce the transformer turns ratio
C. Provide insulation and assist with heat transfer
D. Convert AC into DC
Answer: C. Provide insulation and assist with heat transfer
Rationale: Transformer oil performs two major functions: electrical
insulation between energized components and heat transfer away from
windings and the core. Degraded, contaminated, or moisture-laden oil
can reduce dielectric strength and impair transformer reliability.
7.
A transformer begins operating at a substantially higher temperature
than normal under a load that has historically been acceptable. Which
condition should be considered first?
A. Possible overload or abnormal cooling condition
B. A reduction in magnetic flux caused by low frequency
C. An increase in secondary frequency
D. An automatic increase in insulation resistance
Answer: A. Possible overload or abnormal cooling condition
4
