NABCEP System Maintenance 2 — Questions and Answers
Question 1: During a routine inspection, you find that a string of modules has an open-circuit voltage (Voc) reading 15% below the expected value. What is the most likely cause?
- One module in the string has a failed bypass diode causing it to act as a resistor
- The string combiner fuse for that string has partially failed
- One module in the string is completely disconnected or has a broken cell string (Correct answer)
- The inverter MPPT is limiting the string voltage
Correct answer: One module in the string is completely disconnected or has a broken cell string
A single fully disconnected module or broken cell string reduces string Voc by approximately one module's Voc, which for a typical 20-module string is about 5%, but severe internal failures can drop it further; the most direct cause of a significant Voc drop is an open-circuit internal to one module.
Question 2: What is the recommended torque specification procedure when re-tightening module frame bolts on a racking system during preventive maintenance?
- Use the installer's general preference as no standards exist
- Follow the racking manufacturer's specified torque values using a calibrated torque wrench (Correct answer)
- Hand-tighten only to avoid damaging module frames
- Apply thread-locking compound and tighten to 25 ft-lbs regardless of manufacturer specs
Correct answer: Follow the racking manufacturer's specified torque values using a calibrated torque wrench
Racking manufacturers publish specific torque values for all fasteners, and using a calibrated torque wrench to meet those values ensures structural integrity without over-stressing components.
Question 3: A battery-based off-grid system shows the battery bank voltage sagging excessively under load even though the state of charge reads 80%. What should the technician check first?
- The PV array output current
- Internal resistance or cell condition of individual batteries (Correct answer)
- The inverter's low-voltage disconnect setting
- The charge controller maximum current output
Correct answer: Internal resistance or cell condition of individual batteries
Excessive voltage sag under load with a seemingly high SOC indicates high internal resistance, which is a common sign of battery aging, sulfation, or cell failure.
Question 4: When performing thermographic (infrared) inspection of a PV array, what condition is the inspector primarily trying to identify?
- Modules operating at below-ambient temperature indicating high efficiency
- Hot spots caused by cell damage, soiling, shading, or bypass diode failure (Correct answer)
- Uniform temperature distribution indicating proper current flow
- Inverter thermal performance relative to ambient temperature
Correct answer: Hot spots caused by cell damage, soiling, shading, or bypass diode failure
Thermographic inspection identifies hot spots—localized elevated temperatures in cells or modules—which indicate defects, shading mismatch, soiling, or failed bypass diodes that cause power loss and potential fire risk.
Question 5: An inverter's ground fault protection device (GFPD) has tripped. After clearing the fault and resetting, what must the technician verify before returning the system to service?
- That the DC disconnect is rated for the new fault current level
- That the ground fault has been identified and corrected, not just cleared (Correct answer)
- That the inverter firmware is updated to the latest version
- That the utility interconnection agreement is still valid
Correct answer: That the ground fault has been identified and corrected, not just cleared
Resetting a GFPD without identifying and correcting the root cause of the ground fault creates a fire and safety hazard; the fault must be located and repaired before the system is re-energized.
Question 6: What is the primary purpose of performing an I-V curve trace on a PV module or string during a maintenance inspection?
- To measure the inverter's conversion efficiency
- To verify module nameplate data against measured performance under STC
- To identify performance degradation, shading, soiling, or mismatch by comparing the measured curve to the expected curve (Correct answer)
- To determine the optimal tilt angle for maximum energy production
Correct answer: To identify performance degradation, shading, soiling, or mismatch by comparing the measured curve to the expected curve
An I-V curve trace captures the current-voltage relationship across the module's operating range, and deviations from the expected curve reveal shading, soiling, cell damage, or bypass diode issues.
Question 7: A technician notices white or brown discoloration (delamination or EVA browning) spreading across several PV module cells. What action is most appropriate?
- Clean the modules with isopropyl alcohol to restore transparency
- Document the defect, test electrical performance, and contact the manufacturer regarding warranty claims (Correct answer)
- Shade the affected modules to reduce thermal stress
- Increase the string voltage to compensate for the power loss
Correct answer: Document the defect, test electrical performance, and contact the manufacturer regarding warranty claims
EVA browning and delamination are manufacturing defects or accelerated aging conditions that degrade optical transmission and module performance, warranting documentation and a warranty claim with the manufacturer.
NABCEP System Maintenance 2
During a routine inspection, you find that a string of modules has an open-circuit voltage (Voc) reading 15% below the expected value.
What is the most likely cause?