Licensed Electrical Contractor Electrical Contractor Troubleshooting & Maintenance 1 — Questions and Answers
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?
- Overloaded circuit requiring a larger breaker
- A neutral-to-ground fault downstream of the GFCI device (Correct answer)
- Defective GFCI outlet that needs replacement
- Reversed line and load terminals on the GFCI outlet
Correct answer: A neutral-to-ground fault downstream of the GFCI device
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?
- Voltage imbalance causing excess heat in the windings
- Overload relay sized too low for the actual full-load amperage of the motor (Correct answer)
- Open phase on the supply side of the motor starter
- Failing capacitor causing low power factor
Correct answer: Overload relay sized too low for the actual full-load amperage of the motor
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?
- Replace the fixtures with higher-wattage lamps
- Check for a loose or corroded connection in the circuit, including at the panel (Correct answer)
- Install a voltage regulator at the main service entrance
- Upgrade the circuit to 20A from 15A
Correct answer: Check for a loose or corroded connection in the circuit, including at the panel
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?
- Incorrect wattage element installed for the tank rating
- Sediment buildup causing the element to overheat due to low water level in the tank (Correct answer)
- Improper wire gauge feeding the water heater circuit
- Voltage sag during high-demand periods damaging the element
Correct answer: Sediment buildup causing the element to overheat due to low water level in the tank
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?
- The test equipment is malfunctioning due to electrical interference from the HVAC
- Moisture intrusion in the conduit or junction boxes that worsens when HVAC airflow increases (Correct answer)
- Normal insulation degradation from age that requires full circuit replacement
- A wiring error causing a neutral-to-ground fault on the lighting panel
Correct answer: Moisture intrusion in the conduit or junction boxes that worsens when HVAC airflow increases
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.
Question 6: A 200A residential service panel shows visible arcing marks on the neutral bus bar at one terminal. The conductor at that terminal is a 10 AWG wire from a 20A branch circuit. What is the correct repair procedure?
- Replace only the neutral bus bar and retighten all conductors to manufacturer torque specs
- Clean the arcing marks, apply anti-oxidant compound, and retighten the terminal
- Identify and correct the loose connection, clean the bus bar, and torque the terminal to specification (Correct answer)
- Replace the entire service panel because arcing on the neutral bus indicates a failed bus bar
Correct answer: Identify and correct the loose connection, clean the bus bar, and torque the terminal to specification
Arcing on a neutral bus terminal is caused by a loose or improperly torqued connection creating a high-resistance joint. The correct fix is to identify the loose terminal, clean the contact surfaces of oxidation and arc damage, then torque to the panel manufacturer's specification. Simply replacing the bus without addressing the root cause (improper torque) will result in repeat failure.
Licensed Electrical Contractor Electrical Contractor Troubleshooting & Maintenance 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?