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Topic Title: Light circuit fault
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Created On: 19 December 2012 12:12 AM
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 19 December 2012 12:12 AM
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SKElectrical

Posts: 910
Joined: 01 February 2009

Cust got a shock when touching radiator.
There is a fault on the light circuit - cpc is 230V in some rooms (metal switches too) but only 30V in other rooms, 60V in other rooms. Circuit is over 4 bloody floors too.

I'm assuming... a nail through a cable has severed cpc path back to c/u but has simultaneously touched onto live? Also to explain shock at radiator im assuming they are standing on the offending nail (shoes were being worn tho), or mech pressure onto a pipe.What do you think?
Also why would the voltage on cpc differ switch to switch?

(this fault in two houses im on at moment)
 19 December 2012 12:41 AM
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DOUGIE1000

Posts: 4159
Joined: 13 August 2005

I would be checking Ze and IR

Edited to add thought voltage was on radiator also.

-------------------------
Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!

Edited: 19 December 2012 at 07:35 PM by DOUGIE1000
 19 December 2012 06:53 AM
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normcall

Posts: 8111
Joined: 15 January 2005

Voltage between radiator and earth/neutral?
Type of meter being used?

Depending on answers, would suggest way forward.

-------------------------
Norman
 19 December 2012 09:11 AM
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AJJewsbury

Posts: 11295
Joined: 13 August 2003

cpc is 230V in some rooms (metal switches too) but only 30V in other rooms, 60V in other rooms.

30V/60V sound like c.p.c. isn't connected at all - the meter is just picking up capacitive coupling from nearby line conductors (I'm guessing it's a high impedance (electronic) voltmeter).

I suspect its one of those botched job situations where the original lighting circuit didn't have a c.p.c. (e.g. pre 1970s) and so what c.p.c.s there are don't connect to anything useful. Obviously there's a L-PE fault somewhere near the 230V bit.

What else were they touching at the same time as the radiator? It need not even be obviously metallic - "live wall" syndrome and all that.

- Andy.
 19 December 2012 01:39 PM
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potential

Posts: 1255
Joined: 01 February 2007

This type of job needs a long wander lead connected to an earth rod and a volt meter that requires current (more than microamps) to operate it.
An old moving coil AVO would be ideal.
Connect the meter to the wander lead and a test probe.
Systematically go through house measuring the pd between whatever the test probe is touching and true earth.
 19 December 2012 02:35 PM
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AJJewsbury

Posts: 11295
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volt meter that requires current (more than microamps) to operate it.
An old moving coil AVO would be ideal.

Or any old electronic voltmeter shunted with a decent resistance (say a GLS lamp) - or for the really old school a couple of pygmy lamps and forget the meter

- Andy.
 19 December 2012 02:37 PM
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Zs

Posts: 2789
Joined: 20 July 2006

SK, Wow, 230V...I assume you have told them to go and stay somewhere else until it is found and corrected?

I'd not be assuming the nail in the floor being stepped on. Last one of these I had was a melted connector block under a floorboard, with an added sheared earth compressing into the cable. On the socket circuit. In effect a short circuit (L-E), but no bonding and a voltage in the copper pipes as a result of it. A cocktail of issues which were only bought to light by the lady of the house getting a shock in the shower. It took days. It was a separate shower circuit. Break it into little chunks and re build it as you test.

Good luck, two floors were enough.



Zs
 21 December 2012 01:37 PM
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mapj1

Posts: 2528
Joined: 22 July 2004

First things first - if its not obvious, isolate the circuit that is making the radiator live (pulling circuits off in turn.,). It doesn't have to be the lighting circuit you mention, but let's hope it is.
Then using a wander lead, find the last point in the offending circuit that actually seems to have a sensible resistance back to earth .
Also, I'd expect radiator to be earthed, at least via piping unless its obviously plastic pipes.
As others have noted, high impedance meters, i.e. nearly all digital ones, can read picked-up voltages that don't neccesarily require a direct conection, just wires in the proximity of somthing live. The clue is that the voltage wanders with humidity, where you hold things and then dissappears altogether on the connection of any kind of load. It sounds like the 30v/60v are just symptoms of 'its floating' i.e. not connected at either end.
If you are brave you could try the effect of adding a positive bond between the cpc of a (known good) socket circuit and part of the lighting circuit you belive to be floating and see how much of the circuit now behaves.
This will allow you to work out how may separate earth islands the circuit is broken into - and maybe, just maybe, which fitting or group of fittings has the L_E problem.
In the dark ages (i.e the first 20 years after 1970 when lighting ciurcuits were first supposed to have an earth core), the earthing of lighting circuits was considered a bit of a good advice lifestyle option rather like 'dry clean only'. Many ceiling roses didn't actually have anywhere to terminate a cpc, so the earth cores were sometimes clipped short, sometimes stuffed up into the ceiling void, maybe twisted together hopefully, seldom sleeved, and occasionaly actually connected to each other with a bit of choc-block.
In one awesome cock-up case I've seen with loop-though-rose wiring where the installer/ modifier didn't know what to do with the CPC and so went for the last free screw, which happened to share the brass-bock with the un-switched live. Maybe this house was done by the same chap.
Good luck, and do keep your wits about you.
regards Mike.

edited for inability to type

-------------------------
regards Mike

Edited: 21 December 2012 at 01:52 PM by mapj1
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