NABCEP System Maintenance & Troubleshooting 2 — Questions and Answers
Question 1: A string of PV modules shows significantly lower output than adjacent strings on the same array. The first diagnostic step should be to:
- Replace all modules in the string
- Measure open-circuit voltage (Voc) of each module in the string (Correct answer)
- Bypass the entire string at the combiner box
- Increase the tilt angle of the affected string
Correct answer: Measure open-circuit voltage (Voc) of each module in the string
Measuring Voc on each module individually isolates whether the issue is a single failed module, partial shading, or wiring fault.
Question 2: During routine maintenance, a technician finds that an inverter's ground fault protection device (GFPD) has tripped. The safest next step is to:
- Reset the GFPD and resume operation immediately
- Disconnect the array and inverter, then inspect wiring for ground faults before resetting (Correct answer)
- Bypass the GFPD to restore power generation
- Replace the inverter without further investigation
Correct answer: Disconnect the array and inverter, then inspect wiring for ground faults before resetting
A tripped GFPD indicates a real or suspected ground fault; the system must be de-energized and inspected before resetting to prevent fire or shock hazard.
Question 3: Which condition is MOST likely indicated when infrared thermography reveals a hot spot on a single cell within a PV module?
- Soiling on the module surface
- Cell crack, delamination, or bypass diode failure (Correct answer)
- Incorrect string fusing
- Reverse polarity wiring
Correct answer: Cell crack, delamination, or bypass diode failure
Localized hot spots on individual cells typically indicate physical damage such as cracks, delamination, or a failed bypass diode forcing current through a resistive cell.
Question 4: A battery-based off-grid system shows that batteries are consistently undercharged after several sunny days. The MOST likely cause is:
- Oversized inverter
- Charge controller set-point voltage too low for battery chemistry (Correct answer)
- Undersized AC disconnect
- Array azimuth facing due east
Correct answer: Charge controller set-point voltage too low for battery chemistry
If the charge controller's absorption or float voltage set point is lower than the battery manufacturer's specification, the batteries will never reach full state of charge.
Question 5: When performing IV curve tracing on a PV string, a 'stepped' curve with multiple knee points most likely indicates:
- Normal temperature-related degradation
- Partial shading or module mismatch within the string (Correct answer)
- A blown string fuse
- Correct bypass diode operation under uniform irradiance
Correct answer: Partial shading or module mismatch within the string
Multiple inflection points on an IV curve are characteristic of partial shading or module mismatch, where bypass diodes in some modules are conducting.
Question 6: A grid-tied inverter repeatedly disconnects and reconnects throughout the day. Utility voltage logs show voltage rising above 253V (120% of nominal 211V) during peak generation hours. The BEST corrective action is to:
- Increase the inverter's anti-islanding delay time
- Coordinate with the utility to raise the inverter's overvoltage trip threshold above 253V
- Work with the utility to address the voltage rise issue or reduce array output via curtailment (Correct answer)
- Replace the inverter with a higher-rated model
Correct answer: Work with the utility to address the voltage rise issue or reduce array output via curtailment
Overvoltage on the utility side is a grid-interaction issue; the solution involves utility coordination for voltage regulation or implementing Volt-Watt curtailment, not raising trip thresholds beyond safety limits.
Question 7: What is the primary purpose of performing a 'tug test' on module mounting hardware during a maintenance inspection?
- To verify module electrical output under mechanical stress
- To confirm that racking connections and fasteners are properly torqued and secure (Correct answer)
- To test the flexibility of module frames under wind load
- To check bypass diode response to mechanical pressure
Correct answer: To confirm that racking connections and fasteners are properly torqued and secure
A tug test physically verifies that clamps, bolts, and racking connections have not loosened due to thermal cycling, vibration, or corrosion.
NABCEP System Maintenance & Troubleshooting 2
A string of PV modules shows significantly lower output than adjacent strings on the same array.
The first diagnostic step should be to: