DIY Doctor

Faulty circuit - how do I rectify?

Postby tammas2000 » Sat Feb 21, 2009 12:02 pm

I was replacing an immerser switch with a 3a fused switch for under unit lights in my kitchen and found a fault.
The story goes: the immerser fuse has been withdrawn since we moved into the house 12 years ago.
I tested with a neon tester and the circuit was dead.
I inserted the fuse in the immerser circuit and identified the live incoming cable. Obviously it became live and that was how I knew the feed cable.
This is where it gets complicated!
Directly underneath the switch is a double socket, part of the ring main.
Both the socket and the switch are connected to a backing plate made of metal, which I can only assume is a "converted" cooker circuit's backing plate as it's huge and messy.
The double socket is connected to the lugs on the back plate, the switch was fixed by stuffed plaster around a plasterboard plate!
The immerser cable runs up through the socket, bypassing it as it goes directly to the switch. For some reason the cable was stripped to the bottom of the socket backing plate (as is the ring cable), and they had snipped off the earth at that point!
The earth of the outgoing ring main cable and the earth of the supply cable to the immerser were twisted together and connected to the switch's earth terminal. (How dangerous could that be as that means the earth circuit of the ring is not solid?).
I left the sockets live and had the immerser fuse out, and tested the immerser cable was dead then went to wire up the switch.
The switch was in the "on" position as I wired it. As t was dead, it was convenient to connect the neutral supply to the lights last.
When I touched the neutral wire I got a shock.
I tested the live coming into the switch and it was live too.
I checked the fuse was definitely out and used insulate tools to disconnect the feed to the switch.
On testing it, the feed was now dead.
I inserted the fuse and it became live, withdrew it and it was dead again.
The only alteration I have made was to earth the backing plate of the socket.
Is there an intermittent short somewhere between the immerser cable and another feed, or could there be any other reason for this happening?
I'd understand if you thought I'm mistaken in the events above, but I can assure you that the events happened in the order stated and the testing was accurate.
Any thoughts would be more than welcome!

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Simply Build It

Postby kbrownie » Sat Feb 21, 2009 5:49 pm

Tammas, my thoughts are firstly call your local electrician in and secondly don't use a neon screwdriver for anything other than screwing and unscrewing grub screws or stirring your tea, this is not reconized or suitable test instrument and should not be used or sold as one.
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Postby moggy1968 » Sat Feb 21, 2009 7:31 pm

to reinforce what brownie has said, this looks complex and needs a sparky with the proper diagnostics to sort it out.
If you drop one of these 'tester' screwdrivers by the way the internals can become damaged and the brass end piece can become live when you test, giving you a healthy bang. also, they are not reliable enough to test dead. an apparently dead circuit could be a fault with the screwdriver. using the correct dead tesetr after testing a circuit we would always then prove the tester, either on a live circuit, or preferably usineg a proving unit. the screwdrivers are so unreliable in operation to make even this dodgy. IMHO they shoould be banned!
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