NABCEP Commissioning and Troubleshooting 3 — Questions and Answers
Question 1: During commissioning of a roof-mounted system, you discover the array is tilted 5° south instead of the designed 20°. What is the most appropriate action?
- Document and proceed — 5° difference is acceptable
- Recalculate annual energy yield impact and inform the customer before finalizing (Correct answer)
- Increase string size to compensate for lower output
- Reprogram the inverter to compensate for tilt error
Correct answer: Recalculate annual energy yield impact and inform the customer before finalizing
A significant tilt deviation affects annual yield and must be documented, quantified, and disclosed to the customer before project acceptance.
Question 2: What instrument is most appropriate for verifying a PV module's I-V curve during commissioning?
- True-RMS clamp meter
- I-V curve tracer (Correct answer)
- Digital multimeter set to AC voltage
- Megohmmeter
Correct answer: I-V curve tracer
An I-V curve tracer sweeps the module from Isc to Voc and captures the full current-voltage relationship for performance verification.
Question 3: A technician notices the ground fault indicator light is illuminated on a string inverter. What is the safest immediate action?
- Reset the inverter to clear the fault and resume operation
- Disconnect AC and DC, lock out/tag out, then locate the fault before re-energizing (Correct answer)
- Add an additional grounding electrode to reduce fault resistance
- Increase the inverter's ground fault detection threshold
Correct answer: Disconnect AC and DC, lock out/tag out, then locate the fault before re-energizing
A ground fault is a serious safety hazard requiring LOTO procedures before troubleshooting to prevent arc flash or shock.
Question 4: During insulation resistance testing of PV wiring with a megohmmeter at 500V DC, a reading of 0.5 MΩ is obtained. How should this be interpreted?
- Acceptable — any reading above zero is satisfactory
- Marginal — values typically should be ≥1 MΩ per IEC standards (Correct answer)
- Excellent — 0.5 MΩ indicates near-perfect insulation
- The test voltage is too low to produce a valid result
Correct answer: Marginal — values typically should be ≥1 MΩ per IEC standards
IEC 62446 requires insulation resistance values typically ≥1 MΩ; a 0.5 MΩ reading indicates degraded wiring insulation requiring further investigation.
Question 5: A microinverter system shows one unit continuously reporting zero output in the monitoring portal while all neighboring units operate normally. What is the most likely cause?
- The entire branch circuit has lost AC power
- The specific microinverter or its associated module has failed (Correct answer)
- The monitoring portal software has a display bug
- The branch circuit GFCI has tripped
Correct answer: The specific microinverter or its associated module has failed
A single non-reporting microinverter in an otherwise functioning system most likely indicates a failed microinverter or a defective module connected to it.
Question 6: What is the primary purpose of performing a continuity test on PV array grounding conductors during commissioning?
- To verify the ground wire carries current during normal operation
- To confirm a low-impedance path exists from array components to the grounding electrode (Correct answer)
- To measure ground rod depth below grade
- To test the inverter's ground fault protection circuit
Correct answer: To confirm a low-impedance path exists from array components to the grounding electrode
Continuity testing verifies that all metallic array components are bonded together and connected to the grounding system to provide effective fault current clearing.
Question 7: After commissioning a battery storage system, the battery management system (BMS) shows a state of charge (SOC) that differs from the inverter's SOC display. What should a technician do?
- Trust the inverter reading as it is more accurate
- Check commissioning documentation to determine which device is the SOC master and configure accordingly (Correct answer)
- Replace the BMS as it is faulty
- Disable SOC display on one of the devices to avoid confusion
Correct answer: Check commissioning documentation to determine which device is the SOC master and configure accordingly
Dual SOC readings indicate improper system integration; commissioning documentation specifies which device controls SOC and how the inverter should receive that data.
NABCEP Commissioning and Troubleshooting 3
During commissioning of a roof-mounted system, you discover the array is tilted 5° south instead of the designed 20°.
What is the most appropriate action?