Licensed Electrical Contractor Electrical Contractor Troubleshooting & Maintenance 1

Question 1

A GFCI outlet on a 20A circuit trips immediately when reset. You measure 0Ω between the neutral and ground conductors at the outlet. What is the most likely cause?

Accepted Answer

A neutral-to-ground fault downstream of the GFCI device

Answer

A 0Ω reading between neutral and ground indicates a solid neutral-to-ground connection somewhere downstream, which the GFCI interprets as a ground fault and trips immediately upon reset. The fault must be located and cleared before the device will hold.

Question 2

A three-phase 480V motor runs normally under no-load but trips the overload relay within minutes when connected to its load. Voltage at the motor terminals measures 478V on all three phases. What is the most likely cause?

Accepted Answer

Overload relay sized too low for the actual full-load amperage of the motor

Answer

With balanced voltage and normal no-load operation, the overload relay tripping under load strongly suggests the relay is set or sized below the motor's actual FLA. An open phase would prevent the motor from starting or running smoothly, and voltage imbalance would appear in the terminal measurements.

Question 3

You are troubleshooting a 120V lighting circuit where the lights flicker intermittently. Voltage at the fixture measures between 108V and 122V during the flicker. What should you investigate first?

Accepted Answer

Check for a loose or corroded connection in the circuit, including at the panel

Answer

Intermittent voltage fluctuation between 108V and 122V on a single circuit points to a high-resistance connection that varies under load. Loose wire nuts, corroded terminals, or a loose breaker connection are the primary suspects and should be traced before assuming a utility issue.

Question 4

After replacing a single-phase 240V water heater element, the new element fails within two weeks. The voltage across the element measures 240V and resistance of the element tests good. What is the most probable cause of repeated element failure?

Accepted Answer

Sediment buildup causing the element to overheat due to low water level in the tank

Answer

When an element repeatedly fails with correct voltage and proper resistance, sediment accumulation around the lower element or a low water level exposes it to dry-fire conditions. The element overheats without water to dissipate heat, causing premature burnout.

Question 5

An insulation resistance test on a 277V fluorescent lighting circuit reads 1.2 MΩ. After running the HVAC system for 30 minutes, the reading drops to 0.18 MΩ. What does this indicate?

Accepted Answer

Moisture intrusion in the conduit or junction boxes that worsens when HVAC airflow increases

Answer

Insulation resistance that drops significantly after HVAC operation indicates moisture being driven into conduit or enclosures by air pressure or condensation. The 0.18 MΩ reading falls below the 1 MΩ minimum threshold for 277V systems, confirming moisture as the culprit.

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Licensed Electrical Contractor Electrical Contractor Troubleshooting & Maintenance 1