CEP Installation & Maintenance Practices 3 — Questions and Answers
Question 1: What is the purpose of bonding a metal water pipe system to the electrical service per NEC 250.104?
- To reduce galvanic corrosion on the pipes
- To provide a low-impedance fault current path and equalize potentials (Correct answer)
- To meet plumbing code requirements for grounded pipes
- To protect the pipes from lightning strikes
Correct answer: To provide a low-impedance fault current path and equalize potentials
NEC 250.104 requires bonding metal water piping to limit voltage differences and provide a low-impedance path for fault current to operate overcurrent protection.
Question 2: During a megohmmeter (megger) test on a 480V motor winding, a reading of 1 MΩ is obtained. How should this be interpreted?
- Excellent insulation, no action needed
- Acceptable insulation for a new motor
- Marginal insulation requiring investigation and monitoring (Correct answer)
- Insulation failure; motor must be rewound immediately
Correct answer: Marginal insulation requiring investigation and monitoring
Per IEEE 43, a minimum acceptable insulation resistance for a 480V motor is 1 MΩ, which is marginal and warrants further investigation and trending.
Question 3: When installing a new branch circuit, a technician needs to connect to a panelboard that is energized. What is the FIRST safety step?
- Install the breaker and make connections quickly
- Wear rubber gloves and add a new breaker
- Perform an arc flash risk assessment and don appropriate PPE (Correct answer)
- Verify the voltage with a non-contact tester through the panel door
Correct answer: Perform an arc flash risk assessment and don appropriate PPE
NFPA 70E requires an arc flash risk assessment and appropriate PPE (arc flash suit, gloves, face shield) before working on or near energized electrical equipment.
Question 4: What is the correct installation practice for a cable tray containing both power and signal cables?
- Run all cables together for convenience
- Use a solid divider or separate trays to segregate power and signal cables (Correct answer)
- Power cables must always be on top of signal cables
- Signal cables must be in conduit inside the cable tray
Correct answer: Use a solid divider or separate trays to segregate power and signal cables
NEC 392.46 and good engineering practice require physical separation or solid barriers between power conductors and signal/communication cables to prevent inductive interference.
Question 5: A 3-phase, 208V panel serves a 10 kVA balanced load. What is the approximate full-load current per phase?
- 27.8 A (Correct answer)
- 48.1 A
- 16.0 A
- 33.3 A
Correct answer: 27.8 A
I = kVA / (√3 × V) = 10,000 / (1.732 × 208) = 10,000 / 360.3 ≈ 27.8 A per phase.
Question 6: Under NEC 230.70, where must the main service disconnecting means be located?
- At the utility meter base
- At the nearest readily accessible location to the service entrance conductors (Correct answer)
- In the main electrical room only
- Outside the building at the point of attachment
Correct answer: At the nearest readily accessible location to the service entrance conductors
NEC 230.70(A)(1) requires the service disconnect to be installed at a readily accessible location nearest to the point where the service conductors enter the building.
Question 7: When performing infrared (IR) thermography on electrical panels, what condition indicates a potential problem requiring immediate attention?
- A breaker running 2°C above ambient temperature
- A connection point showing 30°C above adjacent similar components (Correct answer)
- The entire panel surface is 5°C above ambient
- A neutral bar at the same temperature as the phase bus
Correct answer: A connection point showing 30°C above adjacent similar components
NETA and NFPA 70B guidelines classify a temperature difference of ≥30°C above a reference point on similar components as a serious deficiency requiring immediate corrective action.
CEP Installation & Maintenance Practices 3
What is the purpose of bonding a metal water pipe system to the electrical service per NEC 250.104?