Solar panel isolator switches: what they do and when they fail

This guide covers what the AC and DC isolators do, how to tell if one is off, common faults, and where fuses and surge protectors fit in.

• Don’t open an isolator, a fuse holder or any junction box, and don’t touch the cables from the panels.
• The DC side stays live in daylight. Panels produce electricity whenever light falls on them and can’t be switched off. MCS guidance says DC isolator boxes should carry the warning “Danger – contains live parts during daylight”.
• Burning smell, smoke or scorch marks: keep clear. Follow the emergency shutdown procedure on the label near your inverter only if it is safe to do so, then call an engineer. If there is any sign of fire, call 999.

Look near the inverter first: in the loft, garage, utility room or on an outside wall. You will usually see:

• a DC isolator beside or below the inverter, or a DC switch on the inverter itself;
• an AC isolator next to the inverter, often a red-and-yellow rotary switch;
• on some systems, a fused spur next to the inverter doing the AC isolator’s job;
• a breaker for the solar circuit in your fuse board, labelled “solar”, “PV” or “inverter”.

Your handover paperwork should include a wiring diagram showing each isolator, and an emergency shutdown procedure.

Fires involving solar panels are rare. The government’s summary of a three-year BRE study says “the incidence of such fires is very low”: the 80 possible incidents BRE investigated were about 0.01% of UK installations. But DC isolators stood out. BRE found they “present the greatest fire risk”, causing approximately 30% of the incidents it recorded. Where the reason was known, it was usually poor installation.

• Water getting in. Seven incidents involved water in DC isolators, all with cable glands facing upwards. In several cases more than one cable had been pushed through a gland made for one. On four installations, mounting holes had been drilled through the back of weatherproof boxes.
• Loose terminals. Lab examination found loose terminals in at least three cases. BRE lists damp connections and loose screw terminals among the likely causes of arcing.
• The wrong switch. Some isolators were under-rated for the voltage or current of the panels feeding them. BRE also warns that models originally designed for AC are unlikely to be reliable.
• Sun and heat. The IET points out that a switch on a south-facing wall in direct sun gets hotter and takes more UV. A box only keeps water out while it stays sealed, so report a cover that is cracked, brittle or damaged.
• Overheating. Solar Energy UK’s maintenance guidance includes checking for heat damage, discolouration and unusual smells.

On a service or repair visit, our engineers:

• check each isolator works and can be locked off;
• make the system safe, then look inside DC isolators for overheating, moisture and loose terminals, as BRE recommends;
• check terminals are tightened to the correct torque and the switch is rated for your system, as the IET advises;
• check fuses and surge protectors are intact;
• test the DC circuits and read the inverter’s fault log, which BRE notes can show minor issues before they become serious.

If an isolator is wet, damaged or wrongly rated, we replace it with a correctly rated switch. You get a clear written quote before any work starts.

Isolators, fuses and surge protectors are on the 21-point checklist for every maintenance visit. We repair systems fitted by any installer across England, Wales and Scotland. See solar panel repair, and for SolarEdge or Tigo systems, solar panel optimisers.

Sources: BRE National Solar Centre research on fire and solar PV systems for the government; IET; Solar Energy UK.