IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Earthing the armour of SWA
Topic Summary: Unusual earthing arrangement of the SWA armour
Created On: 23 February 2013 11:55 PM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 23 February 2013 11:55 PM
User is offline View Users Profile Print this message



springcrocus

Posts: 21
Joined: 25 March 2009

I came across an unusual arrangement regarding the earthing of the armour in a submain cable recently and wondered if anybody can see a problem with this? My thoughts are that it is fine, if not the usual way things are done. It's a garden supply with waterproof consumer unit to feed a hottub and other peripherals.

After the meter, the tails are split at a DP Henley block and new tails taken to a 50A switchfuse along with an earthing conductor from the side of the PME head. Leaving the switchfuse are the three cores (sheathed) of a 10mm SWA cable (but not the armour) which lead into an adaptable box. Here, the armour is glanded off but there is no connection to the earthing conductor.

At the other end of the SWA cable, the armour is glanded off into a consumer unit. The line and neutral go the 61008 63A 30mA RCD and the third core, earth, is looped back to the banjo of the gland. There is a local earth rod nearby which is connected to the the earthing bar in the CU and the system is TT from here onwards.

The armour is therefore connected to the source earth via the third core of the SWA albeit at the far end, and the third core is not left floating because of its connection at the incomer.

I don't see a problem with this arrangement but thought to pass it by greater minds than mine in case I'm missing something. The niggling doubt I'm having is if damage to the SWA cable could affect safety even though the armour is effectively protecting the core that connects it to earth.

Regards,
 24 February 2013 09:13 AM
User is online View Users Profile Print this message



normcall

Posts: 8147
Joined: 15 January 2005

This might help you with this problem.

When you wire a radial circuit, do you attach both ends back to the MET?

You are right to have concerns that someone might dig the cable up, cut through some 50+ steel strands, carefully split the outer PVC insulation, spread the three cores, carefully cut the inner core used an earth protection. The earthing rod would need to be disconnected (obviously) then the live connection in the external consumer unit connected to earth. Gently suggest to AWMBO that she get ready to use the hot tub and you will be out shortly.

Most of the hot-tubs I've seen have a plastic lining, so you might have to resort another method to prove the danger.

Best I can do (but I no nuffick).

-------------------------
Norman
 24 February 2013 10:54 AM
User is offline View Users Profile Print this message



daveparry1

Posts: 6255
Joined: 04 July 2007

When I make a remote supply TT such as this I earth the armour at the source, I suppose what they've done is ok but it wouldn't me my way of doing it. The problem I see it is that because the armour is relying on the length of cpc for it's earthing something could possibly happen to the swa causing the cpc to get broken so the armour from the source to the damage could become live,

Dave.
 24 February 2013 10:57 AM
User is offline View Users Profile Print this message



alancapon

Posts: 5790
Joined: 27 December 2005

I think there is a reg that requires the armouring to be earthed at the source end. I will have to have a search, as I don't recall its number.

Regards,

Alan.
 24 February 2013 12:39 PM
User is offline View Users Profile Print this message


Avatar for slittle.
slittle

Posts: 3559
Joined: 22 November 2007

There is a reg Alan, and I can't remember it either.

I can see the logic of the guy that done it but it depends as well what he's done with the "3rd" core of the SWA. If he's "earthed that" at the switchfuse end, what has he done with it at the hot tub ??

My version would have been 3rd core and armouring connected to the MET at the switchfuse therefore using the supplier earth to protect the cable.

SWA terminated into plastic compression gland at the hot tub end with third core safely insulated and the enclosure labelled appropriately and then RCD and stake the hot tub.

Stu
 24 February 2013 12:56 PM
User is offline View Users Profile Print this message


Avatar for sparkingchip.
sparkingchip

Posts: 6215
Joined: 18 January 2003

If the CPC third core and the armour are both interconnected at both ends would't it roughly quarter the R2 and if they were just linked at the source halve the R" compared with the present set up?

Andy
 24 February 2013 02:40 PM
User is offline View Users Profile Print this message



springcrocus

Posts: 21
Joined: 25 March 2009

I think there is a bit of misunderstanding of the configuration - probably my poor explanation.

