[Panels are still live]a plain-English guide

This page exists because the instinct that works on every other appliance in a house fails here, and the failure is dangerous. On a fridge, a boiler or a hob, switching off at the wall makes the thing safe. On a solar system, isolating the alternating current side makes the inverter safe to stand next to and leaves the direct current side — the panels and the wiring between them — live for as long as there is daylight. The MCS Guide to the Installation of Photovoltaic Systems, the guide MCS-certified installers worked to before the IET Code of Practice replaced it, states it directly: the supply from PV modules cannot be switched off, so the DC terminals remain live at all times during daylight hours, and the guide requires the DC isolator enclosure to carry the label "Danger — contains live parts during daylight". Look at your own isolator and you will probably find that label. The only thing that stops a module generating is darkness or something opaque covering it. The scale is worth stating with a number rather than an adjective.

GivEnergy’s commissioning check for their PV over-voltage alarm is that the string voltage does not exceed 580 volts direct current, and Fox ESS specify an open-circuit voltage under 600 volts for the strings on their H1 hybrid inverters. Those are the ceilings the inverters are built to accept, not what a typical roof reads — what yours produces depends on how many panels are wired in series — but a domestic string is measured in hundreds of volts, and the ceiling is several times anything else in the building. Direct current at that level is also less forgiving than mains alternating current, because there is no zero crossing to help an arc extinguish itself. SMA give the best statement of how far the hazard reaches, and it is further than people assume: in the event of a ground fault, high voltages can be present, and do not touch the cables, the substructure, the frame or the modules of the PV array. Substructure and frame means the mounting rails and the metal edges of the panels themselves, not just the wiring.

SolarEdge add the timing point that catches installers out, never mind owners: after switching off, wait five minutes for the input capacitors to discharge before disconnecting AC and then DC. Fox ESS publish the same idea in their switch-off sequence, with a five-minute wait before the lid is opened. The unit is dangerous after it has been switched off. Batteries add a third source that is independent of both sun and grid. Tesla instruct owners not to troubleshoot during a grid outage, precisely because the battery is energising the house at that moment, and warn that the DC conductors of a PV system connected to a Powerwall 3 can become grounded without indication while the inverter tests the array’s isolation. GivEnergy’s owner guide says battery systems contain high-voltage components and must not be opened or tampered with. Enphase note that battery indicators keep flashing while the DC switches are off because the units are still receiving AC power. And Fox ESS make the practical point in their standard remedy for a run of inverter faults: disconnect PV, grid and battery — three separate isolation points.