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Topic Title: Bolting earths to metal enclosures
Topic Summary: SWA especially
Created On: 24 July 2015 12:17 PM
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 24 July 2015 12:17 PM
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Baldyhugh

Posts: 353
Joined: 01 June 2007

Hi all

I posted a question a month or two ago along the lines of the following. I had a 100A 230v metal clad fused switch supplied with pvc / pvc tails. Load out was via 3core armoured using one core as earth. An earth terminal was fitted in the switch but I found they were not big enough to accept the cables and poor quality so asked if it was acceptable to join the incoming and outgoing earths at the swa banjo where a 6mm bolt goes through the two lugged cables, through the banjo, through the enclosure with a nut on the outside. I received comments saying that this was ok, however, having a nosey through other threads today, it was mentioned that there was a regulation saying that it was not a good idea where the metal enclosure was used to continue the earth path incase corrosion could impair the connection over time i.e. the incoming earth should terminate at the earth bar and the outgoing should terminate here also, then an earth to the banjo from here also. I only have a copy of the red regs to hand, I know a bit old, 543.2.4 is the closest that I can find relating to this. In my case my two crimped lugs are in direct contact with each other, my armour is getting a good reliable contact with earth, a small amount of paint was removed inside the enclosure under the banjo tag so enclosure has a good earth so although my bolt goes through the metal case, it could be argued that i'm not relying on the case for continuity, just tightness in my opinion. Interestingly, I bought a bare metal enclosure the other day with din rail to mount mcb's rcd's etc and it didn't have an earth terminal with separate holes but came with a bolt and nut, just like my 'amendment'. Opinions welcome

Thanks
 24 July 2015 12:25 PM
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aligarjon

Posts: 3854
Joined: 09 September 2005

What you describe seems fine to me. i suspect the reg is telling you to fit a fly lead from the banjo to the earth bar. as you are not using the earth bar as an earth terminal it doesn't matter.

Gary

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 24 July 2015 12:44 PM
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Baldyhugh

Posts: 353
Joined: 01 June 2007

Thanks Gary

Normally I would use proteus equipment which I find good at there is a good sized earth terminal with three holes. I would fit my incoming and outgoing earths to this and from the third would run an earth to the banjo so that the earth terminal is 'doing the work' however my initial post related to a different brand of accessory which I don't normally use and was of inferior quality hence the question regarding a 'work around'. Another point if you would care to comment on is that many earth connections to metalwork that I have seen involve a bolt coming through said metalwork and then a nut on the other side to tighten it. Your lugged earth cable then goes on top of this nut with another nut on top to tighten it. This always seemed a strange and possibly inferior connection as I always connect my lug directly to the metalwork rather than having a nut in between the two surfaces. Seems to over complicate the thing and leave the possibility of a loose connection. I can see no engineering reason why it would be an advantage

Thanks

Peter
 24 July 2015 08:18 PM
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sparkingchip

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Joined: 18 January 2003

When using the third core of the SWA as a main earth strip back the SWA allowing the core to run to the MET as an unbroken conductor, then bond the SWA armour to the metal enclosure and then run another conductor to the MET to bond both the SWA and the enclosure.

Belt and braces job.

Andy
 24 July 2015 08:31 PM
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perspicacious

Posts: 7854
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When you say bolt and engineering in the same breath, I hope that precludes the common usage of "gutter bolts" to maintain an earth through the "banjo"......

Regards

BOD
 24 July 2015 08:59 PM
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sparkingchip

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If you run the third core out of the enclosure to the MET as the main earth conductor you are not relying on the integrity of any bolted connections or crimps, yes it's a bit of extra SWA, but it overall it saves labour as well as reducing the risk of installation error.

Andy
 24 July 2015 09:16 PM
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perspicacious

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Isn't it a requirement that the SWA itself has to be "earthed" and if it isn't, you might as well use 3093Y/3183Y?

Regards

BED
 24 July 2015 09:29 PM
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sparkingchip

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How about the issues with small scale wind turbine masts and lighting?

The recommendation I have seen is to isolate the SWA from earth between to plastic boxes leaving the SWA armour as mechanical protection that is not earthed as lighting hitting the turbine mast could wreck installation and installed equipment.

Andy
 24 July 2015 09:56 PM
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Brightspark1967

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Hi baldyhugh,

This maybe of some help, some isolators I have installed have a bolt with two nuts inside the enclosure for the earth conductors. The way is see it, is that if you have a seperate bolt for the enclosure and another for the conductors then you are not relying on the enclosure to maintain continuity. I'm sure this same method will be suitable for your isolator. This should be made on the main part of the enclosure not a removable plate.
 24 July 2015 10:41 PM
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Baldyhugh

Posts: 353
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Sparkingchip
With your method how would the sea armour be earthed if you continue the swa Earth on through to the met?
Bod
Hex bolts and nuts not roofing bolts
Brightspark
If I read your post right there is one bolt with two nuts even though you say later that there are two bolts however in the one bolt case I believe having a lug bolted on top of a nut which is in turn bolted to the enclosure is an inferior connection as opposed to what I like to do where I bolt the lug or lugs directly onto the banjo using a hex bolt and locknut. In my opinion two nuts leave more opportunity for slackening and faults than one
 24 July 2015 11:13 PM
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sparkingchip

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With the earth bond that you would install anyway.

Andy

It is the use of the third core of the SWA I am considering, why cut it off to then extend it?
 24 July 2015 11:30 PM
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Baldyhugh

Posts: 353
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You are suggesting running your earth continuously through the enclosure to the met then back from the met to Earth the banjo and hence enclosure. Whilst I agree that it would have the desired result there would be more joints than necessary and all depending that the met is adjacent to the switch. Imo a properly crimped lug bolted directly to a banjo also making good contact with the enclosure is as reliable a connection if not better than a screw terminal of a Henley block
 25 July 2015 01:40 AM
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Baldyhugh

Posts: 353
Joined: 01 June 2007

Originally posted by: sparkingchip

With the earth bond that you would install anyway.



Andy



It is the use of the third core of the SWA I am considering, why cut it off to then extend it?


Sparking chip

You could also argue why run a cable one way to have to run another one back? Unless there is a requirement for the eArth of the armour to be continuous? Thanks for your suggestions however, much appreciated
 25 July 2015 03:55 PM
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mapj1

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The recommendation I have seen is to isolate the SWA from earth between to plastic boxes leaving the SWA armour as mechanical protection that is not earthed as lighting hitting the turbine mast could wreck installation and installed equipment


Often it is only really useful to have a braid break if you are breaking the cores as well !! (like at a substation transformer, where primary and secondary may move relative to one another) .
Such tricks can certainly be played, and the most common is at a TNCS-TT transition, but you need to be very clear what exactly in voltage terms moves with what, and what "stays behind". I'm not sure of the turbine case per-se, but I'd want to see a block diagram that showed exactly where the offending current comes from and where runs to ground, and ideally some tests or at least calculation to back it up - willy nilly breaking earth paths "for lightning" is bit odd and may make a problem worse in many cases.

-------------------------
regards Mike
 25 July 2015 04:51 PM
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sparkingchip

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I'm suggesting that if you are using the third core of a SWA as a main earth conductor it should run unbroken from the MET to the earth bar in the distribution board, cutting it off then extending it again is counter productive.

The earthing of the enclosures and SWA armour is still required and would be a parallel earth path without any doubling back.

How long are you planning the tails to be from the main switch? I would not expect them to be more than a metre or so in most situations.

Andy
 25 July 2015 05:28 PM
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Baldyhugh

Posts: 353
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Hi Andy

I would still be earthing the armour and enclosure with my method just that I wouldn't be continuing through the enclosure and doubling back. Is there a requirement for the earth to remain unbroken or is this just your personal preference? Also what is your preference for footing your earth to the banjo? Do you bolt through the enclosure and banjo or bend the banjo back like you would for a plastic enclosure? I have seen some who put a bolt through the enclosure and banjo then a nut then the earth cable then another nut. I don't particularly like this setup and prefer the earth lug in direct contact with the banjo with the bolt going right through lug, banjo and enclosure with a washer and lock it on the outside

Peter
 25 July 2015 06:00 PM
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weirdbeard

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Originally posted by: perspicacious

When you say bolt and engineering in the same breath, I hope that precludes the common usage of "gutter bolts" to maintain an earth through the "banjo"......



Hi Bod, I like their low profile look on the outside of a box and there's something about the way the square nut digs in that I like.

I wouldn't have thought the shape of nut/ bolt, if it's providing a secure connection would be high on the list of commonly encountered SWA problems?

-------------------------
:beer)
 25 July 2015 07:12 PM
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sparkingchip

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Metal composition not shape.

Andy
 25 July 2015 07:15 PM
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Baldyhugh

Posts: 353
Joined: 01 June 2007

Hi andy

Any opinion of your preference for earthing banjos as per my last reply?

Peter
 25 July 2015 07:20 PM
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sparkingchip

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I have been trying unsuccessfully to post.

Unbroken earth conductors are normally discussed regards main protective bonding tryhere

Andy
IET » Wiring and the regulations » Bolting earths to metal enclosures

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