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Topic Title: Changing old 30 waydistribution board
Topic Summary: Does bonding comply
Created On: 08 June 2018 09:05 PM
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 08 June 2018 09:05 PM
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tillie

Posts: 915
Joined: 03 May 2006

Hi , I have an old 30 way distribution board to change next week.

The board has a mixture of BS88 fuses , fusewire and bits of copper acting as fusewire and all busbars open to touch when you open the door to the board.

The supply comes from a substation 50mtrs away in a carpark and into a small intake room via a 25mm 4c swa installed in 1984.

The swa is being used as the earth conductor.

Zdb is 0.09.

Traced out all circuits and tested. All good.

The installation is a 100 x 100 steel trunking which runs from the distribution board out onto a concourse ( football stadium ) and runs for 80 mtrs down the concourse.

20mm heavy gauge conduit then goes into two kitchens and two toilets to feed lighting and power.

My query is with the bonding.

In each of the kitchens and toilets I have metal Wastes and pipework which comes out of the floor so I assume this is 4x extraneous conductive parts.

The existing bonding is as follows.

A 6mm cable goes from the disboard and bolted to the trunking.

Then in each kitchen and toilet a 6mm cable is the taken from the nearest conduit and bonded to the ECP where it comes up from the floor so that the trunking and also the conduit is acting as the bonding conductor.

Is this acceptable ?

I know the csa of the conduit is about 82mm and the trunking is about 324mm and when the factor for steel (8.8 ) is used it still comes out at over 6mm.

The board has to be changed because it is in a dangerous condition and they want to turn the adjoining room into an office.

Do I recommend running a bonding conductor from the disboard or leave as is ?

Advice please
 08 June 2018 10:51 PM
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mapj1

Posts: 11223
Joined: 22 July 2004

well, the trunking/conduit is acting as both circuit CPC and a bond. It may be worth a high current Zs test, but if the metalwork is in good order, it is no more or less part of the circuit than if there was a length of G/Y in the trunking. So two tests, is the resistance low enough to hold things down to 50V between things you can touch while the main fuse blows, and is it equivalent cross-section large enough to be at least as good as the bond you propose to add. I suggest taking the cross section of the steel, and assuming ten times higher resistance than electrical grade copper. It may very well be OK, if it met the standards of 1980 - it was not much different to today.
It may need a few 'electrical earth do not remove' stickers.

-------------------------
regards Mike
 11 June 2018 10:27 AM
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AJJewsbury

Posts: 17169
Joined: 13 August 2003

So two tests, is the resistance low enough to hold things down to 50V between things you can touch while the main fuse blows

Although the 50V bit is only a requirement for supplementary bonding (and where disconnection times exceed 5s) - not main bonding.

If it's true TN-S then BS 7671's basic requirement is half the required c.s.a. of the installation's earthing conductor (minimum 6mm2) - in any material - the Cu equivalent is only for PME or when calculating the copper equivalent of 25mm2 for the maximum required.

- Andy.
 12 June 2018 08:03 PM
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Weirdbeard2

Posts: 519
Joined: 29 November 2017

Hi tillie, my advice for football clubs is if they can't afford heated seats then the existing arrangement is fine, if they have heated seats then the bonding conductors to to ECPs should be at least upgraded to 10mm
 12 June 2018 10:54 PM
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Dave69

Posts: 742
Joined: 16 July 2011

Just a thought but I'd say,,,
Stuff the bonding to start with, an old board with fuse wire,,,,,, rings alarm bells, hope you have read the asbestos report!
Remember your legal duties,
 13 June 2018 10:18 AM
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tillie

Posts: 915
Joined: 03 May 2006

Hi , have read the asbestos report.

Asbestos arc shields and porcelain fuseholders was highlighted in 2006 along with some asbestos wall cladding.

The wall cladding remains but labelled and the shields were replaced with non asbestos shields.

The fuse holders are now the black plastic 4 pronged affairs with mainly BS88 fuses in .

Some have been replaced with fusewire as and when required.

Regards
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