Troubleshooting & System Testing Flashcards

A transformer secondary voltage reads 10% below nameplate value at full load but is correct at no load. The most likely cause is:

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.

When troubleshooting an intermittent nuisance trip on a 100A molded-case circuit breaker, which test method is most appropriate?

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.

A 480V/277V wye-connected distribution system shows a sustained neutral-to-ground voltage of 8V under balanced load. This indicates:

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.

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:

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.

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:

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.

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:

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.

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:

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.