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Topic Title: SWA
Topic Summary: Earth the Armouring?
Created On: 17 February 2014 09:25 PM
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 17 February 2014 09:25 PM
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SKElectrical

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Is it necessary to earth the armouring of SWA?
I ask because there have been several topics recently concerning unearthed parts owing to double insulation.
Given that the armour is doubly insulated it begs the question.

(I haven't consulted any manufacturer's instructions)




If unearthed can voltages and or currents arise?
 17 February 2014 09:32 PM
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redtoblackblewtopieces

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Drive a metal spike through cable and hit live conductor and if swa earthed fault will be live conductor to earthed armour as opposed to?
Kevin

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 17 February 2014 09:35 PM
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RB1981

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Originally posted by: SKElectrical

Given that the armour is doubly insulated it begs the question.


The armour is an exposed conductive part, and cables are not double insulated.
 17 February 2014 09:36 PM
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leckie

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Can you write that in English please Kevin
 17 February 2014 09:38 PM
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leckie

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Yes you have to terminate the armour to earth, but if you are not using the armour as a cpc you can just bond it at either end.
 17 February 2014 09:38 PM
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Legh

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Given that the armour is doubly insulated it begs the question.

(I haven't consulted any manufacturer's instructions)


Is it ?

I have never known SWA cable to be Double insulated and of course, what about the use of 2-core SWA cable?

Legh

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 17 February 2014 09:41 PM
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mattyatty1204

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I don't think there is any grey area regarding earthing of SWA armouring's! As mentioned previously, if a nail, screw or sharp object was to penetrate the cable and make contact with a live conductor then touch the armouring's these would become live under fault conditions and wouldn't cause a fault condition!

In any SWA installation at least one end of the SWA armouring's should be earthed. I often use the easy earth locknut rings purely for the ease of connecting these to earth without drilling the enclosure and messing about etc.
 17 February 2014 09:45 PM
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redtoblackblewtopieces

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Like what mattyatty 1204 said:-)
Kevin

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 18 February 2014 12:32 AM
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SKElectrical

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Originally posted by: RB1981
The armour is an exposed conductive part, and cables are not double insulated.


The armour would not be an exposed part if it were shrouded at termination but not glanded!

After a long look into manufacturer's technical data, it reveals that the inner sheath is classed as a 'sheath' and not 'insulation'. Although it is made from PVC (usually) which is a good insulator, I presume that manufacturers don't guarantee its continual integrity (that it remains mechanically sound and intact) over a given length.
 18 February 2014 01:11 AM
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geoffsd

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Why is there any doubt?
 18 February 2014 05:50 AM
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redtoblackblewtopieces

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Just to pick something from Leckie's point earlyer (not about my English),I was always taught to earth the armour at the end where the CPD for the cable is. ?
Kevin

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 18 February 2014 07:39 AM
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leckie

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If you are using an extra core as the cpc I don't think it matters as if you connect either end of the swa you are still bonding it.

If you were using an swa from a TNCS system to supply a remote building that was going to become TT, then you would connect at the outgoing distribution end, and then tape off the armour at the TT end and connect via a stuffing gland to an insulated enclosure, or a similar approved method.
 18 February 2014 04:57 PM
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geoffsd

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Originally posted by: leckie
If you are using an extra core as the cpc I don't think it matters as if you connect either end of the swa you are still bonding it.

The armour would be earthed at the supply end for fault protection, not bonded.



I
 18 February 2014 08:10 PM
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daveparry1

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I can't believe that anyone here would even consider not earthing swa armour! (unless the swa was being used just to supply selv garden lights etc. on the secondary side of the tx)
 18 February 2014 09:08 PM
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slittle

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I had an SWA a few weeks ago that a "handy man" had installed to feed a farm toilet about 20 metres. 3 core but no earth connection the armouring.


The farmer isn't using him again after I had a few choice words on the subject !

Stu
 19 February 2014 05:05 PM
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TVRBryan

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According to the NICEIC's Snags & Solution book on earthing, you should earth the armouring at both ends (even if the armour is not being used as the earth), it also used to state this on the online ESC technical guide (run by the NICEIC). However, when I recently called the NICEIC helpline on this subject I was advised that only the supply end requires earthing. I obviously questioned why the snags & solutions book & ESC technical guide stated earthing was required at both ends, I was told they would look into it & get back to me (they never did). 2 days later, the guidance on the online ESC technical guide (stating both ends required earthing) was removed...
 19 February 2014 11:22 PM
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alancapon

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Originally posted by: redtoblackblewtopieces
. . . I was always taught to earth the armour at the end where the CPD for the cable is. . .

Correct. There is a regulation in BS7671 that requires this, although its reference number escapes me at the moment.

Regards,

Alan.
 20 February 2014 07:25 AM
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GeoffBlackwell

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Originally posted by: alancapon

Originally posted by: redtoblackblewtopieces

. . . I was always taught to earth the armour at the end where the CPD for the cable is. . .


Correct. There is a regulation in BS7671 that requires this, although its reference number escapes me at the moment.

Regards,
Alan.


Name that regulation

Have a care here - if we are concerned with earthing the armour as part of an ADS system we are simply trying to ensure effective operation of the circuit protective device and protection of the installation equipment.

If the means of earthing is at the source of the installation - TN system with supplier earth facility or TT systems with the earth electrode connected to a main earth terminal at the source - then, in general, earthing at the CPD end of any circuit makes sense.

However, consider a TN-C-S installation feeding a remote building (pool, shed, etc). We may decide to create a TT system at this building (lets not divert into the rights and wrongs of it) - in this case we might have an RCD as part of our circuit protection and this could be located at the source end so that it protects both the building and its supplying distribution circuit.

We may decide to locate the earth electrode at the remote building (often a good idea as we should aim to keep the electrode outside of the resistance area of any conductive-parts that are connected to the TN-C-S system). In this case then we are earthing the armour at the remote end so any regulation that precludes this would cause us a problem.

Regards

Geoff Blackwell
 20 February 2014 10:15 AM
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OMS

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Wot GB said - remote end earthing via a TT system rather than source end eartrhing isn't that uncommon

Plus - it's often the case that calculations would be based on the armour being "bonded" at both ends - circulating currents, standing armour voltages and cables in parallel (faults flowing in "unconventional" directions) being obvious examples.

In simple terms though - in almost every circumstance where ADS is the means of protection, the armour should be solidly earthed - it is fundamentally an exposed-conductive-part, by definition.

Regards

OMS

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 20 February 2014 10:48 AM
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zeeper

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My GN8 discribes the armouring as an exposed conductive part, which will require earthing (not bonding).If not being used as CPC.
IET » Wiring and the regulations » SWA

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