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Mains supply 100A rcd switch faulty?

Postby portised » Sat Jul 11, 2009 9:41 pm

Our renovated house has just had a full rewire to 17th edition by electrician. We have a split load twin rcd consumer unit with 14 mcbs fitted (big house). The main supply comes in through normal fuse etc to meter, then through a 100A 100ma rcd and then to the consumer unit. House is very old and fuse, meter and 100A rcd were already fitted previously to an old fuse box. The system uses an earth rod in ground. Earth 16mm2 and mains 25mm2. The 100A 100ma RCD main switch is black bakerlite type about 6 inches high by 2 inches square, the main neutral, live and earth are fed through this rcd to the consumer unit. Electrician tells me today main 100A RCD is faulty as on checking circuits he has 75 ohms resistance on earth loop impedance check, with the earth on 100A rcd linked together this is 0.9 ohms. So my question is should it be changed and whose responsibility is it? Should I contact electricity company or do I have to pay and get my electrician to change it? On his fluke test meter circuit tests all mcbs and rcds in consumer unit work OK and main switch 100A rcd works too, so I am a little confused. Thanks for any help.
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Postby sparx » Sun Jul 12, 2009 8:02 pm

Hi portised,
several points here:
First why did your 'electrician' leave the original trip switch there?
as you say the new board has 2 RCD's fitted to 17th recommendations.
Second I don't believe it is an RCD at all but a Voltage operated type due to having earths connected to it.
I am not confident about your electricians knowledge!!!
A typical earth rod reading will be in the region of 75 Ohms in my location and I doubt one would ever be 0.9 Ohms!
I would have the old trip switch replaced by an isolator switch and the incoming & out going earths connected together in an earth block.
Further you say main earth 16mm, is this from rod? max required is 6mm!
If wired from rod to new board it will bypass old trip rendering it inoperative anyway!
Voltage operated types required earth from rod to one term on unit and earth from units other term to consumer unit, thus any earth leakage would be 'seen' by unit and cause trip.
RCD's work on a balance principle whereby any current passing thro' Live should return thro' Neutral, any difference must be by earth leakage, thus no earth connections needed,
There are NO testers that can prove old style VOCB's,
Most older units were made by crabtree or wylex and had a yellow test button,
regards Sparx
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Postby moggy1968 » Sun Jul 12, 2009 10:06 pm

It sounds to me as though when he did the earth loop impedence he didn't disconect the main bonding. The main switch operation has no effect whatsover on this. I had one last week giving almost the exact same results as this with and then without bonding attached (70Ohms and around 1 ohm respectively).
As sparx said, this switch should not be there, it shold have been replaced or, prefereably, removed altogether (assuming your meter is within 3 metres of your DB) about 50% of the time these old units fail to operate and they are now illegal (and have been for sometime, so it should have been removed)
The size of the earth does depend on the nature of the rod, although 16mm does seem somewhat OTT!! dependent on the rod installation method 4 mm is more like it.
basically, as sparx says, if your new board has all circuits protected by the 30ma RCDs then you don't need this main switch, providing the meter is less than 3 meters from your board.
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Postby portised » Mon Jul 13, 2009 7:39 pm

Thank you very much for your knowledge and help with this problem. It is very much appreciated that people like you are prepared to help and take the time. Thanks again, I think we will remove it altogether.
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