Licensed Electrical Contractor Troubleshooting & Maintenance 3 — Questions and Answers
Question 1: A lighting circuit dims noticeably when a large appliance starts. What is the primary cause?
- Shared neutral overloaded
- Excessive voltage drop due to undersized wiring or loose connections (Correct answer)
- Phase imbalance at the panel
- AFCI breaker restricting current
Correct answer: Excessive voltage drop due to undersized wiring or loose connections
Voltage drop caused by undersized conductors or high-resistance connections causes lights to dim when high-inrush loads start.
Question 2: When troubleshooting a tripped thermal-magnetic breaker, you find the breaker is warm but the circuit is lightly loaded. What should you suspect?
- The breaker is correctly sized for the load
- A loose connection at the breaker lug causing resistance heating (Correct answer)
- The ambient temperature in the panel is too high
- The circuit neutral is open
Correct answer: A loose connection at the breaker lug causing resistance heating
A warm breaker under light load suggests a loose lug connection creating resistance, which generates heat even at low current levels.
Question 3: A 120V outlet measures 105V under load and 118V at no load. This is best described as:
- Normal voltage regulation
- Excessive voltage drop indicating a wiring problem (Correct answer)
- Utility supply issue requiring a call to the power company
- Neutral current feedback from adjacent circuits
Correct answer: Excessive voltage drop indicating a wiring problem
A 13V drop under load far exceeds the NEC 5% recommendation (6V) and indicates a high-resistance fault in the wiring or connections.
Question 4: During PM on a motor control center (MCC), you find carbon tracking on the bus insulation. The correct action is to:
- Apply insulating varnish over the tracking
- De-energize, clean with appropriate solvent, and inspect for the arc source (Correct answer)
- Continue operation and schedule repair at next maintenance window
- Apply electrical tape to isolate the affected area
Correct answer: De-energize, clean with appropriate solvent, and inspect for the arc source
Carbon tracking provides a conductive path for future arcing; the bus must be de-energized, cleaned, and the root cause corrected.
Question 5: A circuit breaker fails to trip during an overload test. The most appropriate action is to:
- Adjust the trip setting upward to compensate
- Replace the breaker immediately — a breaker that won't trip is a fire hazard (Correct answer)
- Verify the test equipment is calibrated and retest
- Reset and return to service; test errors are common
Correct answer: Replace the breaker immediately — a breaker that won't trip is a fire hazard
A breaker that fails to trip during a confirmed overload test is defective and must be replaced as it can no longer protect the circuit.
Question 6: What does a 'hot-cold' pattern on a thermal imaging scan of a switchboard indicate?
- The switchboard is operating normally with load variation
- A potential loose connection or overloaded conductor at the hot spot (Correct answer)
- A calibration error in the infrared camera
- A voltage unbalance from the utility supply
Correct answer: A potential loose connection or overloaded conductor at the hot spot
A localized hot spot on thermal imaging relative to surrounding areas indicates increased resistance from a loose connection or overloaded conductor.
Question 7: A 3-phase panel shows a neutral conductor that is significantly hotter than the phase conductors. What is the most likely cause?
- Oversized neutral conductor increasing reactance
- High harmonic content from non-linear loads overloading the neutral (Correct answer)
- Phase-to-neutral fault on one circuit
- Incorrect phasing of 240V circuits
Correct answer: High harmonic content from non-linear loads overloading the neutral
Non-linear loads (computers, VFDs, LED drivers) generate third-harmonic currents that add on the neutral rather than cancel, causing overheating.
Licensed Electrical Contractor Troubleshooting & Maintenance 3
A lighting circuit dims noticeably when a large appliance starts.
What is the primary cause?