Direct current is the part of a solar system that cannot be switched off

The array is live whenever there is light

Switching off an inverter doesn’t de-energise the panels. Photovoltaic cells generate whenever light falls on them, so the conductors between the modules and the isolator remain at voltage regardless of what any switch downstream is doing. There is no off position on the roof, and there cannot be one without extra equipment.

That voltage isn’t trivial either. A domestic string operating at several hundred volts is well beyond the level at which contact is dangerous, and it is present on a cold clear morning before anyone in the house is awake. The isolators exist to break that circuit at a defined point, not to stop it existing.

It is a genuinely unusual property in a domestic electrical installation, where the assumption everywhere else is that removing the supply makes things safe. Anyone working on the roof, and anyone attending an incident there, has to work from a different assumption.

Why a direct-current arc behaves badly

Alternating current passes through zero twice in every cycle, and an arc struck across a gap tends to extinguish itself at that moment. Direct current never crosses zero, so once an arc is established it can sustain itself, drawing power continuously and reaching temperatures that ignite anything nearby.

This is why direct-current switchgear is built differently from its alternating-current equivalent, with longer contact gaps and arc chutes designed to stretch and cool the arc until it fails. It is also why fitting a switch not rated for direct current into a photovoltaic circuit is a serious error rather than a technicality.

The everyday consequence is that a poor connection is more dangerous here than elsewhere. A joint with a little resistance heats, heating degrades the joint, degradation increases the resistance, and the process can end at a small persistent arc inside a connector on a roof. That feedback loop is the origin of most direct-current incidents, and it starts with workmanship rather than with equipment.

What the mitigations do

Isolation is the first layer. There is normally a switch adjacent to the inverter, and depending on local rules there may be another at the array, so that the run through the building can be de-energised from either end. Their location should be known to the household and reachable without a ladder.

Arc-fault detection is the second. Some inverters monitor the electrical signature of the direct-current circuit for the characteristic noise an arc produces and shut down when they see it. It is a genuinely useful protection and it isn’t a substitute for connections that were made properly.

Module-level shutdown is the third and the most divergent between jurisdictions. Some codes require that the voltage on the roof can be reduced to a safe level on command, which in practice means electronics at every panel. Other jurisdictions don’t require it at all. Neither position is unreasonable, and where you live decides which one applies to you.

Signs and documents are not bureaucracy

A solar installation adds a source of energy that an electrician or a firefighter arriving at the property has no reason to expect. Labelling exists to tell them, and the labels that matter identify the presence of generation at the intake position and at the consumer unit, mark the isolators clearly, and warn that the direct-current side remains live after isolation.

In many jurisdictions a diagram of the installation is required at the origin, showing where the array is, where the isolation points are and how the system connects. It costs almost nothing at installation and it is the difference between a rapid decision and a cautious one during an incident.

Emergency services generally do not refuse to attend a building with solar, which is a persistent piece of folklore. What they do is manage the roof as a live electrical area, which may change tactics. Clear labelling and an accessible isolator directly reduce how much caution is warranted.

What an owner can reasonably check

Not much of the electrical work is inspectable without instruments, and a few things are visible to anyone. Labels present and legible. Isolators accessible and marked. Cables clipped and supported rather than resting on tile edges or hanging in loops where wind can chafe them.

Cable ties are worth a look specifically. Ordinary plastic ties degrade under ultraviolet exposure and fail after a few years, at which point cables sag onto the roof surface and abrade. Ultraviolet-stable ties or metal clips are the correct choice, and this is one of the few workmanship details visible from the ground with binoculars.

Beyond that, the honest position is that this is a job for a qualified person with test equipment, at commissioning and periodically afterwards. The interval is set locally and the principle is not: an installation with a hidden defect gives very little warning, and the inspection is far cheaper than the alternative.