Licensed Electrical Contractor Electrical Contractor Troubleshooting & Maintenance 2 — Questions and Answers
Question 1: A 240V circuit is reading 120V at the outlet. What is the most likely cause?
- Open neutral on the circuit (Correct answer)
- Tripped GFCI upstream
- Undersized wire gauge
- Faulty breaker handle
Correct answer: Open neutral on the circuit
An open neutral on a 240V circuit causes each leg to read only 120V to ground instead of the full 240V across both legs.
Question 2: When troubleshooting a motor that hums but does not start, which component should be checked first?
- Run capacitor
- Start capacitor (Correct answer)
- Thermal overload
- Contactor coil
Correct answer: Start capacitor
A humming but non-starting motor typically indicates a failed start capacitor that cannot provide the phase shift needed to generate starting torque.
Question 3: What does a megohmmeter reading of 0.5 MΩ on a 480V motor winding indicate?
- Normal insulation resistance
- Acceptable but marginal insulation
- Failed insulation requiring immediate attention (Correct answer)
- The megohmmeter is malfunctioning
Correct answer: Failed insulation requiring immediate attention
The minimum acceptable insulation resistance for a 480V motor is 1 MΩ plus 1 MΩ per kV, so 0.5 MΩ indicates dangerously degraded insulation.
Question 4: During thermal imaging of a panelboard, one breaker shows significantly higher temperature than adjacent breakers carrying similar loads. What is the most probable cause?
- Normal operating variation
- Loose connection at the breaker terminal (Correct answer)
- Oversized conductor for the breaker
- Higher ambient temperature on that side
Correct answer: Loose connection at the breaker terminal
A loose connection creates high-resistance contact points that generate excess heat, which thermal imaging easily detects.
Question 5: A commercial building experiences intermittent voltage sags. The utility feed tests normal. Where should the contractor investigate next?
- Main distribution panel connections (Correct answer)
- Individual branch circuit loads
- Building grounding electrode system
- Emergency generator transfer switch
Correct answer: Main distribution panel connections
Intermittent voltage sags with a normal utility feed most commonly originate from loose or corroded connections at the main distribution panel.
Question 6: What is the recommended maintenance interval for retorquing electrical connections in commercial switchgear per NFPA 70B?
- Every 6 months
- Annually (Correct answer)
- Every 3 years
- Every 5 years
Correct answer: Annually
NFPA 70B recommends annual retorquing of bolted electrical connections in commercial switchgear to prevent loose connections and thermal failures.
Licensed Electrical Contractor Electrical Contractor Troubleshooting & Maintenance 2
A 240V circuit is reading 120V at the outlet.
What is the most likely cause?