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Topic Title: Trunking Bonding
Topic Summary: Should an earth be run from the earth bar to the metal work
Created On: 16 April 2018 05:24 PM
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 16 April 2018 05:24 PM
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Ruggedscot

Posts: 6
Joined: 20 December 2017

Question -

A section of trunking has been sat on top of a number of distribution boards. These boards are directly underneath the trunking and cables pass through from the boards out to the final circuits via the trunking.

Should there be an earth cable from each earth bar contained in the distribution boards out and attached to the trunking directly. No reliance being placed on one earth or through any mechanical fixing from distribution board frame to trunking. I believe that there needs to be an earth conductor from each DB earth bar to the trunking and this secured by proper nut and bolt.
 16 April 2018 07:26 PM
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sparkingchip

Posts: 11457
Joined: 18 January 2003

So you want multiple main protective bonding conductors to the trunking?
 16 April 2018 07:53 PM
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Alcomax

Posts: 399
Joined: 12 November 2009

Ruggedscott, change the title of the post to Trunking Earthing as you are likely querying the need to ensure adequate earthing to an exposed conductive part, likely the metal trunking?
 16 April 2018 08:00 PM
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dustydazzler

Posts: 2772
Joined: 19 January 2016

exposed conductive parts

Yes they will need 'earthing'
 16 April 2018 09:40 PM
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mapj1

Posts: 11568
Joined: 22 July 2004

but earthing via the conduit or boxes that screw into to it may be enough - test the impedance.

-------------------------
regards Mike
 16 April 2018 09:49 PM
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leckie

Posts: 4705
Joined: 21 November 2008

Well yes it wants a suitable earth, but it does not need a separate earth from the earth bar of each DB, which is what the OP is suggesting if I am understanding the post correctly.
 17 April 2018 07:54 AM
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Ruggedscot

Posts: 6
Joined: 20 December 2017

I may not have described it properly. Trunking above dis board enclosures. dis board enclosures not bolted to trunking. holes in trunking align with dis board enclosure holed holes are protected. cables pass from dis board through into trunking on way to final circuits as expected. Supplies to dis boards come in to bottom of dis boards cables terminating through swa glands etc. usual installation.

I would have thought that the trunking metal would should be bonded to the dis board earth. I said using the db earth bar and running a earth cable out would be the best way to do this. now as there are multiple supplies from the main switchboard to this I would have said that either linking the earth bars together from the 4 distribution boards and then to the trunking or a link form each earth bar to the trunking what ever way there would need to be an earth provided to the trunking and bonding between the four supply earths. ( my view what happens to the earth if one supply cable is damaged and that one was being used as the earth )
 17 April 2018 09:14 AM
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potential

Posts: 1736
Joined: 01 February 2007

What are the impedances of the trunking to the earth bars?
Are all the earth bars bonded together?
If not I don't think you should use the trunking to bond them, which is what you are proposing.
 17 April 2018 09:25 AM
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AJJewsbury

Posts: 17445
Joined: 13 August 2003

The bottom line is that while (steel) trunking need to be adequately earthed, there's no requirement in BS 7671 to do it in any particular way. Adding copper wire(s) as you suggest is certainly one option, but there are many other possible methods allowed - see regulation 543.2.1. Typically steel trunking is earthed indirectly - e.g. by being bolted to the earthed enclosures of other equipment or even via steel conduit to steel clad accessories. For example if each of outgoing circuits has its own copper green/yellow protective conductor, connected to downstream accessories, and those accessories are in good contact with the trunking, the trunking is probably perfectly adequately earthed, even if there's no direct contact at the DB end. If on the other hand there are no G/Y wires and the trunking itself is being relied upon to provide an earth to downstream equipment you'd certainly want a good connection directly between the DBs and trunking at the DB end.

In general there's no requirement to consider the case of a broken protective conductor (it's just one of the possible faults in the two-faults-to-danger approach) - duplicate earthing is normally only required where there is large protective conductor currents in normal service (usually >10mA) and where the c.p.c.s are small enough that there's a reasonable expectation that they might become broken (e.g. <4mm if mechanically protected, <10mm2 otherwise).

- Andy.
 17 April 2018 09:26 AM
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broadgage

Posts: 3016
Joined: 07 August 2007

With 4 DBs I would be inclined to run an earth from each of the 4 onto the trunking.
If I have understood correctly, the supplies INTO the DBs are not routed via the trunking, so the only risk is the trunking becoming live via a damaged final circuit cable within the trunking.
These 4 earthing conductors could therefore be sized as being at least equal to the largest sub circuit conductor.

I would not be keen on earthing the trunking via only one or two of the four incoming supplies lest this be removed perhaps to replace it, and the trunking then being left not earthed.
 17 April 2018 09:42 AM
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dustydazzler

Posts: 2772
Joined: 19 January 2016

As above

IF it were me , I would just grab some off cuts of 10mm G/Y and earth lug the trunking form each individual board

a bit over kill I know but at least it done up like a kipper and you can sleep well
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