NABCEP Commissioning 2 — Questions and Answers
Question 1: During commissioning of a grid-tied PV system, the installer measures an open-circuit voltage significantly higher than the STC rating. What is the most likely cause?
- Low ambient temperature increasing Voc (Correct answer)
- High irradiance causing voltage spike
- Faulty inverter input stage
- Undersized DC combiner fuses
Correct answer: Low ambient temperature increasing Voc
PV module Voc increases as temperature decreases, so cold ambient conditions can push array voltage well above STC (25°C) nameplate values.
Question 2: Which instrument is most appropriate for verifying that PV source circuits are not cross-connected at the DC combiner box during commissioning?
- Clamp-on ammeter under irradiance
- Insulation resistance tester at 1000 V
- Low-range DC voltmeter with polarity check (Correct answer)
- Digital multimeter set to AC voltage
Correct answer: Low-range DC voltmeter with polarity check
Measuring DC polarity and voltage at each source circuit input confirms correct wiring before energizing the combiner.
Question 3: A commissioning checklist item requires torquing all DC lugs. Why is under-torquing particularly dangerous in a PV system?
- It reduces array Voc
- It can cause high-resistance connections leading to arcing and fire (Correct answer)
- It voids the module warranty
- It increases inverter standby losses
Correct answer: It can cause high-resistance connections leading to arcing and fire
Loose connections create resistance, which generates heat and can produce DC arcs that are difficult to extinguish.
Question 4: When commissioning a battery-based off-grid system, what state of charge should the battery bank be at before first connecting PV source circuits?
- Fully discharged (0% SOC)
- At least 50% SOC as supplied by manufacturer
- Fully charged to 100% SOC via AC charging (Correct answer)
- SOC does not matter at initial commissioning
Correct answer: Fully charged to 100% SOC via AC charging
Manufacturers typically require batteries to be fully charged before initial PV connection to verify charge controller set points and prevent sulfation.
Question 5: During AC-side commissioning, the installer observes that the utility revenue meter is spinning backward before the interconnection agreement is in place. What should be done immediately?
- Note it as a positive sign of system production
- Disconnect the system from the grid immediately (Correct answer)
- Call the AHJ to report the production figures
- Reduce inverter output power
Correct answer: Disconnect the system from the grid immediately
Exporting power before the utility has approved interconnection violates the interconnection agreement and can create safety and billing issues.
Question 6: What does the commissioning step of verifying 'anti-islanding' confirm in a grid-tied inverter?
- The inverter disconnects from the grid during a utility outage (Correct answer)
- The array produces maximum power at all irradiance levels
- Ground fault protection activates within 1 second
- The battery backup kicks in within 20 ms
Correct answer: The inverter disconnects from the grid during a utility outage
Anti-islanding ensures the inverter shuts down when the utility grid goes down, protecting utility workers from energized lines.
Question 7: A string inverter commissioning log shows Vpv = 420 V but the MPPT window is 200–480 V. What does this indicate?
- String voltage is within the MPPT operating range — normal (Correct answer)
- The string is over-voltage and will damage the inverter
- The inverter will clip power at this voltage
- A ground fault has shifted the midpoint voltage
Correct answer: String voltage is within the MPPT operating range — normal
420 V falls within the 200–480 V MPPT window, so the inverter will operate normally and track maximum power.
NABCEP Commissioning 2
During commissioning of a grid-tied PV system, the installer measures an open-circuit voltage significantly higher than the STC rating.
What is the most likely cause?