NABCEP - Certified North American Board of Certified Energy Practitioners Specialist Commissioning and Troubleshooting Questions and Answers — Questions and Answers
Question 1: A technician measures the open-circuit voltage (Voc) of a 15-module string on a cool, sunny day. The expected Voc is 675V based on temperature-corrected module data, but the multimeter reads 630V. What is the most probable cause of this discrepancy?
- The ambient temperature is significantly higher than was anticipated.
- The multimeter's battery is low, causing an inaccurate voltage reading.
- A single module in the string has failed with an internal short circuit.
- One module in the string is completely bypassed due to shading or a faulty bypass diode. (Correct answer)
Correct answer: One module in the string is completely bypassed due to shading or a faulty bypass diode.
The expected voltage per module is 675V / 15 modules = 45V. The measured voltage drop is 675V - 630V = 45V. This indicates that exactly one module is not contributing its voltage to the string total, which is a classic symptom of a module being fully bypassed. High temperatures lower voltage, but typically not by the exact amount of one module. A shorted module would likely cause more significant issues or zero voltage. While a low battery in a meter is possible, the precise voltage drop points strongly to a specific module issue.
Question 2: During the commissioning of a new PV system prior to energization, what is the primary purpose of conducting an insulation resistance test, often called a 'megger' test, on the DC conductors?
- To ensure there are no unintentional, low-resistance paths for current to flow to ground, which could cause a ground fault. (Correct answer)
- To verify that the short-circuit current (Isc) rating of the modules matches the datasheet specifications.
- To confirm the equipment grounding conductor is properly and continuously bonded from the modules to the inverter.
- To measure the open-circuit voltage (Voc) of the strings under a simulated load condition to check for anomalies.
Correct answer: To ensure there are no unintentional, low-resistance paths for current to flow to ground, which could cause a ground fault.
An insulation resistance test applies a high DC voltage between the current-carrying conductors and ground to detect any leakage current. A low resistance reading indicates that the conductor's insulation is compromised, creating a potential path for a ground fault. This is a critical safety and operational test performed before energizing the system. It does not measure Isc, Voc, or the continuity of the equipment grounding conductor.
Question 3: A grid-tied inverter is displaying a 'Ground Fault' error and has ceased operation. Which of the following is the most logical and safe first step in the troubleshooting process?
- Immediately replace the inverter's ground fault detection and interruption (GFDI) fuse and restart the system.
- Disconnect the AC breaker and test the utility grid voltage to ensure it is within the inverter's operating range.
- Follow lockout/tagout procedures, de-energize the system, isolate the DC strings, and perform insulation resistance testing on each string-to-ground. (Correct answer)
- Check the production monitoring software to analyze historical performance data for trends leading up to the fault.
Correct answer: Follow lockout/tagout procedures, de-energize the system, isolate the DC strings, and perform insulation resistance testing on each string-to-ground.
A ground fault error indicates a hazardous condition where a DC conductor is contacting ground. The standard and safest procedure is to completely de-energize the system, then isolate each string (typically at the combiner box) and use an insulation resistance tester to identify which string has the fault. Replacing the fuse without finding the root cause will likely result in the new fuse blowing and does not resolve the safety issue. AC voltage and historical data are irrelevant to locating a DC ground fault.
Question 4: A commissioning agent is testing the rapid shutdown functionality of a PV system installed on a building. After initiating the rapid shutdown switch, what voltage must the controlled conductors *inside* the array boundary be limited to within 30 seconds, according to NEC 690.12?
- The voltage must be reduced to exactly 0V.
- The voltage must be limited to not more than 80V. (Correct answer)
- The voltage must be limited to not more than 30V.
- The current must be reduced to less than 0.5 amps.
Correct answer: The voltage must be limited to not more than 80V.
According to NEC 690.12(B)(2), controlled conductors located inside the array boundary must be limited to not more than 80 volts within 30 seconds of rapid shutdown initiation. The 30V limit applies to controlled conductors *outside* the array boundary. A limit of 0V is not required.
Question 5: A client reports that their newly commissioned 8 kW PV system is only showing 3.5 kW of production on their monitoring app at solar noon on a clear, cool day. The technician confirms the inverter is operating without errors and that DC string voltages and currents are normal. What is the most likely cause of the discrepancy?
- The inverter's AC output circuit breaker has tripped in the main service panel.
- A string of modules was accidentally reverse-polarized in the combiner box.
- Multiple modules in the array have failed or shaded bypass diodes.
- The system's production monitoring current transformers (CTs) are installed incorrectly. (Correct answer)
Correct answer: The system's production monitoring current transformers (CTs) are installed incorrectly.
When the inverter is operating correctly with normal DC inputs and showing no faults, but the reported production is inaccurate, the issue often lies with the measurement equipment. Production monitoring CTs that are installed on the wrong conductors, facing the wrong direction, or not fully clamped can lead to significant under-reporting of system output. A tripped AC breaker would result in zero production. Reverse polarity or multiple failed diodes would cause abnormal DC voltage/current readings and likely trigger inverter faults.
Question 6: What is the correct and safest sequence for energizing a grid-tied string inverter PV system for the first time during the commissioning process?
- Close the AC disconnect first to provide a grid reference, then close the DC disconnect to provide PV power. (Correct answer)
- Close the DC disconnect first to power the inverter's components, then close the AC disconnect to allow export.
- Close the DC and AC disconnects simultaneously to prevent voltage potential differences.
- Close the main service panel breaker first, then close the DC disconnect, then finally close the AC disconnect.
Correct answer: Close the AC disconnect first to provide a grid reference, then close the DC disconnect to provide PV power.
The standard and manufacturer-recommended procedure is to apply AC power from the grid to the inverter before applying DC power from the PV array. This allows the inverter to power up its internal controls, perform self-diagnostics, sense the grid's voltage and frequency, and prepare for synchronization. Once the inverter is stable and has a grid reference, the DC disconnect is closed to allow it to begin the process of converting DC power and exporting it to the grid.
NABCEP - Certified North American Board of Certified Energy Practitioners Specialist Commissioning and Troubleshooting Questions and Answers
A technician measures the open-circuit voltage (Voc) of a 15-module string on a cool, sunny day.
The expected Voc is 675V based on temperature-corrected module data, but the multimeter reads 630V.
What is the most probable cause of this discrepancy?