NABCEP Commissioning 4 — Questions and Answers
Question 1: What is the correct sequence for de-energizing a grid-tied PV system during a commissioning fault investigation?
- Open DC disconnect, then AC disconnect, then utility disconnect
- Open AC disconnect first, then DC disconnects from array (Correct answer)
- Open utility disconnect first, then AC, then DC from array
- Open all disconnects simultaneously
Correct answer: Open AC disconnect first, then DC disconnects from array
The inverter AC disconnect is opened first to stop current flow, then DC disconnects are opened to safely de-energize the array side.
Question 2: A newly commissioned microinverter system shows that one microinverter is offline in the monitoring portal. Which commissioning step would identify this before system handover?
- Ground continuity test of all module frames
- Comparison of expected vs. actual microinverter count in the monitoring system (Correct answer)
- Insulation resistance test of AC branch circuits
- Verification of rapid shutdown voltage levels
Correct answer: Comparison of expected vs. actual microinverter count in the monitoring system
Commissioning should include verifying that every installed microinverter is communicating and visible in the monitoring platform before owner handover.
Question 3: During commissioning, a string of 10 modules measures 385 V Voc instead of the expected 410 V. Which condition would most likely explain this discrepancy without indicating a fault?
- One module has a failed bypass diode
- Ambient temperature is 35°C instead of the STC 25°C (Correct answer)
- A module is wired in parallel instead of series
- The combiner fuse for this string is undersized
Correct answer: Ambient temperature is 35°C instead of the STC 25°C
Higher ambient temperatures reduce Voc; at 35°C the array voltage will be lower than the STC (25°C) nameplate calculation.
Question 4: Which document must the installer obtain from the utility before energizing the point of common coupling (PCC) on a grid-tied system?
- Certificate of completion from the module manufacturer
- Utility permission to operate (PTO) or interconnection approval (Correct answer)
- NABCEP certification number of the lead installer
- Building department rough-inspection approval
Correct answer: Utility permission to operate (PTO) or interconnection approval
Utility permission to operate (PTO) is required before the system can export power to the grid; energizing without it violates the interconnection agreement.
Question 5: What is the primary purpose of measuring the short-circuit current (Isc) of each source circuit at the combiner during commissioning?
- To set the inverter's power factor correction
- To verify each string is producing and confirm no open-circuit failures (Correct answer)
- To calculate the annual energy yield of the system
- To size the battery bank correctly
Correct answer: To verify each string is producing and confirm no open-circuit failures
Measuring Isc at the combiner confirms each string is producing current and helps identify strings with failed modules, bad connections, or incorrect wiring.
Question 6: A commissioning report for a ground-mount system requires documenting the system grounding electrode. Which electrode type is specifically permitted for PV systems under NEC 690?
- Isolated ground electrode separate from the building system
- Any grounding electrode permitted under NEC Article 250 (Correct answer)
- Driven copper rod at least 4 feet in length
- Metal water pipe only
Correct answer: Any grounding electrode permitted under NEC Article 250
NEC 690.47 requires PV system grounding electrodes to comply with Article 250 Part III, using any grounding electrode type permitted therein.
Question 7: During owner training at system handover, which monitoring metric should the installer specifically explain to help the owner detect system underperformance early?
- Inverter cooling fan RPM
- Specific yield (kWh/kWp) compared to historical local irradiance (Correct answer)
- Battery electrolyte level indicator
- DC bus capacitor voltage
Correct answer: Specific yield (kWh/kWp) compared to historical local irradiance
Specific yield normalizes production by system size and allows the owner to compare output against expected values based on local solar resource data.
NABCEP Commissioning 4
What is the correct sequence for de-energizing a grid-tied PV system during a commissioning fault investigation?