Solar PV NABCEP Exam Questions & Answers 2026 (11–20)

Question 1

The measured Isc for a string is 10% above the nameplate Isc at STC. The irradiance is measured at 950 W/m². What is the most accurate interpretation?

TheoryPractice · Accepted Answer

Irradiance alone cannot explain the discrepancy; further investigation is needed. At 950 W/m² the expected Isc would be about 95% of nameplate STC Isc. A measured value 10% above STC nameplate at lower irradiance (meaning roughly 15–16% above what is expected) is anomalous and warrants investigation for wiring errors such as mis-connected parallel strings.

Question 2

A technician clamp-meters a combiner box output and reads an Isc that is exactly 3× the per-string Isc at the same irradiance, but the combiner is designed for 4 strings. What does this indicate?

TheoryPractice · Accepted Answer

One string is not contributing current, likely due to an open circuit or blown fuse. At the combiner output, total current equals the sum of all parallel string currents. If the measured total equals 3× per-string Isc instead of 4×, one string is contributing zero current—most likely an open fuse, disconnect, or broken conductor in that string.

Question 3

What instrument is used to perform an insulation resistance test on PV wiring?

TheoryPractice · Accepted Answer

Megohmmeter (megger). A megohmmeter applies a high DC test voltage (typically 500–1000 V for PV systems) and measures the resulting leakage current to calculate insulation resistance in megohms, which a standard multimeter cannot do.

Question 4

Before performing an insulation resistance test on PV array conductors, the technician should:

TheoryPractice · Accepted Answer

Disconnect the inverter and all loads, and short the positive and negative conductors together at the array end. Shorting the positive and negative together at the array end and disconnecting the inverter allows the megohmmeter to test all conductor insulation simultaneously against ground from a single measurement point without back-feeding voltage into the inverter.

Question 5

IEC 62446-1 recommends a minimum insulation resistance value for PV strings. Which value is typically cited as the minimum acceptable for a system under 1 kVdc?

TheoryPractice · Accepted Answer

1 MΩ. IEC 62446-1 specifies a minimum insulation resistance of 1 MΩ for systems with a maximum system voltage ≤1000 Vdc. Values below this indicate degraded insulation that could lead to ground faults or leakage currents.

Question 6

A technician tests a 1000 V string and obtains 0.6 MΩ insulation resistance. After drying out wet conduit and retesting, the value rises to 4.8 MΩ. Which statement best describes the situation?

TheoryPractice · Accepted Answer

The first reading failed the 1 MΩ minimum; the second passes, and moisture was the root cause. IEC 62446-1 requires ≥1 MΩ for systems ≤1000 V. The initial 0.6 MΩ reading failed that threshold. After drying, the 4.8 MΩ value clears the minimum, confirming moisture in the conduit was temporarily compromising insulation resistance.

Question 7

During grid-tied inverter commissioning, the inverter performs an automatic anti-islanding check. What is the purpose of this check?

TheoryPractice · Accepted Answer

To ensure the inverter will shut down when utility grid power is lost. Anti-islanding protection ensures the inverter disconnects from the grid when utility power is lost, preventing the inverter from energizing a de-energized utility line and protecting utility workers performing maintenance.

Question 8

The correct sequence for starting up a string inverter from a completely de-energized state is:

TheoryPractice · Accepted Answer

Close AC disconnect → close DC disconnect → allow inverter to self-start. The AC disconnect is closed first to establish the grid reference the inverter needs. Then the DC disconnect is closed to provide PV power. The inverter then performs its internal checks (grid voltage/frequency, anti-islanding delay) and self-starts without a separate manual enable step.

Question 9

An inverter displays a 'Grid Frequency Out of Range' fault on first startup. The grid voltage and waveform look normal on an oscilloscope. What should the technician check first?

TheoryPractice · Accepted Answer

The inverter's programmed grid frequency setting. Grid-tied inverters have configurable frequency windows. If the inverter was configured for 50 Hz (e.g., exported from a European market) but the grid is 60 Hz, the inverter will fault. Checking and correcting the programmed frequency setting resolves this.

Question 10

After initial startup, a three-phase inverter shows power output on two phases but not the third. What is the most likely cause?

TheoryPractice · Accepted Answer

A loose or open connection on the third AC output conductor. An open connection on one AC phase will prevent current flow on that phase only, while the other two phases operate normally. A DC-side issue would typically affect all phases equally.

Solar PV NABCEP Exam Questions & Answers 2026 (11–20)

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