NABCEP Electrical Component Inspection 3 — Questions and Answers
Question 1: During inspection of a battery-based PV system, you find that the battery bank negative terminal is directly bonded to the equipment grounding conductor at the charge controller. According to NEC Article 690, what type of system is this?
- An ungrounded DC system
- A solidly grounded DC system (Correct answer)
- A resistance-grounded DC system
- A functionally grounded system
Correct answer: A solidly grounded DC system
A solidly grounded DC system has one conductor of the circuit directly connected to the grounding electrode system without any impedance.
Question 2: An inspector observes that a string of 10 modules in series produces 380V Voc at STC but only 310V during morning inspection. What is the most likely cause?
- One module in the string has a failed bypass diode
- The system has a partial shading condition or failed module (Correct answer)
- The inverter is limiting string voltage
- The temperature coefficient is causing voltage increase
Correct answer: The system has a partial shading condition or failed module
A significantly lower-than-expected string Voc during clear conditions typically indicates a failed module or partial shading that reduces voltage contribution from part of the string.
Question 3: During a thermal imaging inspection of PV modules, you identify a cell-level hot spot that is 40°C above surrounding cells. What is the most probable root cause?
- The module is operating at maximum power point
- A cracked or degraded cell with increased series resistance (Correct answer)
- The module is oversized for the inverter MPPT
- Ambient temperature is too high for proper operation
Correct answer: A cracked or degraded cell with increased series resistance
Cell-level hot spots indicate locally increased resistance, most commonly from cracked cells, delamination, or soiling that causes current to flow through a high-resistance path.
Question 4: An inspector finds that the service entrance panel where the PV system interconnects has a main breaker rated at 200A and the bus bar is also rated at 200A. The PV interconnection breaker is 40A. Does this installation comply with NEC 705.12(B)(2)?
- Yes, if the sum of all breaker ratings does not exceed 120% of the bus rating
- No, the PV breaker must be located at the opposite end from the main breaker (Correct answer)
- Yes, but only if the main breaker is a backfed breaker
- No, the bus rating must be increased to 240A
Correct answer: No, the PV breaker must be located at the opposite end from the main breaker
NEC 705.12(B)(2) requires the PV supply-side breaker to be at the opposite end of the bus from the main breaker when using the 120% rule, to limit worst-case bus loading.
Question 5: During inspection, you measure insulation resistance of a PV array using a megohmmeter and find a reading of 0.5 MΩ between the positive conductor and ground. What does this indicate?
- Normal resistance for a large PV array
- Significant insulation degradation requiring further investigation (Correct answer)
- The array grounding is functioning correctly
- The array has too many parallel strings
Correct answer: Significant insulation degradation requiring further investigation
A megohmmeter reading of 0.5 MΩ is far below the minimum acceptable insulation resistance (typically ≥1 MΩ for PV systems), indicating compromised insulation that must be investigated.
Question 6: A PV system inspection reveals that USE-2 conductors are used for the module inter-string wiring inside the junction box. Is this installation correct?
- Yes, USE-2 is approved for all PV wiring applications
- No, PV wire or THWN-2 must be used inside junction boxes (Correct answer)
- Yes, as long as the conduit fill calculations allow it
- No, only aluminum conductors are permitted in junction boxes
Correct answer: No, PV wire or THWN-2 must be used inside junction boxes
USE-2 cable is approved for outdoor exposed wiring between modules but is not listed for use inside enclosures or raceways; PV wire or THWN-2 is required inside junction boxes.
Question 7: During commissioning, an inspector verifies that a grid-tied inverter's anti-islanding protection is functional by simulating a utility outage. The inverter fails to disconnect within 2 seconds. What is the safety significance?
- The inverter efficiency is reduced during grid interruptions
- The energized conductors could pose an electrocution hazard to utility workers (Correct answer)
- The battery backup system will not activate properly
- The inverter will sustain damage from the outage condition
Correct answer: The energized conductors could pose an electrocution hazard to utility workers
Failed anti-islanding protection means the inverter continues to energize utility conductors after grid disconnect, creating an electrocution hazard for utility lineworkers who assume conductors are de-energized.
NABCEP Electrical Component Inspection 3
During inspection of a battery-based PV system, you find that the battery bank negative terminal is directly bonded to the equipment grounding conductor at the charge controller.
According to NEC Article 690, what type of system is this?