extending a sub main

[tbs1983]

hi guys, I have a job in a flat where a wall is being removed to make it open plan, the wall contains a 10mm T&E sub main that feeds the DB for the flat.
In order to re-route the sub main, I will have to extend it very slightly (2m max), I'm unable to run a whole new sub main due to routes/logistics just not possible.

What is the best way to extend the cable bearing in mind it may be carrying 50A or more.
I have seen large coffin junction boxes but am not overly keen on the idea of using a JB for this purpose bearing in mind the load it may have to carry.
Any better suggestions? thanks in advance

 

[GLENNSPARK]

thanks for replies guys, joint would be under a floorboard therefore deemed accessible, sub main will be RCD protected. just looking for advice on the best possible way i can carry out this joint as new sub is out of the question (we have explored the idea thouroughly!!!)

if...and i say if you are going to joint the cable under the floorboard then at least create a proper access hatch for it...
and make its wereabouts known on the dis-board schedule...

 

[GLENNSPARK]

whats the earthing arrangements at origin ?

 

[dazza1981]

Are you sure the submain is protected by a 100ma rcd??? where is it fed from that you cant run a new supply

 

[tbs1983]

valid point Dave, thanks for the advice

 

[GLENNSPARK]

has anyone been down to the switchroom/intake position whatever and caried out visual confirmation of supply characteristics & earthing arrangements together with associated aperatus?

 

[tbs1983]

TNS glenn, as is a lot of my area. No RCD at present to protect sub main Dazza, just a 63A MCB from when it was installed a few years back,

 

[dazza1981]

I think the problem you have is that if you are not going to run a new supply in and extend whats already there then its down to you to meet the current regs as you are altering the distribution cable,so i would really take an in depth look at how this 10mm is installed and if its suitably protected,whether your extending it by 2 metres or 20 the same rules apply

 

[Engineer54]

Afraid just adding a 100mA RCD device to this supply cable ain't gonna work too well!! You'll find that in most cases, any earth fault in the flat will take out both the respective 30mA RCD and the up-stream 100mA. Only a delayed S type RCD will be suitable for discrimination purposes!!


You're in a catch 22 position really aren't you, with a cable that is part run in wall(s) without mechanical protection, and unable to comply with BS7671 as far as providing a 30mA RCD to the cable!!!! The only way to comply, would mean a change of cable to a SWA, which you're saying isn't an option. ....Maybe it will become the only option!!

 

[dazza1981]

Im guessing you will have to extend some of the circuits also if the boards being moved.....
Good luck tbs hope it goes well mate

 

[tbs1983]

cheers guys thanks for the help, I take your valid points and will have a good look into it!!! looks like a new SWA may be the only option I have.
This will also eliminate my original problem of the cable joint.
Will have to investigate further on site and hopefully find a solution!!!

Thanks again

 

[GLENNSPARK]

Afraid just adding a 100mA RCD device to this supply cable ain't gonna work too well!! You'll find that in most cases, any earth fault in the flat will take out both the respective 30mA RCD and the up-stream 100mA. Only a delayed S type RCD will be suitable for discrimination purposes!!


You're in a catch 22 position really aren't you, with a cable that is part run in wall(s) without mechanical protection, and unable to comply with BS7671 as far as providing a 30mA RCD to the cable!!!! The only way to comply, would mean a change of cable to a SWA, which you're saying isn't an option. ....Maybe it will become the only option!!

this is it eng...
i would suggest an SWA is the only option for compliance here...

let us not forget that if that sub is still buried within a wall...for part of its length..
then you couldn`t connect up to it without either confirming it is has earthed metallic protection (unlikely) or providing a 30mA RCD top end ...
so to connect up to this without the above would constitute a non-conformance...

 

[GLENNSPARK]

cheers guys thanks for the help, I take your valid points and will have a good look into it!!! looks like a new SWA may be the only option I have.
This will also eliminate my original problem of the cable joint.
Will have to investigate further on site and hopefully find a solution!!!

Thanks again

basically what i said at #2

 

[Guitarist]

if...and i say if you are going to joint the cable under the floorboard then at least create a proper access hatch for it...
and make its wereabouts known on the dis-board schedule...

Exactly. You can't simply say it's under a floorboard, and therefore accessible. Agree with the others...SWA is the way to go.

 

[Spoon]

joint would be under a floorboard therefore deemed accessible,QUOTE]

I thought that under the floorboards was deemed as not accessible??

GLENNSPARK your post 16 says "if...and i say if you are going to joint the cable under the floorboard then at least create a proper access hatch for it...
and make its wereabouts known on the dis-board schedule..."

This is still classed as not accessible, isnt it? I thought that all connections done under floorboards had to be maintenance free. Or have I just read the post wrong ... again...

 

[Darkwood]

cheers guys thanks for the help, I take your valid points and will have a good look into it!!! looks like a new SWA may be the only option I have.
This will also eliminate my original problem of the cable joint.
Will have to investigate further on site and hopefully find a solution!!!

Thanks again

Nice to see the OP taking advice from our well educated guys the last thread of this type where the OP refused to believe he had done wrong fitting a time delayed 30mA to the T&E sub-supply without adequate mech' protection and having a 17th edition board up front .... i even showed why with regulation no's and appendix ref' at which point its clear the OP had realised but never replied since..... these were new installs too and he'd said his boss had been doing it this way all the time.... tragic!
Keep it up tsb1983 ... the best way to learn is to listen