Licensed Electrical Contractor Troubleshooting & Maintenance 2 — Questions and Answers
Question 1: A motor repeatedly trips its overload relay shortly after reset. What is the most likely cause?
- Overload relay set too low
- Motor running in a high-resistance fault condition (Correct answer)
- Incorrect phase rotation
- Loose terminal at the motor disconnect
Correct answer: Motor running in a high-resistance fault condition
A motor tripping overloads repeatedly after reset typically indicates a high-resistance fault (such as a failing winding or bearing friction) drawing excess current.
Question 2: When using a clamp-on ammeter to troubleshoot a single-phase 240V circuit, you clamp around both conductors and read zero. What does this indicate?
- An open circuit on the load side
- Normal balanced current flow (Correct answer)
- A ground fault on the circuit
- A high-resistance connection
Correct answer: Normal balanced current flow
Clamping both conductors of a single-phase circuit cancels opposing magnetic fields, resulting in a zero reading, which indicates normal balanced current.
Question 3: A GFCI outlet tests correctly but will not reset. The most likely cause is:
- A downstream ground fault is still present (Correct answer)
- The GFCI module is wired line-to-load reversed
- The circuit breaker is undersized
- Neutral-to-ground bond is missing at the panel
Correct answer: A downstream ground fault is still present
A GFCI that trips immediately upon reset indicates a ground fault condition still exists on the circuit or load side.
Question 4: During a megohmmeter test on a cable, the insulation resistance reads near zero. The best next step is to:
- Replace the entire cable run immediately
- Disconnect all equipment and retest the cable alone (Correct answer)
- Increase test voltage to obtain a more accurate reading
- Apply dielectric grease to the conductors
Correct answer: Disconnect all equipment and retest the cable alone
Disconnecting all equipment isolates whether the low reading is from the cable itself or connected loads before condemning the cable.
Question 5: A 3-phase motor is running but drawing unequal currents on all three phases. Single-phasing has been ruled out. What is the most likely cause?
- Voltage unbalance on the supply (Correct answer)
- Incorrect overload heater size
- Open delta transformer bank
- Motor winding shorted turn
Correct answer: Voltage unbalance on the supply
Even a small supply voltage unbalance causes disproportionately larger current unbalance in a 3-phase motor.
Question 6: An arc-fault circuit interrupter (AFCI) breaker is tripping on a bedroom circuit. Which tool helps identify the cause?
- Megohmeter at 1000V
- Receptacle tester with arc-fault indicator (Correct answer)
- Clamp meter measuring neutral current
- Voltage drop tester across each outlet
Correct answer: Receptacle tester with arc-fault indicator
A receptacle tester with an arc-fault indicator can simulate arcing conditions to help locate the fault on the circuit.
Question 7: What is the purpose of performing a 'wiggle test' on electrical connections during maintenance?
- To verify torque on all lugs meets spec
- To identify loose or intermittent connections that cause heating (Correct answer)
- To test insulation flexibility in cold environments
- To check for conductor corrosion under insulation
Correct answer: To identify loose or intermittent connections that cause heating
Physically wiggling connections while monitoring for voltage changes or sparks identifies loose or intermittent connections before they fail.
Licensed Electrical Contractor Troubleshooting & Maintenance 2
A motor repeatedly trips its overload relay shortly after reset.
What is the most likely cause?