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Topic Title: rcd test button works with no supply ?
Topic Summary: strange fault
Created On: 05 November 2012 05:59 PM
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 05 November 2012 05:59 PM
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aligarjon

Posts: 2828
Joined: 09 September 2005

i had a weird fault this morning. customer had no power. in her meter box is a time delay rcd and a switch fuse. the rcd is an mem with a white fly lead. i pushed the rcd test button and the rcd operated. i put my tester on the outgoing terminals and there is no supply. simples i thought, dodgy rcd. i dapped my tester on the incoming terminals. no supply.

It just so happens that there were no seals on the main fuse or the meter so i dropped the covers off and there was no supply to the meter, yet with the main fuse out i had 240v from the incoming live to the neutral on the meter. blimey the main fuse has gone, i haven't seen that before, i tested the fuse and its fine. .

I'm presuming she has lost her neutral but can't understand why the rcd operates. I'm guessing its something to do with the fly lead connected to the TT system ? I also can't understand why i was getting 240v from the incoming supply cable with the fuse out, but nothing with the fuse in .

She was contacting her supply company when i left.

Any ideas ?

Gary

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Specialised Subject. The Bleedin Obvious. John Cleese
 05 November 2012 07:24 PM
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mikejumper

Posts: 1714
Joined: 14 December 2006

Originally posted by: aligarjon
I also can't understand why i was getting 240v from the incoming supply cable with the fuse out, but nothing with the fuse in .
Gary

What were you using to measure the voltage?
 05 November 2012 07:39 PM
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aligarjon

Posts: 2828
Joined: 09 September 2005

I was using a Steinel 2 prong tester Mike. she phoned me back just now and apparently there was a problem with one of the connectors on the pole in the garden. 4 vans and 4 men turned up.

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Specialised Subject. The Bleedin Obvious. John Cleese
 05 November 2012 07:43 PM
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markwatson

Posts: 14
Joined: 26 May 2006

Hi Gary,

I have come across something like this before. It was also a TT system. When I arrived they had no power. I checked connections at the consumer unit with it on and there was no voltage between live and neutral, but with the consumer unit switched off the lcd display on the meter came back on and the rcd tripped and I did get a voltage between live and neutral. I worked out that it was a nearly burnt out neutral connection at the pole outside that with no load on was just about making contact but as soon as any load was applied it was breaking again.
The board came out, verified it was a dodgy neutral connection and sorted it.
Cheers,
Mark

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sparkymark
 05 November 2012 08:10 PM
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daveparry1

Posts: 6201
Joined: 04 July 2007

Simply a high resistance connection on the supply, with no load applied there was enough current to give you a reading, once there was a load there wasn't! Just like measuring a dead battery, off-load the normal voltage would probably be measured, put a load on and it would be dead,

Dave.
 06 November 2012 05:32 PM
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cmatheson

Posts: 211
Joined: 16 January 2003

Originally posted by: daveparry1

put a load on and it would be dead,



Dave.


A burned neutral on a 3 phase supply gives even more interesting results. Put a load like the OP on it and I wonder if he would live to forget to verify he has an earth and measure voltage live to earth and neutral to earth.

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Chris Matheson MInstMC
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