Terminating SWA at insulated consumer unit

[Andy Hall]

I don't believe that this one has come up in quite this form before so should be worth a thread.

I'm completing a summer house in the garden and adding the electricity supply.

This is being run from the main CU via SWA and enters the building inside next to the outer wall. There is an inner studded and clad wall, which over most of the building is insulated, but in the area of the electrical services will be kept mainly clear. Thus there is a compartment formed by a partly fixed and partly removable panel with a

65mm gap to the outside wall.

My original plan was to bring the SWA up inside the cavity and through to the front just below a CU fitted to the front panel, terminating in a 20mm gland fitted to the CU.

However, the CU is an insulated MK type which has T&E shaped knockouts around the edges and some rectangular knockouts on the back. There are no round knockouts for glands and not really a suitable place to drill for one.

I thought of simply bringing the SWA into the back of the CU, running the red and black core cables to the incomer breaker. However, this does not seem that mechanically satisfactory. I can clip the cable as far as a point close to the entry through the back.

An alternative idea is to use a metal box with 20mm knockouts at both ends. This would be fitted in the cavity (screwed down) and use a gland to terminate the SWA cable with the conductors going into a 30A terminal block. 6mm^2 T&E would continue from there into the CU.

Has anyone had to deal with a similar situation, and come up with a neat solution? .andy

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[Andy Wade]

Could you mount an adaptable box close up to the CU housing (directly behind it, say) and couple it mechanically with a long pattern conduit bush (would probably need to be 25mm) and a few lockrings? Then take the SWA into the adaptable box with a gland in the usual way, and feed the L&N thro' the bush and into the CU itself.

It's not clear whether you're using the armour to provide an earth or whether you need to isolate it at this end because the summer house will be a TT system. If the latter, a plastic adaptable box may be preferable to metal (since the armour should be considered as potentially live within the separately earthed zone, IYSWIM).

[BigWallop]

You should have been given a brass earth tag when you bought the glands for the SWA. Yes ? Then you will fit the brass tag (that's the thing with a big hole in one end and small hole at the other) over the threaded end of the brass gland before you stick it through the hole you've drilled for the fitting, and then tighten it in with the locking nut of the gland. Next you'll need the two nuts, the four washers and the one bolt you bought when you got the rest of the fittings. You did get them, didn't you ?

Anyway. You can now drill a hole through the box and through the small hole of the brass earth tag. Yes, that's right, so you can bolt the small hole end of the earth tag to the side of consumer unit too. You only use two washers at this bit, one on the outside and one on the inside. Then you put one of the nuts on and tighten it all up so you make a sandwich of the wall of the consumer unit. Next. Take another washer and put it on top of the nut on the inside of the box. You then take the 6 mm green/yellow insulated cable you got, you did get that as well, didn't you ?

Anyway. You crimp on the loop to one end of 6 mm cable and then slip it over the bolt. What do you mean "I didn't get any crimp on loop" well this is were you're you gonna' need one to do the job right. OK. You've now got the crimp on loop on the end of the 6 mm green/yellow cable. Right. You put the last washer on the top of that, and then the last nut on the top of that.. Don't tighten it up to far just now though, as you've got the other end of the cable to terminate.

Trim down the cable to the length you need and slip off some insulation, enough to let you connect into the earth bar of the consumer unit. Yes, the brass bar thing screwed to the side with all the holes and bolt in it. No !!! Not the one for the neutrals, the other one. Now, you've got that end secure, you can go back and tighten up the nut and make sure that the cable isn't fouling anything and that the lid will go back on without mishaps.

Right !!! You're ready to tackle the red and black conductors to the main switch now ? Good !!!

Now !!! What about the other end of the SWA. :-))

[BigWallop]

What do you mean " You didn't get any shrouds to fit over the glands" ?

[Andy Wade]

I'd use a brass screw and a multiplicity of nuts (as a patent agent would put it) through the small hole in the tag in conjunction with a crimped ring terminal lug on the end of the earthing wire.

With care it's possible to bend the end of the tag over thro' 90° so that the connection is entirely inside the enclosure. Do this if you're using a plastic box because you don't really want a joint relying on pressure applied through plastic parts which might creep.

