NABCEP System Maintenance & Troubleshooting 3 — Questions and Answers
Question 1: A microinverter-based system's monitoring portal shows one microinverter reporting zero production for three consecutive days while all others are normal. The FIRST field action should be to:
- Replace the microinverter immediately
- Verify AC circuit continuity at the affected branch circuit and check for tripped GFCI or breaker (Correct answer)
- Reboot the entire system at the main disconnect
- Re-aim the module to improve irradiance
Correct answer: Verify AC circuit continuity at the affected branch circuit and check for tripped GFCI or breaker
Before replacing hardware, verify that AC power is present and the branch circuit is not open, which is the most common cause of a single microinverter going offline.
Question 2: Electroluminescence (EL) imaging is BEST used during maintenance to detect:
- Module soiling patterns
- Invisible cracks, inactive cell areas, and delamination not visible to the naked eye (Correct answer)
- Bypass diode temperature rise
- String voltage imbalance
Correct answer: Invisible cracks, inactive cell areas, and delamination not visible to the naked eye
EL imaging applies a forward current to modules and captures emitted near-infrared light, revealing micro-cracks and inactive cell regions invisible under normal conditions.
Question 3: An installer notices that string fuses in a combiner box have blown repeatedly after replacement. The MOST likely root cause is:
- Modules are generating above their rated Isc
- A ground fault in one string is causing backfed current from parallel strings to exceed fuse rating (Correct answer)
- The combiner box is improperly ventilated
- The string cable gauge is too large
Correct answer: A ground fault in one string is causing backfed current from parallel strings to exceed fuse rating
Repeated fuse failure usually indicates excessive backfed fault current from parallel strings flowing through the faulted string, which exceeds the fuse's continuous current rating.
Question 4: A flooded lead-acid battery bank in an off-grid system shows specific gravity readings that vary significantly between cells in the same battery. This condition is BEST corrected by:
- Adding distilled water to equalize electrolyte volume
- Performing an equalization charge at the manufacturer-specified voltage (Correct answer)
- Replacing only the cells with low specific gravity
- Discharging the battery bank fully before recharging
Correct answer: Performing an equalization charge at the manufacturer-specified voltage
An equalization charge applies a controlled overcharge that stirs the electrolyte and balances cell state-of-charge, correcting specific gravity stratification and imbalance.
Question 5: After a hailstorm, a technician visually inspects a PV array and finds no broken glass. However, output is down 15% compared to pre-storm data. The MOST appropriate next diagnostic tool is:
- A multimeter to check string voltage
- Infrared thermography or EL imaging to identify impact-induced micro-cracks (Correct answer)
- A clamp meter to measure string current under load
- A power quality analyzer at the inverter output
Correct answer: Infrared thermography or EL imaging to identify impact-induced micro-cracks
Hail can cause micro-cracks that are invisible to the naked eye but significantly reduce cell performance; IR or EL imaging reveals these sub-surface defects.
Question 6: During commissioning verification of a new grid-tied system, the measured short-circuit current (Isc) at the combiner box is noticeably lower than the calculated value. Which factor should be checked FIRST?
- Inverter firmware version
- Actual irradiance level at time of measurement compared to STC irradiance (Correct answer)
- Battery state of charge
- Utility voltage level
Correct answer: Actual irradiance level at time of measurement compared to STC irradiance
Isc is directly proportional to irradiance; measurements taken at less than 1000 W/m² will yield lower Isc values than STC-rated specifications, which is normal.
Question 7: A grid-tied string inverter's display shows an 'Isolation Fault' error. This indicates:
- The utility grid voltage is out of acceptable range
- The DC resistance between the PV array conductors and ground has dropped below the minimum threshold (Correct answer)
- The inverter's cooling fan has failed
- AC output voltage is exceeding the inverter's rated output
Correct answer: The DC resistance between the PV array conductors and ground has dropped below the minimum threshold
An isolation fault means the impedance between ungrounded DC conductors and earth ground has fallen below the inverter's minimum threshold, indicating a potential ground fault in the array.
NABCEP System Maintenance & Troubleshooting 3
A microinverter-based system's monitoring portal shows one microinverter reporting zero production for three consecutive days while all others are normal.
The FIRST field action should be to: