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Topic Title: Can anyone explain
Topic Summary: Mystery earthing
Created On: 31 January 2013 02:28 PM
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 31 January 2013 02:28 PM
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DR2366

Posts: 705
Joined: 09 April 2006

Came across an installation where the supply cable and earthing is a standard TNS as the braid goes to a separate MET block into which is the bond to the gas meter.

However:

Out of the side of the service head is a nice bit of 16mm G/Y which ends in the MET of the consumer unit along with the bond to the water which makes it TNCS.

So did the electrician who rewired the property some 20yrs ago decide to make it bit of both or is there another explanation such as a decision to convert from one to the other due to high readings etc by the supplier?

It can't be both and you can't tick 2 boxes on the certificate

The fuse is sealed and would a quick call to the supplier resolve matters as they could tell you what system it should be?

What would your course of action be?

Thanks in advance.

Edited: 31 January 2013 at 02:38 PM by DR2366
 31 January 2013 02:56 PM
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geov

Posts: 173
Joined: 22 February 2004

What Ze are you getting from each main earth?

In my experience there's no such think as a "quick call" to the DNO and once you do get through they often have no record of the main earthing arrangement!

Regards.
 31 January 2013 02:58 PM
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Parsley

Posts: 960
Joined: 04 November 2004

Probably a high Ze on the sheath and the spark possibly got feed up of waiting on the phone to be put through to someone who understood the issue, so decided to make his own connection but if it's been sealed since it might be legit.

In my recent experience the suppliers are now better than they were a few years ago.

Regards
 31 January 2013 03:00 PM
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AJJewsbury

Posts: 11269
Joined: 13 August 2003

Out of the side of the service head is a nice bit of 16mm G/Y which ends in the MET of the consumer unit along with the bond to the water which makes it TNCS.

It might not be - I understand some cutouts have a facility no omit the N-PE link and connect the side terminal to the cable sheath only for TN-S setups.

Two different supply earth connections, with separate connections to different extraneous-conductive-parts seem very odd though - certainly BS 7671 should object as you don't have one single MET for the installation.

Other possibility is that someone's connected to the terminal on the side of the cut-out because it's there - when in fact it's not actually connected to anything internally. Worth a quick check with a meter I'd suggest.

- Andy.
 31 January 2013 03:31 PM
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DR2366

Posts: 705
Joined: 09 April 2006

The builder is just starting the loft conversion so not even due to visit the site for a few weeks.

Here is a pic he sent me if I have worked out how to add it!

Earthing Pic

Testing the Ze of each earth conection would be my first action.
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