Lockout/Tagout on the DC Side: What a Locked Disconnect Does Not Do

Question

Lockout/Tagout on the DC Side: What a Locked Disconnect Does Not Do

Safety Application Domain 1 · Task 4.A

It is early afternoon under a clear sky. A service call has been raised to replace a blown string fuse in a rooftop combiner box on an eight-year-old 250 kW commercial system. The site's written de-energization plan, left behind by the original installer, says to open and lock both the AC disconnect and the DC disconnect at the inverter. The technician follows it exactly: both disconnects are opened, locked and tagged, and a meter reads 0 V at the inverter's DC input terminals. What is the correct assessment of the rooftop combiner box the technician is about to open?

Answer 1

Reveal answer and explanation

Correct answer: B) Still energized — the modules produce voltage whenever they are illuminated, so absence of voltage has to be verified at the combiner box itself

A PV module is a source that cannot be switched off. As long as light falls on it, it produces voltage, and in a commercial design the strings feeding that combiner box can sit near their open-circuit voltage of several hundred volts DC. Opening and locking the DC disconnect isolates the inverter from the array. Everything on the array side of that disconnect, including the rooftop combiner box, stays live.

The 0 V reading is a true measurement taken on the wrong side of the isolation point. The lock proves that nobody will re-close the disconnect. It says nothing about what is upstream of it. OSHA and NFPA, the two bodies the JTA names for safety regulations, both require that a de-energized state be verified by test at the point of work rather than inferred from the position of a switch. Here the point of work is the combiner box, so that is where the test belongs.

The O&M point: that written procedure is not wrong, it is incomplete, and it was written by someone whose job was to build the system rather than to service it. Isolating the inverter is what most work on a new installation needs. On a system you inherited, you are the one who has to notice what the procedure does not cover, and that nobody has confirmed the as-built drawing still matches the roof after eight years of repairs. Following an inherited procedure exactly is not the same as establishing an electrically safe condition where your hands will be.

Why the other options are wrong

A) De-energized — the only DC source has been isolated and the disconnect is locked and tagged: This is the habit that transfers from AC-only work, and it is the most dangerous one to bring onto a roof. On the AC side, isolating and locking really does kill the conductors, because both the utility and the inverter are switchable sources. On the DC side there is no switch between a module and its own output conductors. "Isolated and locked" describes the disconnect, not the array.
C) De-energized once the manufacturer’s stated wait time for the inverter DC bus capacitors to discharge has elapsed: The capacitor wait time is a real requirement, and it applies to different equipment. The inverter's DC bus holds charge after the disconnects open, which is why the manufacturer gives a wait time before you open the inverter enclosure. It has nothing to do with the roof, where the source is not a capacitor that discharges but a 250 kW array that keeps generating. Half-right answers like this one survive a quick read, which is exactly why they appear on the exam.
D) De-energized at the fuse holders, because they are touch-safe and pulling the fuse opens the string circuit: Touch-safe holders reduce the chance of contact; they do not remove voltage. And "open" is close to the opposite of "de-energized": pulling a fuse raises the string to its full open-circuit voltage across the holder, the highest voltage that string will produce, and draws a DC arc if current was flowing when you pulled it. Confusing an open circuit with a dead one is one of the most common ways people get hurt on the DC side.

References

NABCEP OMAT JTA v.2026.5 — Domain 1, Task 4.A: "Electrical hazards and control methods (e.g., DC/AC, electrical shock, arc flash, de-energization plan, lockout/tagout, working clearance, ground fault)"
NABCEP OMAT JTA v.2026.5 — Domain 1, Task 3.A names OSHA and NFPA as the safety regulations in scope. Both require a de-energized state to be verified by test at the point of work, not inferred from the position of a disconnect.