NABCEP System Maintenance 5 — Questions and Answers
Question 1: A battery-based system's charge controller is in float mode but the battery voltage is consistently 0.3V below the float setpoint. What is the most likely cause?
- The PV array is undersized for the load and cannot maintain float
- The float voltage setpoint is incorrectly programmed too low in the charge controller
- The battery bank has a shorted cell causing a constant voltage depression
- The battery temperature compensation sensor is reading incorrectly high temperature (Correct answer)
Correct answer: The battery temperature compensation sensor is reading incorrectly high temperature
Temperature-compensated charging reduces voltage setpoints as temperature rises; a sensor reading falsely high temperature causes the controller to reduce its float voltage target, making actual measured voltage appear low.
Question 2: What is the purpose of performing a 'string voltage' measurement at the combiner box on each individual string separately (with other strings disconnected)?
- To measure the combined array Vmp for MPPT verification
- To identify a shorted string fuse or reverse-polarity connection that would be masked by parallel string measurements (Correct answer)
- To calibrate the inverter's DC input voltage monitoring
- To verify that all strings have identical module counts
Correct answer: To identify a shorted string fuse or reverse-polarity connection that would be masked by parallel string measurements
Testing each string individually in open-circuit while other strings are disconnected reveals shorted fuses (which would show lower-than-expected voltage) and reverse-polarity wiring that would be masked by parallel combination.
Question 3: When replacing a failed string inverter, which of the following must be verified before energizing the new unit?
- That the new inverter is the same brand as the original to maintain warranty
- That the DC input voltage and current ratings of the new inverter are compatible with the PV array's Voc, Vmp, and Isc values (Correct answer)
- That the new inverter has a higher power rating than the original to allow for future expansion
- That the new inverter is connected to a dedicated utility meter for production tracking
Correct answer: That the DC input voltage and current ratings of the new inverter are compatible with the PV array's Voc, Vmp, and Isc values
The replacement inverter must be verified compatible with the existing array's electrical characteristics—particularly that array Voc does not exceed the inverter's maximum DC input voltage and that Isc is within the input current rating.
Question 4: A monitoring system shows that a rooftop PV system's daily energy production peaks at a lower value every few weeks in a repeating pattern. What is the most likely explanation?
- Periodic inverter firmware updates throttling performance temporarily
- Cyclic soiling and cleaning pattern, or seasonal tree/vegetation shading growing then being trimmed (Correct answer)
- The utility grid undergoes planned voltage reductions on those days
- The inverter's MPPT algorithm re-calibrates on a fixed schedule
Correct answer: Cyclic soiling and cleaning pattern, or seasonal tree/vegetation shading growing then being trimmed
A repeating cyclic pattern in production dips most commonly reflects accumulating soiling (pollen, dust, bird droppings) between cleaning events, or seasonal shading from vegetation that follows a growth and trimming cycle.
Question 5: What information must be documented and retained after completing any corrective maintenance on a PV system?
- Only the cost of parts replaced, for accounting purposes
- A record of the fault found, work performed, components replaced, test results, and technician credentials (Correct answer)
- Only the serial number of the replaced component for warranty tracking
- The system's total lifetime energy production up to the date of service
Correct answer: A record of the fault found, work performed, components replaced, test results, and technician credentials
Comprehensive maintenance records documenting the fault, corrective actions, replaced components, post-repair test results, and technician information are required for warranty support, regulatory compliance, and future troubleshooting.
Question 6: During routine maintenance, a technician discovers that the equipment grounding conductors (EGCs) connecting module frames to the racking system have visible corrosion at several bonding points. What is the correct action?
- Apply corrosion-inhibiting grease over the existing connections and leave them in place
- Clean or replace the corroded bonding connections to ensure low-resistance grounding continuity, then verify with a ground continuity test (Correct answer)
- Replace only the EGC wires and leave the racking bonding hardware in place
- Increase the system's ground fault protection sensitivity to compensate for the higher resistance
Correct answer: Clean or replace the corroded bonding connections to ensure low-resistance grounding continuity, then verify with a ground continuity test
Corroded bonding connections increase grounding resistance, compromising personnel safety in fault conditions; the connections must be cleaned or replaced and then tested to verify continuity meets code requirements.
Question 7: A grid-tied PV system passes all electrical tests during a maintenance inspection, but the system owner reports that the monthly utility bills have not decreased as expected since installation. What should the technician investigate?
- Whether the utility company has changed its net metering rate structure since installation
- System production monitoring data versus expected yield, shading analysis, soiling, and potential metering issues (Correct answer)
- Whether the inverter's warranty has expired, reducing performance guarantees
- Whether the homeowner has added new electrical loads since the system was sized
Correct answer: System production monitoring data versus expected yield, shading analysis, soiling, and potential metering issues
Investigating production data against modeled expectations, irradiance records, shading, soiling, and utility metering accuracy identifies whether the shortfall is from the PV system or from energy use changes, and points to corrective action.
NABCEP System Maintenance 5
A battery-based system's charge controller is in float mode but the battery voltage is consistently 0.3V below the float setpoint.
What is the most likely cause?