NABCEP System Maintenance 4 — Questions and Answers
Question 1: During a visual inspection of a PV array, you notice that several module junction boxes have covers that are cracked or missing. What is the priority concern?
- Aesthetic non-compliance with building codes
- Exposure of live DC conductors and bypass diodes to moisture and environmental contamination (Correct answer)
- Reduced module efficiency due to heat loss through the open junction box
- Potential for animals to nest inside the junction boxes reducing module weight
Correct answer: Exposure of live DC conductors and bypass diodes to moisture and environmental contamination
Open or damaged junction boxes expose energized DC terminals and bypass diodes to moisture, insects, and debris, creating shock hazards and potential for arcing or corrosion-related failures.
Question 2: A flooded lead-acid battery bank requires regular watering. When should distilled water be added relative to the charging cycle?
- Before charging begins to ensure electrolyte covers the plates throughout the charge
- Immediately after a full charge when gassing has subsided and electrolyte levels can be accurately assessed (Correct answer)
- At any time since the electrolyte level does not change during charging
- Only when the battery voltage drops below the low-voltage disconnect threshold
Correct answer: Immediately after a full charge when gassing has subsided and electrolyte levels can be accurately assessed
Watering after a full charge prevents overfilling since electrolyte expands during charging, and it allows accurate measurement of the settled electrolyte level once gassing has stopped.
Question 3: What is the most effective method for detecting a high-resistance connection in a PV array string without disconnecting the system?
- Measuring the string open-circuit voltage with a multimeter
- Using an infrared camera to identify hot spots at connectors and terminals under operating conditions (Correct answer)
- Measuring the array ground fault current with a clamp meter
- Checking the inverter's real-time DC input resistance reading
Correct answer: Using an infrared camera to identify hot spots at connectors and terminals under operating conditions
A high-resistance connection dissipates power as heat, which is detectable with an infrared camera while the system is operating at load, without requiring disconnection.
Question 4: A grid-tied inverter is repeatedly disconnecting from the grid and reconnecting every few minutes. Which of the following is the MOST likely cause?
- The inverter's DC input capacitors are failing
- Grid voltage or frequency excursions outside the inverter's permissible operating window (Correct answer)
- The PV array is producing too little current for the inverter to operate
- The inverter's cooling fan has failed, causing thermal cycling
Correct answer: Grid voltage or frequency excursions outside the inverter's permissible operating window
Grid-interactive inverters are required by standards such as IEEE 1547 to disconnect when utility voltage or frequency moves outside defined limits, then reconnect after a mandatory reconnection delay.
Question 5: When checking specific gravity of flooded lead-acid battery cells with a hydrometer, what does a reading of 1.265 at 77°F (25°C) generally indicate?
- The battery is fully discharged and needs immediate recharging
- The battery cell is approximately 50% state of charge
- The battery cell is in good condition and at or near full charge (Correct answer)
- The electrolyte concentration is too high and water must be added immediately
Correct answer: The battery cell is in good condition and at or near full charge
A specific gravity of approximately 1.265 at 25°C is the typical fully-charged value for standard flooded lead-acid batteries, indicating healthy, fully-charged electrolyte.
Question 6: A technician is reviewing a string inverter's event log and finds repeated 'insulation fault' or 'Riso low' error codes. What does this indicate?
- The inverter's internal transformer has degraded insulation
- There is a low-resistance path between the DC conductors and ground, indicating a ground fault or degraded insulation (Correct answer)
- The AC output cables have insufficient insulation for the installed conduit type
- The inverter's monitoring communication cable has a fault
Correct answer: There is a low-resistance path between the DC conductors and ground, indicating a ground fault or degraded insulation
Riso (insulation resistance) monitoring measures the resistance between DC circuit conductors and ground; a low Riso value indicates moisture ingress, damaged wiring insulation, or a ground fault that requires immediate investigation.
Question 7: During a maintenance visit, the technician finds that module mounting clips on a ballasted flat-roof system have shifted and some modules are no longer secure. What is the immediate safety and maintenance priority?
- Recalculate the system's energy yield with the new module positions
- Secure the modules to prevent wind uplift and damage, then assess cause of movement and verify compliance with wind load design (Correct answer)
- Electrically disconnect the affected modules until the racking manufacturer is consulted
- Add additional ballast blocks to the entire system as a precaution
Correct answer: Secure the modules to prevent wind uplift and damage, then assess cause of movement and verify compliance with wind load design
Unsecured modules on a rooftop are an immediate wind uplift hazard that could cause modules to become airborne; securing them and then investigating the root cause and design compliance is the correct priority sequence.
NABCEP System Maintenance 4
During a visual inspection of a PV array, you notice that several module junction boxes have covers that are cracked or missing.
What is the priority concern?