Before You Connect the Megohmmeter

Correct answer: B) Disconnect or isolate the surge protective devices, because they will conduct at the test voltage, which both corrupts the reading and risks damaging them

An insulation test is different from every other measurement in the bag, because the instrument does not observe the circuit, it drives it. The megohmmeter applies a DC voltage well above normal operating levels and measures what leaks. Anything in the circuit that is designed to conduct above a threshold will therefore take part in the test, and a surge protective device is exactly that kind of component: its whole purpose is to become conductive when the voltage rises.

So leaving it connected spoils the measurement in the most misleading direction. The instrument reads a low resistance, the technician concludes the insulation is compromised, and the fault is the test setup. Worse, the device may be degraded by being driven this way, which means the test has damaged a protective component while telling you something untrue. Isolating it is not fussiness about accuracy, it is a condition of the test meaning anything.

The same reasoning extends to whatever else is on the circuit. Inverter input stages, monitoring equipment and anything else with semiconductors across the conductors under test belong on the same list, and the manufacturer's instructions for each are where the list comes from.

The O&M point: isolating and verifying de-energised is the preparation that safety training drills into you, and it is the preparation for protecting the person. This test needs a second kind of preparation, for protecting the measurement and the equipment, and nothing about standing there with a locked-off string prompts you to think of it. The two get conflated because both are called "prep", and the first one being complete feels like being ready.

Why the other options are wrong

A) Nothing further — the string is isolated, locked and verified, which is the whole of the preparation

Everything listed has been done and all of it protects the technician, which is why this answer feels complete. None of it addresses what happens when a test voltage is applied to a circuit that still has voltage-sensitive components sitting across it. Being safe to work on and being ready to test are two different states, and reaching the first is not evidence of the second.

C) Wait until after dark, since the array’s own voltage in daylight makes an insulation measurement impossible

A PV array is a source in daylight and that is a real complication, and it is one the established methods handle rather than a reason to stop. Insulation testing of illuminated arrays is ordinary field work, done with procedures that account for the array being live. Waiting for darkness would also mean working on a roof at night, which trades a manageable complication for a worse one.

D) Set the instrument to its highest test voltage, so that marginal insulation is certain to be found

More test voltage is not more diagnosis. The value is chosen for the system and the equipment being tested, following the manufacturer's instructions, and going above it can break down insulation that was sound, which converts a test into the fault it was looking for. This is also the answer that sounds rigorous, and rigour in an insulation test means using the specified voltage and isolating what should not be in the circuit, not turning the dial up.

References

• NABCEP OMAT JTA v.2026.5 — Domain 4, Task 3.B: "Insulation testing devices (e.g., megohmmeter)". The JTA names the instrument and its application; it does not specify test voltages or minimum resistance values.
• No test voltage and no minimum insulation resistance figure is quoted here, deliberately. The tables those values come from sit in IEC 62446-1, which appears nowhere in the JTA and therefore cannot carry an answer, and secondary sources disagree with each other about what the table says for higher-voltage systems. The correct answer depends only on what a surge protective device does when a test voltage is applied to it.