NABCEP - Certified North American Board of Certified Energy Practitioners Specialist Commissioning and Troubleshooting 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?

Accepted Answer

One module in the string is completely bypassed due to shading or a faulty bypass diode.

Answer

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?

Accepted Answer

To ensure there are no unintentional, low-resistance paths for current to flow to ground, which could cause a ground fault.

Answer

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?

Accepted Answer

Follow lockout/tagout procedures, de-energize the system, isolate the DC strings, and perform insulation resistance testing on each string-to-ground.

Answer

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?

Accepted Answer

The voltage must be limited to not more than 80V.

Answer

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?

Accepted Answer

The system's production monitoring current transformers (CTs) are installed incorrectly.

Answer

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.

(NABCEP) Certified North American Board of Certified Energy Practitioners Specialist Practice Test

(NABCEP) Certified North American Board of Certified Energy Practitioners Specialist Practice Test

NABCEP - Certified North American Board of Certified Energy Practitioners Specialist Commissioning and Troubleshooting Questions and Answers