Of all the components in a solar system, the humble DC isolator has the most outsized safety reputation — both as an essential protective device and, when done poorly, as a genuine fire risk. This is the more detailed companion to the isolators basics: a closer look at what the standards require, why DC isolators specifically have caused fires, and what a safe installation looks like.
Why isolators exist, and why DC is the hard part
Solar panels generate voltage whenever there’s daylight — you can’t switch them off at the source. So isolators are the switches that let the system be safely disconnected for maintenance, faults, or emergencies. A compliant install includes them so the array can be cut off from the inverter and the building.
The reason they’re specialised comes down to physics. Breaking a DC circuit under load is much harder than breaking an AC one. AC reverses direction 100 times a second, so any arc that forms as the contacts open naturally extinguishes itself. DC flows steadily in one direction, so an arc can sustain itself across opening contacts — a hot, persistent arc that ordinary AC-rated switches aren’t built to quench. This is why solar isolators on the DC side must be specifically DC-rated; an AC switch in that role can fail dangerously.
What AS/NZS 5033 requires
The relevant standard, AS/NZS 5033 (the standard for solar PV arrays), sets out where isolators must be placed and how they must perform. In practical terms, a compliant install provides means to isolate the array from the inverter, with isolators that are:
- DC-rated for the array’s voltage and current.
- Lockable, so a system being worked on can’t be re-energised by accident.
- Clearly labelled, so an electrician or firefighter can identify and operate them quickly.
- Weatherproof and correctly installed, since many sit outdoors exposed to the elements.
The standard has evolved over the years partly in response to the fire problem, including changes around rooftop DC isolators specifically.
The fire history — and its cause
This is the part to take seriously. Rooftop DC isolators have been one of the most documented causes of solar fires, in New Zealand and internationally. The cause is rarely the concept and almost always the execution: a poor-quality or badly installed isolator lets water ingress over time. Moisture inside an enclosure carrying live DC leads to corrosion and tracking, which can develop into a sustained arc — and on a hot roof in a timber structure, that’s a fire risk. So the danger isn’t isolators as such; it’s cheap, poorly sealed, or badly installed ones degrading in the weather.
This history is exactly why some jurisdictions and standards have moved to reduce reliance on exposed rooftop DC isolators, and why quality and correct installation matter so much here.
What a safe installation looks like
For an isolator to do its job safely for decades:
- Quality, properly DC-rated units from reputable makers — not the cheapest available.
- Correct, weatherproof installation so water can’t track in — the single biggest factor in the fire history.
- Periodic inspection. Isolators should be checked when the system is serviced; discolouration, cracking, or water staining around one is a warning sign to act on.
As a homeowner, you should never remove or bypass an isolator, and you should get any that looks damaged or discoloured checked by a solar electrician rather than poking at it yourself.
The verdict
DC isolators are the safe-shutdown switches that make a solar system serviceable and emergency-ready, and AS/NZS 5033 dictates that they be DC-rated, lockable, labelled, and weatherproof. They’ve earned a fire reputation not because the idea is flawed but because cheap, poorly sealed, or badly installed units let water in and arc over time. The lesson is consistent with everything else in solar: quality components and a competent installer matter — and here, they’re a genuine fire-safety issue, not just a performance one.
Get a free assessment and we’ll make sure your system is specified and installed to standard.
Sources: Isolator placement and DC-rating requirements per AS/NZS 5033; DC isolator fire history per regulator and industry safety guidance. Installation is prescribed electrical work for a registered electrician.