RCD


Was taking down strip light in the garage conversion tonight, in order to paint ceiling.
Turned off lighting circuit MCB. Then disconnected light, checking with electric neon test screwdriver that all was dead. As I prepared to remove the light, I must have connected together the neutral and earth - and the whole garage RCD tripped.
Is that normal? Frightened the life out of me! :-)
JW
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Hi Quite normal proves the system works. Others on group will probably site technicalities but simply the breaker monitors difference between earth potential and line/neutral. Can be a pain on PME systems IIR. CJ

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Yes.
The MCB only switches live (some would say a failing of the UK way of doing things.)
If you are not on a PME supply - and clearly you are not - there can be a small potential between N and E (usually only a volt or two) but there is sufficient current when shorted to earth for it to trip the RCD.
--
Woody

harrogate three at ntlworld dot com
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Thanks - I'll tape the neutral before fiddling next time! :-)
JW
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Why do you think that the OP is not on a PME supply? The RCD will still trip on a PME supply under the circumstances he gave.
Adam
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A PME supply has local earthing, i.e. the neutral and earth are locally bonding, hence it is unlikely that there would be enough potential present to cause in excess of 30mA to be drawn.
On the other hand if it is not PME and the local earth is at the substation some distance away then there could be enough induced potential present for that level of current to be drawn.
--
Woody

harrogate three at ntlworld dot com
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Woody wrote:

large currents coming back down the neutral could give enough voltage drop to have a significant current down the earth lead if joined to the neutral some way away from the sub station. Years ago a friend of mine lived in an old house and could feel a small voltage from the neutral when working on his wiring.
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My house has PME and you neutral to earth shorts knock out the RCD every time.
---
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Hi Woody
In the case of an RCD tripping with a PME supply it is not the neutral having a higher potential than the earth that causes the trip. It is the alternative path to earth the short circuit created that allowed enough of the current from the other circuits using the RCD to pass to earth creating an imbalance in the RCD.
Cheers
Adam
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ARWadsworth wrote:

But being pedantic the only reason that current will take the alternative root down the earth wire is because there must be a potential difference (all be it fairly small). In other words no potential difference no current flow.
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Sorry - just resurrecting this thread. I managed to trip everything out again yesterday, whilst painting. To get into some awkward corners, I removed some sockets, leaving the lighting on. But in order to get the power back on again, I re-connected the socket again, rather gingerly, whilst the paint was still wet, intending to leave it loose, but connected. As I fumbled around (why the hell did I put the socket there?), I must have stuck my finger between the neutral and earth. Momentarily, it felt similar to sticking a PP3 on your tongue, and the RCD covering the whole CU went out PDQ. I guess that's normal - but thought I'd check. When I use neon screwdriver on neutral, there is no glow.
Beginning to wish there were seperate RCDs now! :-)
Cheers
JW
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That should not happen. There must be an alternative explanation.
Adam
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Just re-read my message, and it sounds as though I got a tingle on my tongue. What I meant was I felt a similar tingle in my finger.
Having cleared that up, does your response still apply? Is there anything I can do to check this out? It was all professionally installed for me.
Cheers
JW
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You need to measure the voltage between neutral and earth.
Adam
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Thanks Adam - will do that v. soon.
JW
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