RCD Trip - Dead Short?

andrewukr6 · 1 Nov 2017

Hi All. Caveat: I am a trained electrician but do not work in the field. Getting advice before I call in my old boss if necessary!

Ok, so RCD started tripping on a kitchen power circuit (ring), nothing new has changed. I unplugged and switched off any spurs so should be nothing connected.

I removed all the conductors for this circuit in the CCU, and was going to use my insulation resistance tester that I've never had reason to use up until now...however first I checked it with a basic fluke continuity tester, and I'm getting continuity between all conductors (L-N, L-E) which I'm pretty sure would mean a dead short.

I would think my next step is to remove all socket faceplates and start testing the runs between cables - would this be correct?

Thanks for any help

andrewukr6 · 1 Nov 2017

Right to update, First socket I opened, one direction + - no continuity, the other way continuity. The wire looks like it disappears under the concrete, possibly into an attic then back to CCU....

gblades · 1 Nov 2017

Can you elaborate on your last statement. Are you connecting the tester +vw output on L and the other terminal on N and then doing it the other way round?
And one way you get a short and the other you don't?

If you are getting continuity one way round but not the other then it's not a simple wiring fault. Most likely there is something still connected that has a fault. Often power supplies have a bridge rectifier (https://en.wikipedia.org/wiki/Diode_bridge) which consists of 4 diodes. When a diode fails it can fail as a short in both directions which then causes the device to fail short circuit but only on one half of the mains cycle. Hence continuity only one way round.

andrewukr6 · 1 Nov 2017

Yes sorry. To elaborate. In the CCU that ring circuit has been disconnected. On a plug socket I removed the facia, and tested between L - N going one direction and it was fine, (no continuity) then I tested the other leg, heading in the other direction (either to another socket in the chain or back to the CCU) and it did have continuity between L - N....

gblades · 1 Nov 2017

Ah ok. I would be tempted to leave that two cables disconnected from the 1st socket and then go round the others. Insert a mains plug with the meter across the L and E terminals and mark which sockets are still showing as a short. Then go about half way between the faulty ones and disconnect the wiring and do the same again for all the marked sockets. If the socket is ok then remove the mark. That should enable you to identify the faulty patch of cable without having to remove every single socket (hopefully).

andrewukr6 · 1 Nov 2017

I think I've actually narrowed it down...I think the first socket I tested was the first point in the ring, and the dodgy cable is running behind kitchen cupboards, behind a structural beam, plastered into ceiling, into attic, then to the CCU so pretty much the worst cable in the house to need replacing! I'm going to see if I can see if a rodents been eating on it under the cupboards.... thanks for help!

ban-all-sheds · 1 Nov 2017

andrewukr6 said:
the dodgy cable is running behind kitchen cupboards, behind a structural beam, plastered into ceiling, into attic, then to the CCU

And in safe zones all the way!

jg321 · 2 Nov 2017

andrewukr6 said:
'm getting continuity between all conductors (L-N, L-E) which I'm pretty sure would mean a dead short.

It could also mean you have a load still connected. What resistance were you getting for L-N/L-E?

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RCD Trip - Dead Short?

jg321

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