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 ambient temperature is significantly higher than was anticipated.

Answer

The multimeter's battery is low, causing an inaccurate voltage reading.

Answer

A single module in the string has failed with an internal short circuit.

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

To verify that the short-circuit current (Isc) rating of the modules matches the datasheet specifications.

Answer

To confirm the equipment grounding conductor is properly and continuously bonded from the modules to the inverter.

Answer

To measure the open-circuit voltage (Voc) of the strings under a simulated load condition to check for anomalies.

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

Immediately replace the inverter's ground fault detection and interruption (GFDI) fuse and restart the system.

Answer

Disconnect the AC breaker and test the utility grid voltage to ensure it is within the inverter's operating range.

Answer

Check the production monitoring software to analyze historical performance data for trends leading up to the 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

The voltage must be reduced to exactly 0V.

Answer

The voltage must be limited to not more than 30V.

Answer

The current must be reduced to less than 0.5 amps.

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

The inverter's AC output circuit breaker has tripped in the main service panel.

Answer

A string of modules was accidentally reverse-polarized in the combiner box.

Answer

Multiple modules in the array have failed or shaded bypass diodes.

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?

Accepted Answer

Close the AC disconnect first to provide a grid reference, then close the DC disconnect to provide PV power.

Answer

Close the DC disconnect first to power the inverter's components, then close the AC disconnect to allow export.

Answer

Close the DC and AC disconnects simultaneously to prevent voltage potential differences.

Answer

Close the main service panel breaker first, then close the DC disconnect, then finally close the AC disconnect.

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?

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?