@revk do you have anything giving off ultraviolet light? My LED bulbs glow when the mosquito lamp is on.

@revk my second guess was going to be ghosts.

But more likely to be a mismatch between what the sensors are seeing and reality.

@revk It's an interesting question as I see the same thing here (cheap Christmas lights and a dubious plug-in relay). I would have suspected earth leakage, but that particular socket is CEE 7/1 (yay for 1960s properties which haven't been rewired in decades) so there is no earth.

Another hypothesis is that the relay is only switching one pole, and further, that it's become reversed somehow so is switching neutral rather than live and so the whole thing is at 240V, giving lots of encouragement for those sneaky electrons to find a return path through whatever they can find. However, I think I tested this by turning the plug round 180° (since Schuko is unkeyed) and the fault remained, but I'm not fully sure, and it's daytime now so I can't check.

Still, it's nice that you get a free night light when it's all turned off :)

@revk I've seen this described as capacitive coupling in the switch wiring. The usual solution is to put a load (resistor) after the smoothing cap after the bridge rectifier.

Here's a beardy Scot talking about it in bulb schematics.

youtu.be/QmyzcBAwcBU?t=226

@avatastic Yeh, except we don't have switch wiring going to the light, it is using a shelly (dry contact relay) in the loft by the light.

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