NABCEP Commissioning and Troubleshooting 5 — Questions and Answers
Question 1: A battery-based PV system frequently reaches 100% SOC early in the day and then diverts excess power, but the customer reports nightly power outages. What should the technician evaluate?
- Increase inverter AC output capacity
- Verify battery capacity is sufficient for the nightly load and that SOC calibration is accurate (Correct answer)
- Add more PV modules to increase daytime generation
- Replace the charge controller with a higher-rated model
Correct answer: Verify battery capacity is sufficient for the nightly load and that SOC calibration is accurate
Early charge completion with nightly outages suggests either battery capacity is undersized for the load or the SOC sensor is inaccurately reading 100% before the bank is truly full.
Question 2: During commissioning, a technician measures 240 VAC between L1 and L2 at the inverter output but 0 VAC between L1 and neutral. What does this indicate?
- The inverter is producing split-phase power correctly
- The neutral conductor is open or disconnected (Correct answer)
- The inverter's AC output is undersized
- L2 has higher impedance than L1
Correct answer: The neutral conductor is open or disconnected
Correct 240V L1-L2 with 0V L1-N indicates the neutral conductor is interrupted, creating a floating neutral condition that can damage 120V loads.
Question 3: What is the purpose of a 'string health check' report generated by a modern monitoring platform?
- It verifies the physical mounting torque of each module
- It compares actual string energy output to expected output to identify underperforming strings (Correct answer)
- It confirms the inverter firmware is up to date
- It tests the arc-fault circuit interrupter function
Correct answer: It compares actual string energy output to expected output to identify underperforming strings
String health reports use performance ratio algorithms to compare measured vs. expected output, flagging strings that are underperforming relative to modeled predictions.
Question 4: A newly commissioned PV system passes all DC electrical tests but the inverter immediately faults on 'AC Frequency Out of Range.' What is the likely cause?
- The DC string voltage is too low for the inverter to start
- The inverter's grid frequency parameter has been configured incorrectly for the US 60 Hz grid (Correct answer)
- A module in the array has failed open
- The inverter is overheating
Correct answer: The inverter's grid frequency parameter has been configured incorrectly for the US 60 Hz grid
Inverters configured for 50 Hz markets will fault immediately when connected to the US 60 Hz grid; commissioning must verify all grid parameters match local utility specifications.
Question 5: During troubleshooting, a technician suspects a failed bypass diode in a module. Which test method can confirm this without removing the module?
- Measure the module's Voc under full sun
- Use a clamp meter on each substring under load to check for anomalously low current through bypass diode paths (Correct answer)
- Measure insulation resistance from positive lead to frame
- Use a power quality meter to assess voltage harmonics
Correct answer: Use a clamp meter on each substring under load to check for anomalously low current through bypass diode paths
A shorted bypass diode creates an alternative current path; measuring current distribution with a clamp meter under load can reveal abnormal conduction through diode circuits.
Question 6: Why is it important to verify the arc-fault circuit interrupter (AFCI) function during PV system commissioning?
- AFCI prevents module overheating in high-irradiance conditions
- NEC 690.11 requires AFCI protection on DC circuits, and its correct operation must be verified before system acceptance (Correct answer)
- AFCI protects the inverter from grid surges
- AFCI prevents overvoltage from occurring in the string
Correct answer: NEC 690.11 requires AFCI protection on DC circuits, and its correct operation must be verified before system acceptance
NEC 690.11 mandates AFCI protection on PV DC circuits operating above 80V; commissioning must confirm the device activates correctly to prevent fire from arcing faults.
Question 7: A customer reports that their grid-tied PV system export to the grid has dropped by 30% over 6 months with no obvious equipment faults. What systemic issue should the technician investigate?
- The utility may have reduced the feed-in tariff rate
- Progressive soiling, shading from vegetation growth, or module degradation affecting energy yield (Correct answer)
- The monitoring system's energy export meter has drifted
- The inverter's MPPT algorithm has changed due to a firmware update
Correct answer: Progressive soiling, shading from vegetation growth, or module degradation affecting energy yield
Gradual export reduction over months commonly results from accumulating soiling, new shading sources such as tree growth, or accelerated module degradation.
NABCEP Commissioning and Troubleshooting 5
A battery-based PV system frequently reaches 100% SOC early in the day and then diverts excess power, but the customer reports nightly power outages.
What should the technician evaluate?