NABCEP Commissioning and Troubleshooting 2 — Questions and Answers
Question 1: During PV system commissioning, you measure an open-circuit voltage significantly higher than the STC-rated Voc. What is the most likely cause?
- Shading on one or more modules
- Ambient temperature is much lower than 25°C STC (Correct answer)
- A bypass diode has failed short
- The inverter is in sleep mode
Correct answer: Ambient temperature is much lower than 25°C STC
PV module Voc increases as temperature decreases below 25°C STC, so cold conditions yield higher-than-rated Voc readings.
Question 2: A string of 10 modules shows an Isc that is 15% lower than expected while all other strings test normally. What is the first diagnostic step?
- Replace the string combiner fuse
- Inspect each module in the string for shading or soiling (Correct answer)
- Reprogram the inverter MPPT settings
- Check the AC output voltage at the main panel
Correct answer: Inspect each module in the string for shading or soiling
A proportionally low Isc in a single string commonly points to shading, soiling, or a failing module reducing current generation.
Question 3: When commissioning a battery-based off-grid system, what parameter must be verified to ensure the charge controller protects the battery bank?
- Maximum PV array open-circuit voltage vs. charge controller input rating (Correct answer)
- AC output frequency set to 60 Hz
- Nominal battery voltage matches the inverter output voltage
- Ground fault detection threshold on the inverter
Correct answer: Maximum PV array open-circuit voltage vs. charge controller input rating
The PV array Voc at minimum temperature must not exceed the charge controller's maximum input voltage rating to prevent damage.
Question 4: An inverter repeatedly faults with an 'AC voltage high' error during afternoon peak. What is the most probable cause?
- The DC input voltage is too low
- Utility grid voltage rises above the inverter's upper trip threshold (Correct answer)
- The inverter needs a firmware update
- PV modules are overheating
Correct answer: Utility grid voltage rises above the inverter's upper trip threshold
Backfeed from distributed generation during low-load periods can raise local utility voltage, triggering AC overvoltage faults.
Question 5: During commissioning, an IR thermal scan of a module shows one cell running 20°C hotter than surrounding cells. What does this indicate?
- The module has the highest irradiance exposure
- A localized cell defect such as a crack or hot spot (Correct answer)
- The bypass diode is operating correctly
- Soiling on the module surface
Correct answer: A localized cell defect such as a crack or hot spot
An isolated hot cell in an IR scan is a classic sign of a cracked or defective cell creating a resistive hot spot.
Question 6: What is the correct procedure when an electrolytic capacitor is suspected of failing inside an inverter?
- Power cycle the inverter three times to reset it
- De-energize the inverter, wait the manufacturer's specified discharge time, then inspect (Correct answer)
- Measure capacitor voltage while the inverter is operating
- Replace the inverter's DC disconnect and retest
Correct answer: De-energize the inverter, wait the manufacturer's specified discharge time, then inspect
Capacitors store dangerous charge; the inverter must be de-energized and fully discharged before any internal inspection.
Question 7: A grid-tied PV system produces less energy than modeled but all electrical measurements appear normal. What non-electrical factor should be investigated?
- String fusing size
- Actual site shading vs. design assumptions (Correct answer)
- Inverter transformer tap setting
- Ground electrode resistance
Correct answer: Actual site shading vs. design assumptions
Discrepancies between modeled and actual energy production often stem from shading sources (trees, chimneys) not captured in the original design.
NABCEP Commissioning and Troubleshooting 2
During PV system commissioning, you measure an open-circuit voltage significantly higher than the STC-rated Voc.
What is the most likely cause?