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Topic Title: Armoured sub main
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Created On: 21 April 2014 01:19 PM
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 21 April 2014 01:19 PM
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Hi All, I've just worked in a large, very old, house where the intake is in the garage, from the meter, tails go to a henley block then go through a 60A 30mA obsolete ELCB to a DP isolation switch that then feeds a twin & earth cable (looks like 16mm) which then runs 21m through the house to the laundry room, connects via another henley to another piece of T&E (unearthed!), which runs to the pantry to another henley that then feeds 3 distribution boards, and 1 sub-main off of one of those. Apparently the intake used to be in the laundry but was moved to the garage about a year ago and an earth rod was installed by the garage (it's a TT system) There are young children in the house, and golf clubs and tennis rackets are swung around a lot. A lot of the T&E run is within 6 ft from the floor and accessible. My view is that if the intake was moved to the garage about a year ago, then the obsolete elcb should not have been reused, that the cable should have been run using swa (when I was there the plumber almost drilled into the cable from the other side of the wall, if I hadn't been in the room and shouted, he would have); that the cable is undersized (it is just OK for volt drop etc at 60A but given that it supplies 4 DBs supplying the large house, 21m away, I can see the elcb being replaced in the future. It may be that as the original intake was in the laundry, that there is an earth rod going to the pantry where the DBs are, but shouldn't the earth still come from the earth rod at the garage (which is the installations earth point)? And of course that last run of T&E should be earthed. About 8m of the run goes outside and T&E is made to BS 6004 which states "cables for lighting and internal use". I know we've all seen security lights wired using a couple of feet of T&E but it does degrade considerably due to the UV from the sun and this is a bit different from just supplying a light. I've raised my concerns with the customer who is trying to get hold of the electrician who did the work to get him to talk to me. I'd be grateful therefore of your views and whether I'm right or not. Thanks in advance and sorry for the long post.

Edited: 21 April 2014 at 01:42 PM by dksanders
 21 April 2014 02:05 PM
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My employer (a DNO) would require overcurrent protection within the first 2.5m of meter tails (most DNOs state 3m), and certainly before anything (SWA / T&E etc) that was not meter tails. The overcurrent protection needs to be sized to protect the extended submain, not necessarily to grade with what you believe the DNOs fuse is. An "old obsolete 60A 30mA ELCB" may well be ok, as long as you are sure it is a current operated device, not voltage operated. You will have to time it to find out. Regarding the earth rods, there should not be an issue connecting them together - you'll lower the earth resistance by doing this anyway.

Regarding drilling through the wall into something the other side, you cannot really guard against that. A few years ago I had someone drill through a wall into the back of their cutout. They realised something was wrong when the drill stopped. The soot marks from the explosion in what was once a white garage was truly impressive and fortunately contained within the garage. .


 21 April 2014 02:59 PM
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Hi Alan, thanks for the reply. I don't know what the DNO fuse is as it's still sealed and I didn't look, but as the meter is new it could be 100A. It's a usuall intake i.e. DNO cable to head with fuse, tails to a meter then tails to the first henley I mentioned and then the elcb (totall length less than 2m from the meter). I obviously didn't explain it clearly about the earth rods. I apreciate they can be joined together for better earth, but my question is shouldn't they be already as the one at the garage is the main installation earth. I have no idea what type the elcb is nor will I be testing it as the customer won't be paying me to, they'll be getting the other electrician to sort out any defficiencies. Again, I know that drilling into cables can't be helped sometimes, but given the environment and that this cable is carrying the main suplly current to the whole house, I would expect it to have been an armoured cable.
 21 April 2014 08:42 PM
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It sounds like they have just used the originals cables that were originally feeding the garage and just fed them the other way to save money,


Specialised Subject. The Bleedin Obvious. John Cleese
 21 April 2014 08:47 PM
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Hi Gary, thanks.

The client said the work was done about a year ago when the intake was moved to the garage which had just been added otherwise I'd agree with you.
 22 April 2014 12:42 AM
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AFAIK BS 7671 makes little distinction between final circuits and submains in terms of allowable cable types - if you're happy with T&E for the rest of the installation, it would be hard to find a principle to object to it being used for a submain. It it was concealed in a wall then 30mA RCD protection is probably the biggest issue (either lack of protection or lack of discrimination). A PVC sheathed cable is quite able to stand up to the occasional bash from a child's tennis racket.

And of course that last run of T&E should be earthed.

I'm not sure that it does. Having an earth present inside an insulating sheath cable doesn't really do anything much for shock protection. BS 7671 generally only requires that an earth is provided at each point & accessory (e.g. 411.3.1.1 & 412.2.3.2) - so not necessarily continuously along every cable. Reg 543.6.1 asks for for a protective conductor to be incorporated into the same wiring system as the live conductors, or immediate proximity - but only where overcurrent devices are used for ADS - very unlikely in a TT system.

- Andy.

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