DIY Doctor

Electrics trip once after replacing a few light switches

Postby Sytra » Fri Sep 25, 2020 11:52 am

Anyone got an idea what could be wrong? I have replaced switches, wiring etc before with no problems however, in our new house I have replaced a couple of light switches.

The problem is that after the switch has been changed and power turned back on, as soon as the light is turned on it trips, it only does this once, reset it and it's perfect from then on. It's done this with the last couple of switches I've replaced.
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Postby ericmark » Fri Sep 25, 2020 9:34 pm

I know with my own house the 4 switches at the front door are fed from two different supplies, and there was an error when I moved in, where the wrong neutral had been used.

30 mA is about 7 watt, so with small LED bulbs one may get away using the wrong neutral, there are two tests, one uses an insulation meter the other a clamp on ammeter, although the insulation resistance at 500 volt may show 1MΩ which should equal 0.23 mA this is measured using DC, so with 50 Hz AC the leakage may be well above this.

We are told for a 30 mA RCD the limit for all circuits using the RCD is 9 mA however my clamp on will not measure that low, so never measured, I just fitted a RCD on every circuit combined with the MCB as a RCBO. So there are 14 RCD's in my house, but many houses combine circuits through the RCD, so common only to have 2, which will increase the likelihood of them tripping.
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Postby Sytra » Fri Sep 25, 2020 10:17 pm

ericmark wrote:I know with my own house the 4 switches at the front door are fed from two different supplies, and there was an error when I moved in, where the wrong neutral had been used.

Hmm, makes sense, it's an ex council property and it's been bodged up all over the place. We have found 1 socket upstairs on the downstairs ring and 2 downstairs that are on the upstairs along with the old red / black mixed with new blue / brown but it's not marked anywhere so it's been getting confusing lol.

We are changing wire where we can but obviously being DIY its limited to what we can do ourselves.
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Postby ericmark » Sun Sep 27, 2020 1:20 pm

Our house split front to back with sockets, and up/down for lights. It makes sense to do it that way as less cable used, so better loop impedance, and if you can't reset one, the you don't need extension up/down the stairs.

However this is all well and good with RCBO's but with RCD on the cheap, with only two, if lights and sockets not split the same, then can't arrange so every room has two independent supplies.

I only had two RCD's with last house, a real pain, specially when in the garage, so this house all RCBO protected.
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