The SWA is supplying the outdoor consumer unit with a line and neutral only. The earthing is obtained from an earth rod and the CU, hot tub and other peripherals are all TT'd.

The suppliers earth is ONLY being used to protect the feeder cable by using the third core and connecting it to the armour.

I can see where he was coming from. The wholesaler probably only had 3-core in stock and you cant leave a floating core in the cable, therefore he connected it to earth at the supply end. At the other end, he has connected it to the armour via the banjo, thus earthing the armour.

I would probably have used an earth block and run separate G/Y's to both the switchfuse and the adaptable box where the armour is glanded off.

If I get the work I've quoted for, I'll take a couple of photos. I only saw it when opened the cabinet to see what the supply was and noticed that there was no (obvious) earthing to the SWA so checked further.
 24 February 2013 03:09 PM
User is offline View Users Profile Print this message



alancapon

Posts: 5790
Joined: 27 December 2005

The reg is 542.1.3.3 which would require the armouring to be earthed at the source end. At the far end, I agree with Stu, such that the PME earth is not available to be used and/or touched within the consumer unit.

You arrangement not only seems to make the PME earth available at the consumer unit at the far end, but in the event of the cable being severed leaves an armouring very possibly at 230v either immediately, or after replacing the fuse when it is discovered that the hot tub is not functioning.

Regards,

Alan.
 24 February 2013 03:59 PM
User is offline View Users Profile Print this message



daveparry1

Posts: 6255
Joined: 04 July 2007

It's simple to fix though isn't it, just put a banjo on the armor at the supply end and run an earth lead to the met or main c/unit, maybe the original installer just forgot to do it!

Dave.
 24 February 2013 04:13 PM
User is offline View Users Profile Print this message



alancapon

Posts: 5790
Joined: 27 December 2005

I agree, the issues are easily fixed at both ends.

Regards,

Alan.
 24 February 2013 04:13 PM
User is offline View Users Profile Print this message



aligarjon

Posts: 2873
Joined: 09 September 2005

I don't see a problem with it myself. The regulation Alan gives does not prohibit the armour being earthed at the far end so long as it originates from the source end. The gland would be in a rubber shroud and not likely to be accidently touched.

Gary

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 25 February 2013 07:24 AM
User is offline View Users Profile Print this message


Avatar for spinlondon.
spinlondon

Posts: 4439
Joined: 10 December 2004

The main problem with this set up, is that the resistance/impedance of the earth fault loop has unecessarily been increased.
A line to armour fault occuring near the supply end of the cable will have not only the impedance of the armour to contend with, but also that of the third core.
It may well be that the impedance is such that the protective device will not operate within the prescribed time.
 25 February 2013 10:14 AM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11583
Joined: 13 August 2003

The reg is 542.1.3.3 which would require the armouring to be earthed at the source end.

I didn't read it that way Alan - for my money both ends of the SWA are part of the TN installation - the TT installation starting just after the load-end gland.

The main problem with this set up, is that the resistance/impedance of the earth fault loop has unecessarily been increased.

Agreed - although a simple Zs test at the supply end L-armour should prove that one way or another. (At least it's easier to check than with the armour earthed at both ends, where the point of max Zs would be somewhere in the latter half of the SWA.)

you cant leave a floating core in the cable

That's a new one on me (in ordinary non-Exe circumstances at least) - which reg is that?

- Andy.
 25 February 2013 11:20 PM
User is offline View Users Profile Print this message



springcrocus

Posts: 21
Joined: 25 March 2009

Thank you, people, for your comments on this arrangement.

Alan - the PME earthing is not available for use in the consumer unit but the banjo is exposed so could be touched.

Spin - the run of SWA was about 15 metres and that length of 10mm would only increase the loop impedance by about 0.03 ohms so not a big deal, I would have thought.

Andy - floating cores. I don't know of a reg but was taught years ago to always connect any spare cores of a multicore cable to earth at the source end to kill any induced voltages. I've never really questioned the validity of that advice.
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.