The Most Dangerous Part of Your Solar System

What a DC isolator is, and why you have more than one

Your solar panels make direct current. Your house runs on alternating current. The inverter sits between the two and converts one into the other.

Everything on the panel side of the inverter is the DC side, and it is the part most people never think about. It is also the part that is live whenever there is daylight, because a solar panel has no off switch. It is not connected to anything you can turn off at the meter box. Sun hits the glass, the panel makes voltage. That is the whole design.

A DC isolator is a switch whose only job is to break that circuit so the system can be worked on safely. In a conventional Australian install there are usually two: one on the roof next to the array, and one at the inverter, often on the wall right beside it. The photo at the top of this article is that second one. You can still read the label on the surviving unit next to the burnt hole: PV ARRAY DC ISOLATOR.

So far, so boring. It is a switch in a plastic box. The interesting part is what happens when you open it at the wrong moment.

Direct current does not let go

Here is the bit that matters, and it is physics rather than opinion.

When you open any switch under load, the current does not stop politely at the moment the contacts part. It jumps the gap. That jump is an arc, and an arc is a channel of ionised air that conducts electricity at thousands of degrees. Every switch in your house does this every time you use it. You never notice, and there is a very specific reason why.

Australian mains power is alternating current at 50 Hz, which means the current reverses direction fifty times a second and therefore passes through zero one hundred times a second. An arc cannot survive a moment of zero current. So an AC arc lights up, gets snuffed out within ten milliseconds by the waveform itself, and the switch opens cleanly. The physics does the hard work for free.

Direct current never crosses zero. There is no moment of nothing. A DC arc, once struck, has no reason to stop and every reason to continue: it is hot, hot air is ionised, ionised air conducts, so it keeps burning and it stretches as the contacts separate. It will happily sustain itself across a growing gap until something runs out. Usually what runs out is the enclosure.

Now add the voltage. A residential solar string typically sits somewhere between about 300 and 600 volts DC in full sun. That is not a spicy tingle. That is more than enough to strike and hold an arc that will melt copper and set fire to a plastic box, which is exactly what you are looking at in the photos on this page.

Never turn the DC isolator off first

This is the practical rule, and it is the one thing I would like every solar owner in the country to know.

Turn the AC off first. Always.

If you open the DC isolator while the system is generating and feeding the inverter, you are asking a switch to interrupt the full output current of your array at several hundred volts DC. That is the worst possible thing you can ask it to do, and it is the moment a tired ten year old isolator turns into the photo at the top of this page.

Kill the AC side first and the picture changes completely. The inverter loses the grid, stops converting, and stops drawing current from the panels within a few seconds. The DC circuit goes to near zero current while staying at voltage. Now when you open the DC isolator, there is barely any current to interrupt, so there is barely any arc. Same switch, same sunshine, entirely different event.

The correct order is not a preference. It is what the inverter manual says, it is what we are trained to do, and it is written on the shutdown procedure label that should be on your switchboard or your inverter.

Why the rooftop ones fail

The isolator at your inverter usually lives under an eave or inside a garage. The one on your roof does not. It sits in the open, in Queensland, for twenty years.

Think about what that actually means. Full UV all day, which turns plastic enclosures chalky and brittle. Surface temperatures that swing from freezing before dawn to well over 70 degrees by mid afternoon, then back again, roughly seven thousand times a decade. Every one of those cycles expands and contracts the housing, the lid, the seal and the cable glands by a slightly different amount.

Eventually a seal hardens, a gland cracks, a lid warps or a screw backs off, and driving rain finds the gap. Once water is inside a live DC enclosure you get corrosion on the terminals, then tracking across the surfaces, then heat, then carbon, and carbon conducts. From there it is a slow motion short circuit inside a plastic box on a roof, and nobody is watching.

That is the whole story of the photo below. It is not exotic. It is a seal that gave up.

Australia mandated these, then changed its mind

This part surprises people, including a lot of solar owners who assume the rules must have always been sensible.

From 2012, Australian standards required a DC isolator on the roof next to the array on conventional systems. We were the only country in the world with that requirement. The intention was good: give firefighters and technicians a way to break the circuit at the source.

What happened in practice was that we put tens of thousands of plastic switch enclosures out in the weather, and inspection data over the following years made the result clear. Rooftop DC isolators became the single largest source of failures on conventional solar systems in this country, and the largest single cause of solar related fires.

In November 2021 the standard was revised and the rooftop isolator mandate was removed, with alternatives such as a rated disconnection point at the array permitted instead. After a transition period where either version could be applied, the new standard took full effect in 2022.

Two things worth being clear about, because this gets misread constantly. First, this does not mean the isolator on your roof is illegal or has to come off. A system that was compliant when it was installed remains compliant, and removing or altering solar DC equipment is licensed work and a design decision, not a weekend job. Second, it does not mean isolators are bad in themselves. It means putting one on a roof, in the weather, for two decades, turned out to create more risk than it removed.

So how worried should you actually be?

I would rather give you the honest version than scare you into a phone call.

Solar fires are rare. The commonly cited New South Wales figure is that in 2020 fewer than 0.022 percent of all solar systems ever installed in that state had a fire related incident, which is roughly one in four and a half thousand. Your house is far more likely to have a kitchen fire. Solar is a safe technology and I install it for a living because I believe that.

But the composition of that small number is the point. When an Australian solar system does catch fire, the DC isolator is very often the part that started it. More than half of solar related fires in New South Wales have been attributed to them, and state fire services have reported the annual counts climbing as the fleet ages: Victoria went from fifteen isolator fires in one financial year to twenty seven in the next, and the Northern Territory from four to eleven across 2022 and 2023.

That last detail is the one I would pay attention to, because it is about age. The systems installed during the boom under the old mandate are now ten to fourteen years old. Seals do not fail on day one. They fail somewhere around now.

So: low probability, and rising with age, and concentrated almost entirely in one component that is cheap to inspect. That is a very good argument for having someone look at it, and a very poor argument for panicking.

What to look for, and what to do

From the ground, with your feet on the ground, you can check more than you would think. Look at the isolator beside your inverter and at anything you can see of the array from outside.

The short version

Your solar array is live every daylight hour and has no off switch. The DC side cannot rely on the waveform to snuff an arc the way the AC side can, so a switch opened under load at several hundred volts DC can strike an arc that sustains itself.

Turn the AC off first, wait for the inverter to stop, then the DC. Never the other way round.

And if there is an isolator sitting on your roof that has been out in the weather since the last solar boom, that is the single most likely thing on your system to fail, and the cheapest thing to have checked. Get someone up there before the weather finds the gap.

Common questions