[Andy Hall]

I have all of the above, but didn't think of using my crimp on terminals. Yes I do have shrouds with the glands. Thanks for the suggestion.

.andy

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[Rick Hughes]

Andy - I did the latter fixed a small galv box alongside the CU, terminated the SWA as normal on the galv box - and run the cores direct through the box (with suitable grommet) into the CU.

Seemed simplest to me.

Rick

[Andy Hall]

Thanks Rick.

This, or perhaps a variant with a conduit coupler or something similar does look to be the way to go. I have the space behind the CU and can make that panel removable for access to the terminals in order to comply in that area.

I couldn't think of a regulatory reason why the SWA cable couldn't be brought into the CU, the steel wires sleeved and run to the earth bus bar. However, it seemed to be a rather shoddy way to implement.

.andy

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[Ed Sirett]

Apologies for the p[revious blank post.

I had much the same sort of problems when wired my store room.

I used plastic boxes both end and a short length of T&E. The armouring was earthed using the tag in the house end box. I used 3 core 2.5mm^2 SWA so the third core was used to export the earth. [This practice is probably wrong since I used the yellow core sleeved with green&yellow for the earth. ]

[BigWallop]

The wording I used Andy, is roughly a conversation I had with a young 4th yearer on a site quite recently. He was left to make all the termination for the first time on his own. I just couldn't resist going through it again when it was still fresh enough in my mind.

After four years on the job and he couldn't it right, SHEESH !!! I don't think he'll go far.

[Andy Hall]

Oh dear.

I could think of various alternatives that I felt would be satisfactory. I was really looking for additional possibilities.

As we've seen, there are several more options.

.andy

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[BigWallop]

Yes there are other options, but which ones are the safest ? After many years of trials on site, I've always found the correct way to be the safest way every time. :-)) Aesthetics don't always come into play when safety is involved.

[BigWallop]

Just an extract from the bible of electrical engineers:

Toothed and serrated lock washers of phosphor bronze must be used where any cable termination is used to carry fault signalling. The basic principle of these washers is to use the spring reaction of their hardened teeth to bite into both the work piece and the locking nut and gland body, so effectively ensuring the tightening torque is maintained even under pull strain or vibration conditions and to improved electrical contact (e.g earthing terminals).

And.

Where PVC/SWA/PVC cables are used as main or sub-main cables, all termination's shall comprise of compression glands of correct size for each cable and fitted with earth bonding washers, star washers and separate copper bonding directly to the earth terminal of the equipment. The whole gland assembly shall be fitted with a PVC shroud after the cable is fixed and earth bonded. If calculations show that the armouring of the cable is insufficient to be used as the sole circuit protective conductor, then a separate additional C.P.C. shall be installed. This may be in the form of an additional core of the cable itself, or alternatively a separate conductor secured to, and in parallel with, the cable and connected directly to the earth terminal of the equipment.

So there is a correct way to do it Andy.

[Andy Hall]

Which particular one?

I was asking about suggested solutions for terminating the SWA and transitioning to the (plastic) CU which did not have an appropriate way to install a cable gland. I did not consider that taking the cable into the CU and running the armour wires or the third conductor used as a CPC would be appropriate, which is why I had rejected it.

The issue was to look at a suitable transition enclosure and methods for terminating cables going into and out of it. I've been doing electrical and electronics work for long enough to know that a star washer should be fitted to bolts and fittings through panels - that wasn't really the point although you didn't mention it in your initial post.

All of the suggested methods included appropriate termination for the CPC going to the earth bus bar, including my own, so you didn't have exclusivity on "correctness".

The point was the suitable enclosure for the task and suggested methods for going from there to the CU terminals. Your post about the conversation that you had had with your apprentice, or whatever he was, was amusing but didn't really cover the issue in question.

Thanks for your contribution, though.

.andy

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[ARWadsworth]

I did something similar recently on a garage and I used an outdoors/waterproof MK one gang socket (I had one spare) to terminate the SWA. As the supply to the garage was fed from the house CU by a 40 amp MCB I changed the socket for a 45 amp DP switch and used the round knockouts on the MK backbox to terminate the incoming SWA. The switch is now an isolator for the garage CU.

Adam