I have a strange problem : when it's raining heavily, the RCD breaks on one line. It is quite large a differential : 0.3 A.

This is in a country where effective voltage is 230 V.

There are quite a few lights on the offending cable. But they are all internal except 2 that are outside, but well sheltered (they are not getting wet):

light 1 light 2

As you can see, it is exposed wires, outside, so it seemed a reasonable guess that these were the offenders. I took my multimeter, and measured the voltage. I was surprised to see that even when I turn the switch off, I still have a voltage of 1.2 V in these cables. This surprised me.

Is it normal for these to have a low voltage even when switch is off? Could that be what causes my RCD to break? If this is abnormal, how do I go about fixing it?


PS: (I have read other questions about phantom voltage, but my question pertains to the RCD breaking in that context)

  • For clarity, these are exposed wiring connections on the outside of your house? Are they normally covered with something and simply exposed for the pics or are they always like this?
    – FreeMan
    Mar 4, 2021 at 17:43
  • Correct, it is outside my house, but under an outside roof. They are always exposed :/
    – DevShark
    Mar 4, 2021 at 17:46
  • Just so that I do not look TOO incompetent, I didn't do that. It was like this when I bought the house :D
    – DevShark
    Mar 4, 2021 at 17:46
  • I'm sure weather is getting into them. Rain does not only go straight down. Putting appropriate fixtures there will surely help weather-seal them. Or just blanking covers, well caulked. Mar 4, 2021 at 20:06
  • Ok, do you think that is what could cause the breaking of the RCD? I am not sure what the mechanism could be.
    – DevShark
    Mar 4, 2021 at 20:07

1 Answer 1


Is it normal for these to have a low voltage even when switch is off?

Yes. The internal resistance of your multimeter is very high (mega ohms), so even a tiny leakage current (fractions of microamps) through the capacitance that exists between wires running in the same conduit will be enough to read something on the multimeter.

Could that be what causes my RCD to break?

Nope, by 5-6 orders of magnitude.

I don't think the culprit is that wire nut. 300mA is a lot of current. Even if the wire nut was submerged in a bucket, I'm not sure it would pass that amount. I mean, it's pretty hard to trip a 300mA RCD unless a wire gets loose and you get an actual short.

The other wire nut only has two holes, so unless they're Live and Earth, if water gets on it it won't trip the RCD.

I'd suspect a junction box somewhere, or maybe a wall socket, or a switch, or something that gets really wet. Or it's not the rain, it's the wind instead that rattles some loose wires. Or the rain causes the mice to go back home, and as they get bored they chew on your wires. Or the insulation on some wires has been damaged, and they touch something that gets earthed much better when it rains than when it doesn't, like a metal gutter.

PS: you should really install a 30mA RCD, they're lifesavers.

  • Thanks for your help! In the 3-way wire nut, there are earth, live and neutral. In the 2-way one, there are live and neutral. Does this extra info change your assesment? I tried to look around for the causes you suggested, but I can’t find candidates. I feel these 2 wire nuts remain my best candidates. What do you think?
    – DevShark
    Mar 4, 2021 at 20:54
  • Well, you could stick the cable or conduit ends into a waterproof junction box and wait for the next rain...
    – bobflux
    Mar 4, 2021 at 20:59
  • Yes. Now that I think more about it though, the RCD was tripping even though the switch to these wire nuts was off. So there was no current going to it (apart from the 1V, but you indicated this was likely a measurment error). Does the fact the switch was off rule out these wire nuts?
    – DevShark
    Mar 4, 2021 at 21:06
  • It's always a good idea to check if the switch cuts the live wire... If it does then yeah, when it's off there's only neutral and Earth in the wire nuts. That can still trip the RCD on a hard short (not just water) in France because earth and neutral are not connected inside the house so there's always a few volts between them, so a short will allow enough current to flow to trip the RCD. In other countries where earth and neutral are tied at the electrical panel in the house, probably not.
    – bobflux
    Mar 4, 2021 at 21:11

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