DIY Doctor

So what on earth??

Postby Peter88 » Sun Feb 12, 2012 4:47 pm

My downstairs power circuit has a fault. Doesn't affect any other circuit.
Originally it blew the circuit breaker, attempst to reset were met with instant back off.
On checking at the fuse only ONE wire goes to live phase. Although I thought it was supposed to be a ring.
Can't find any other 'spare' wire.
I disonnected the first socket (at least I belive it is the first in line. I then introduced a fly lead from the CB to carry power. My theory being to establish at which point there was a fault. I worked around the kitchen (1st room on the circuit) this was all ok. Fault seemed to be in my lounge. Tried a particular socket and found I was good up to that point, but not beyond.

Anyway, I then find an obvious issue which I sort out. (A connection had overheated). Restored sockets to find I now have no power from the main board. ( definetly HAD had at the start of this.)
So I put in a heavy duty bypass wire. Power restored.
I later realise that power is off in a group of sockets part way around the lounge. Bear in mind that if its all a spur then sockets beyond the fault ARE working. I investigate and secure a loose wire. I now have pwer at this small group of sockets......... BUT NO power anywhere else.
Further I am getting a "earth live reverse" warning at one socket.
Frankly I hvae no idae what I do next apart from check the wires I just fixed. Anyway... everything else comes back on.. but teh group of sockets goes back off.
Yeah. I'm getting tired at this point.
Bit more hitting it with a hammer (joke) and it all comes on.

So... whats up? Clearly I hvae a fault and even though I am half minded to just accept its working I know I can't. And anyway.... I don't have a genuine feed through the proper cable.

Any ideas? I just ca't figure out what could be wrong. how can power get past a no power fault to start? How come a previously live phase is now dead?
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Simply Build It

Postby BLAKEY1963 » Mon Feb 13, 2012 5:18 pm

get in an electrician to help you

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Postby ericmark » Mon Feb 13, 2012 9:40 pm

One can't do fault finding with a lenght of wire and a hammer. You need a full test set and clearly more knowledge than you have.

Full marks for trying but the most important thing is to accept when it has become a problem beyond your ability and pass the work on to some one who has the skills and equipment required.

Although in time we may be able to help you gain the skills without a full test set (around £750 to buy and £75 to hire) it still would not help. DIY is unlikely to be cost effective or safe.
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Postby Peter88 » Thu Feb 16, 2012 12:18 pm

I assumed my reference to a "hammer" would be understood to be humour.

What's wrong with using a 'fly' lead'? I was unable to put any power to the circuit, to do any test, the CB would not energise. Using a fly lead I was able to quickly establish what part of the circuit was ok and what wasn't.

I am actually qualified. I've passed the technical scope for 'Domestic Installer" and Wiring regs 16th edition. But it was a while ago.. and I have never desired to use the qualification. I've been ill for some years.

Anyway, I thought this was "DIY" doctor? I just wanted some help and advice fellas.

I'll do it myself..
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Postby ericmark » Thu Feb 16, 2012 7:39 pm

If you work out from the consumer unit with an impedance tester one can normally see a point where the reading become erratic. The load used with the tester is quite high so any bad connection is likely to fail.

Not really the way one should test but it does work. I had one call where I could not work out what had gone wrong which transpired to be caused by lose connections in the consumer unit where the supplier had put the MCB's loosely clipped into the consumer unit and the electrician had not checked the screws so they were still lose.

Comparing the line and neutral readings with a low ohm meter drawing at least 200ma will also often high light problems as will insulation tests using a 500v ohm meter.

In theroy we should follow the tests laid down when we took our C&G2391 inspection and testing but it does take quite a time to go through all the tests so we start by looking for likely faults.

So I will try and list what you have told us so far.
1) Started with a short circuit
2) Wired as radial not as a ring
3) Lost part of radial
4) Started working again.
So we will assume the sockets have been overloaded due to being wired as a radial with a fuse/MCB designed for a ring possibly was originally wired as radial with 4mm then some one thinking it was a ring has used 2.5mm cable in some part. If this is the case likely the 500v insulation tester will confirm there is some cable which has melted then cleared the short circuit.
In the main walls tend to cool cables so other the faults can be seen at the socket. But this is not always the case. With poor readings at my father's house I was able to isolate with the meters which cable was at fault.
With luck you may see a fault but again you may not and if the fault is under floor boards the next time it may be the fire brigade doing the investigation.
So what do you think we should be saying? Open up every socket and have a look and if you see nothing don't worry? or you may have a serious fault you need to use the appropriate test gear and investigate further? Clearly the latter.
With a modern house where near every circuit is protected with a RCD DIY is not so much of a problem. And with old houses with fuses it was not as much of a problem as after we moved to using MCB's. The typical MCB used on a ring main is the B32 which will in the fullness of time trip at 32 amps but for it to trip quickly it needs 160 amp to trip the magnetic part of the trip. So using impedance testers is now far more important then in the days of the fuse.
So all I can suggest is get some good insurance.
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