NABCEP Commissioning 5 — Questions and Answers
Question 1: Which commissioning test confirms that a ground fault detection and interruption (GFDI) device on a grounded PV system is functioning correctly?
- Applying a temporary resistive ground fault and verifying the device trips (Correct answer)
- Measuring Voc of each string and comparing to spec
- Verifying the DC disconnect opens under full load
- Testing continuity of the equipment grounding conductors
Correct answer: Applying a temporary resistive ground fault and verifying the device trips
Functional GFDI testing requires simulating a ground fault (per manufacturer procedure) and confirming the device detects it and interrupts the faulted circuit.
Question 2: When commissioning a battery energy storage system (BESS) paired with PV, what is verified by a 'round-trip efficiency' test?
- That the battery charges and discharges at the rated power without tripping protection
- The ratio of energy retrieved from the battery to energy stored, confirming acceptable losses (Correct answer)
- That the BMS prevents overcharging beyond 100% SOC
- The time required for the battery to reach full charge from PV alone
Correct answer: The ratio of energy retrieved from the battery to energy stored, confirming acceptable losses
Round-trip efficiency measures the percentage of energy stored that can be retrieved, revealing excessive losses that may indicate cell or BMS issues.
Question 3: A commissioning technician notices the PV system's production monitoring shows data but the revenue-grade meter installed for incentive programs is not logging. What is the immediate next step?
- Accept the monitoring data as a substitute for the revenue meter
- Verify communication wiring and configuration of the revenue meter separately (Correct answer)
- Replace the revenue meter with a higher-grade model
- Notify the AHJ that the system cannot be commissioned
Correct answer: Verify communication wiring and configuration of the revenue meter separately
Revenue-grade meters often use separate communication protocols (Modbus, pulse output) that must be independently verified during commissioning.
Question 4: Which voltage measurement during commissioning most directly confirms correct module-level power electronics (MLPE) operation on a DC-optimized string?
- Each module output at rated voltage under irradiance
- String open-circuit voltage equal to number of modules times fixed output voltage (Correct answer)
- Array Isc at the combiner equals sum of all MLPE rated currents
- Inverter input power equals PTC-rated array output
Correct answer: String open-circuit voltage equal to number of modules times fixed output voltage
DC optimizers output a fixed voltage per module; the string Voc should equal the number of modules multiplied by the optimizer's fixed output voltage.
Question 5: What is the significance of recording the 'commissioning irradiance level' (W/m²) at the time of performance measurements?
- It determines the applicable electrical code edition
- It allows measured power and current values to be corrected to STC for comparison with rated specs (Correct answer)
- It triggers automatic warranty registration with the manufacturer
- It sets the inverter's low-light threshold parameter
Correct answer: It allows measured power and current values to be corrected to STC for comparison with rated specs
Irradiance at measurement time is used to translate field measurements to STC conditions so they can be meaningfully compared to module and system specifications.
Question 6: During commissioning, the installer verifies conductor ampacity for the PV source circuit. Which multiplier is applied to the module's rated Isc when sizing conductors per NEC 690.8?
- 1.15× Isc
- 1.25× Isc
- 1.56× Isc (Correct answer)
- 2.0× Isc
Correct answer: 1.56× Isc
NEC 690.8 requires conductors to be sized at 125% of Isc for continuous duty, and then an additional 125% factor is applied, yielding 1.56× (1.25 × 1.25) the module Isc.
Question 7: A newly commissioned rooftop system produces expected power but the utility interconnection inspection reveals the system lacks a utility-visible AC disconnect. What must occur?
- The system may remain energized while the disconnect is ordered
- The system must be de-energized and the disconnect installed before PTO is granted (Correct answer)
- A signed waiver from the owner satisfies the requirement
- The inverter's internal disconnect is acceptable as a substitute
Correct answer: The system must be de-energized and the disconnect installed before PTO is granted
A utility-accessible AC disconnect is required by most interconnection standards and utility tariffs; the system cannot receive PTO without it.
NABCEP Commissioning 5
Which commissioning test confirms that a ground fault detection and interruption (GFDI) device on a grounded PV system is functioning correctly?