CEP Troubleshooting & System Testing 3 — Questions and Answers
Question 1: A transformer secondary voltage reads 10% below nameplate value at full load but is correct at no load. The most likely cause is:
- Incorrect primary tap selection
- Excessive transformer winding impedance causing voltage drop under load (Correct answer)
- An open delta connection on the primary
- Improper grounding of the secondary neutral
Correct answer: Excessive transformer winding impedance causing voltage drop under load
Voltage regulation — the difference between no-load and full-load voltage — is directly caused by the transformer's internal impedance interacting with load current.
Question 2: When troubleshooting an intermittent nuisance trip on a 100A molded-case circuit breaker, which test method is most appropriate?
- Primary injection test to verify trip curve
- Secondary injection test using relay test set
- Hipot test at 2500V DC
- Infrared thermography scan under normal load conditions (Correct answer)
Correct answer: Infrared thermography scan under normal load conditions
Infrared thermography can identify loose connections or high-resistance joints that cause localized heating, which is a common cause of intermittent MCCB trips.
Question 3: A 480V/277V wye-connected distribution system shows a sustained neutral-to-ground voltage of 8V under balanced load. This indicates:
- Normal operation within acceptable limits
- A high-resistance connection in the neutral conductor (Correct answer)
- Ground fault on one phase
- Excessive capacitive reactance on the feeder
Correct answer: A high-resistance connection in the neutral conductor
A neutral-to-ground voltage above ~2–3V under balanced load indicates a high-resistance neutral connection creating a voltage drop along the neutral conductor.
Question 4: During commissioning of a large UPS system, the static bypass switch fails to transfer load from UPS output to bypass. The FIRST check should be:
- Replace the static switch thyristors
- Verify the bypass source voltage and phase angle are synchronized with UPS output (Correct answer)
- Test the battery capacity under full load
- Measure the UPS output frequency only
Correct answer: Verify the bypass source voltage and phase angle are synchronized with UPS output
The static bypass switch requires the bypass source and UPS output to be phase-synchronized before a bumpless transfer can occur.
Question 5: A resistance temperature detector (RTD) on a motor bearing reads 4000 Ω at room temperature when it should read approximately 109.8 Ω. This suggests:
- The motor bearing is dangerously overheated
- An open circuit in the RTD or its lead wires (Correct answer)
- A short circuit across the RTD element
- Normal variation within PT100 tolerance
Correct answer: An open circuit in the RTD or its lead wires
A PT100 RTD should read near 100 Ω at 0°C and rises only slightly with temperature; a reading of 4000 Ω indicates an open circuit in the element or lead wiring.
Question 6: A newly installed 600A service entrance shows correct voltage at the meter but only 340V between phases A and B at the load center. Phases A–C and B–C read 480V. The problem is most likely:
- A blown fuse or open connection on phase B at the service (Correct answer)
- Reversed phase rotation
- Ground fault on phase A
- Incorrect service transformer tap
Correct answer: A blown fuse or open connection on phase B at the service
A low voltage reading between two specific phases while other phase combinations read normal is a classic symptom of an open or high-resistance connection on one of those two phases.
Question 7: After replacing a contactor coil, a motor starter hums loudly and draws excessive coil current but the contacts close fully. The cause is most likely:
- A shading ring is broken or missing on the magnetic core (Correct answer)
- Incorrect wire gauge on the coil circuit
- The overload relay is set too high
- Excessive spring tension on the contacts
Correct answer: A shading ring is broken or missing on the magnetic core
Shading rings on AC contactor cores reduce magnetic flux pulsation at 60 Hz; a missing or broken shading ring causes the core to chatter, producing a loud hum and high coil current.
CEP Troubleshooting & System Testing 3
A transformer secondary voltage reads 10% below nameplate value at full load but is correct at no load.
The most likely cause is: