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Posted By Topic: SWA earthing

hamburger
Sep 15 2016 16:53

a bit of a debate raging in my part of the world over using the steel armour of a cable as an earthing conductor. In this case its an existing submain and a new sub db is being installed. The question is can the SWA be regarded as an adequate earth? 3000 section 5.5.2.2 allows it but am I missing anything that dis allows its use??
   

DougP
Sep 15 2016 17:15

I think 5.3.2.2(ii) and 5.3.2.3(b) are what you are looking for.
   

hamburger
Sep 15 2016 18:02

You've picked my typo there doug thanks. Thats the clause i was trying to quote (5.3.2.2 ii). I can't see what the deal is with disallowing its use??
   

AlecK
Sep 15 2016 19:00

the clause says you can - but only if you comply with the applicable special condition(s) in the following clause.
For SWA, the special condition is (b).

Basically you can only use SWA armour as earth for the circuit supplied by the actives within that cable.
Then of course you STILL have to meet the EFLI requirements saME AS ALWAYS
   

Sparkznz
Sep 15 2016 21:37

Sorry to hijack the thread but in regards to the SWA being used as an earth, do the cable manufacture need to confirm it can be used? I know that the SWA here is made differently to that in the uk as we don't have any copper in the SWA? I could be wrong 🙃
   

AlecK
Sep 16 2016 08:44

What matters is not what the PEC is made of but whether it has a suitably low impedance
   

DougP
Sep 16 2016 09:25

I had a quick look on a couple of manufacturers data and I couldn't find any information about resistance for SWA.

And a quick look through 3008 doesn't seem to have data either.
   

Sarmajor
Sep 16 2016 19:27

In 38 years of electrical practice I have never seen any copper in the armour component of steel wire armoured cable.
I have seen aluminium wire armoured cable though.
You could always install it and then test the EFLI.

But as Aleck has pointed out the SWA can only be used to earth the equipment supplied by the cable, so using it as the earth in a submains is not a go.
It's ok to use a SWA cable to supply a single piece of equipment like a machine as long as the EFLI is low enough for the protection.
   

DougP
Sep 16 2016 19:31

Sorry Sarmajor. Serious misinterpretation there. Everything supplied by the submain is earthed through the SWA so nothing wrong with that at all.
   

justsuppose
Sep 16 2016 20:52

http://www.electriciansforums.co.uk/threads/example-of-use-of-adiabatic-equation.28128/

this link shows an example of a UK adiabatic equation to work out whether swa is ok to use as cpc
   

Sarmajor
Sep 16 2016 22:20

Doug, I disagree with your interpretation of 5.3.2.3.(b)
The equipment connected to the circuits supplied from the distribution board is fed from active conductors originating at the individual MCB's in the switchboard. This does not fit with my interpretation of 5.3.2.3.(b).

If the cable supplies a machine once you get to the main switch of the machine all the wiring after that is not installation wiring and 3000 no longer applies.

With a submains / switchboard situation it is still installation wiring and 3000 applies.

BUT because no one from the departments that are responsible for writing and enforcing the rules will ever actually come out and clarify things like this we can both be correct.
   

AlecK
Sep 17 2016 08:45

I never looked at it quite the way Sarmajor does.
The key is what "supplied only by live conductors incorporated in the cable" actually means.

I believe it is intended to mean that you can't use other cables in parallel.
Which makes perfect sense and aligns with other resrictions on earthing conductors not incorporated in same cable. generally you can use a separate copper PEC regardless of whether it's a submain or a final subcircuit; but if you have a PEC incorporated into the cable you are not allowed to use it to earth things not supplied through the cable.

But now that Sarmajor has raised it, I can see the other interpretaion; which is that if you have a SWA submain you can't earth the final subcircuits using the SWA submain because they are supplied not "only" by the SWA but also by the subcircuit cable.

I don't think it's a correct interpretation, but I can see how someone can interpret it that way.

I looked on the SA FAQs but there isn't one there to cover this point, which indicates it hasn't caused much confusion to date.

I remain of the view that it's OK to use SWA as the PEC for a submain; and that because the final subcircuits fed from the DB originate from the DB (ie from the circuit protection device(s) and from the DB's N-bar) the equipment they supply is supplied only by the final subcircuit cabling and not by the submain.

And as Sarmajor notes, unless it's High Risk PEW you make up your own mind and certify accordingly. Nobody else's interpretation to worry about. Unless you get audited.