NABCEP System Maintenance & Troubleshooting 5 ā Questions and Answers
Question 1: A string inverter fails to start in the morning even though there is sufficient irradiance. The inverter display shows 'DC Voltage Too Low.' The MOST likely cause is:
- A shorted module in the string
- One or more modules are shaded or disconnected, reducing string Voc below the inverter's minimum start voltage (Correct answer)
- The AC disconnect is open
- The inverter has reached its thermal shutdown temperature
Correct answer: One or more modules are shaded or disconnected, reducing string Voc below the inverter's minimum start voltage
If string Voc falls below the inverter's minimum operating voltageādue to shading, a disconnected module, or wiring faultāthe inverter will not start until the voltage threshold is met.
Question 2: During maintenance of a rooftop PV system, a technician identifies roof penetrations that show signs of moisture ingress around the flashing. The MOST appropriate action is to:
- Seal the penetrations with silicone caulk and document as complete
- Refer the issue to a licensed roofing contractor and notify the system owner immediately (Correct answer)
- Tighten the lag bolt to compress the flashing and stop the leak
- Remove the affected modules to allow the roof to dry
Correct answer: Refer the issue to a licensed roofing contractor and notify the system owner immediately
Roof penetration leaks involve the building envelope and must be assessed and repaired by a licensed roofing contractor; improper DIY repairs can void roof warranties and worsen damage.
Question 3: A performance ratio (PR) calculation for a grid-tied system over a quarter comes out significantly lower than the design estimate. After confirming the monitoring data is accurate, which of the following is the LEAST likely cause?
- Inverter clipping due to array oversizing
- Higher-than-expected system soiling losses
- Irradiance data errors in the monitoring system
- The AC utility voltage being within normal operating range (Correct answer)
Correct answer: The AC utility voltage being within normal operating range
Normal utility voltage within operating range does not significantly affect performance ratio; clipping, soiling, and irradiance data errors are all direct contributors to low PR.
Question 4: A technician is troubleshooting an off-grid inverter/charger that shows the battery voltage is normal but the inverter output is 0V AC. With the AC loads disconnected and a known-good battery, the MOST likely fault is:
- Battery cables are undersized
- The inverter's internal transfer switch, output fuse, or power stage has failed (Correct answer)
- The battery SOC display is inaccurate
- The charge controller set point is too high
Correct answer: The inverter's internal transfer switch, output fuse, or power stage has failed
If battery voltage is normal and loads are disconnected but AC output is absent, the fault is internal to the inverterāmost commonly a failed output fuse, transfer relay, or power electronics stage.
Question 5: Which NEC article provides the primary requirements for photovoltaic system wiring methods, equipment, and installation?
- NEC Article 230
- NEC Article 690 (Correct answer)
- NEC Article 705
- NEC Article 480
Correct answer: NEC Article 690
NEC Article 690 specifically governs PV system installation requirements including wiring methods, overcurrent protection, grounding, and disconnecting means.
Question 6: When using a clamp meter to measure DC current in a PV string combiner, the technician must ensure that:
- Both the positive and negative conductors of the string are clamped simultaneously
- Only a single conductor (positive OR negative) of the string is inside the clamp jaw at one time (Correct answer)
- The clamp meter is set to AC current mode
- The string fuse is removed before taking the measurement
Correct answer: Only a single conductor (positive OR negative) of the string is inside the clamp jaw at one time
Clamping both conductors of a DC circuit simultaneously produces equal and opposite magnetic fields that cancel each other out, yielding a zero reading; only one conductor may be in the jaw.
Question 7: A NABCEP-certified technician reviewing long-term performance data notices that a system's annual degradation rate is 1.8% per year, significantly exceeding the manufacturer's warranted 0.5%/year. The BEST course of action is to:
- Accept the degradation as unavoidable and adjust the production model
- Document the findings with supporting data and contact the module manufacturer to initiate a warranty claim (Correct answer)
- Immediately replace all modules at the owner's expense
- Increase the inverter's MPPT voltage range to compensate
Correct answer: Document the findings with supporting data and contact the module manufacturer to initiate a warranty claim
Degradation significantly above the warranted rate is grounds for a manufacturer warranty claim; the technician's role is to document the evidence and initiate the claim process on the owner's behalf.
NABCEP System Maintenance & Troubleshooting 5
A string inverter fails to start in the morning even though there is sufficient irradiance.
The inverter display shows 'DC Voltage Too Low.' The MOST likely cause is: