Do you have to protect submain if wired in SWA?

[rosewood]

Hi Guys,
Sorry title should have been 'do you have to protect a submain with an rcd if wired in SWA?'

Please excuse me if this has already been asked but i cant find a straight answer to question...

I need to change a fuseboard for a split load board (2x 30ma 63amp RCD's). off this board there will be a sub main to a garage split load board which feeds some stables and a barn etc.

The board i have to change is already got 3x32a circuits and 4x6a circuits (re-wireable fuses). If i change it to a new 17th board where the main RCD's are 63amp can i still put a 63amp mcb on this board (protecting 16mm SWA to garage buried 600mm)?

Thanks

 

[Engineer54]

its a bit of both believe it or not...
The swa feeding the main board uses the steel armoured for the earth but they have also put a stake down obviously because the reading wasn't good enough.

Please be precise, ....Do you mean the DNO supply cable?? What is the earthing system at the origin of this installation?? What main board?? Where is the earth electrode conductor connect too??

The original installation could well have been a TT earth that has now been converted to PME and connected the earth rod to the EMT. Or It's a TN-S and has a supplementary rod installed. An earth electrode isn't going to make much of a difference , if any on any readings that you maybe talking about, not unless it's a good, deep driven rod, and they are few and far between these days in UK!!

 

[JamesBrownLive]

Careful E54, I had salt and pepper pots glued to my helmet for coming out with a statement like that!

Sounds very painful :bigcry:

 

[rosewood]

Right sorry i havent explained very well. Its a old farm house that has been converted into a mansion like thing! 2.5million i think matey just bought it for. At the end of the 1/2 mile long drive way is the meter in a wooden box where the drive way meets the lane. plaque on the meter says this is connected to PME installation. There is 3 phase that comes into the cut-outs then up to the meter. Then out the meter into a great big MEM isolator. Then back down into the ground and thats when it goes somewhere. (we'll come back to this). Now it looks like a TN-C system as the earthing conductor is coming out of the neutral block.

Im changing the D/B in the annex (or outbuilding) which needs a submain to garage across the yard. The supply into the isolator in the outbuilding is three phase and has nothing going out except tails upto the D/B im changing.

The main house is fed with a single two-core cable and has no three phase supply to the house at all. So some where it must split to feed the house and feed the annex or is it more likely there is a joint underground on one of the phases that goes to annex and fed the house with it?

Its a 20m run from garage to annex building. And looks like there is an earth stake coming into the isolator for the annex.

On the garage board:

16a Radial
6a lighting
6a lighting
16a radial
6a lighting
(they are having more buildings put up which is why i wantd to putin on a 63a mcb and in turn i had to put in on a 16 mm swa?

 

[Knobhead]

You sure three phases set off from the MEM switch and not two phases and two neutrals? I could see someone doing that and splitting in a joint somewhere along the way. A bit of a weird way of doing it but no more weird than loosing a phase in a joint.

 

[rosewood]

No way of telling. cant open isolator as its boxed in. im going to th place again tonite so il get a photo!

 

[Knobhead]

This just gets better, there are probably fuses in the isolator (well there should be).

Tools required to change fuse: screwdriver and crowbar.

A photo would be good.

 

[rosewood]

the distributors cut out fuses, meter and that MEM isolator is all in an 'overgrown' little brick enlosure at the end of the property as you come off the road with a tiny little door in so all you can see in front of you is main cut out fuses and meter and isolator it on the right where there is not a door so you cant open the isolator. will get pics tonite!

 

[Guest123]

the distributors cut out fuses, meter and that MEM isolator is all in an 'overgrown' little brick enlosure at the end of the property as you come off the road with a tiny little door in so all you can see in front of you is main cut out fuses and meter and isolator it on the right where there is not a door so you cant open the isolator. will get pics tonite!

It's VERY important you know what you're dealing with supply wise before you think about adding to anything to anything.


Knock the cabinet down if you have to, but you need to be 100% sure whats there before anything else.

Looking forward to seeing the piccies.

 

[Deleted member 19807]

the distributors cut out fuses, meter and that MEM isolator is all in an 'overgrown' little brick enlosure at the end of the property as you come off the road with a tiny little door in so all you can see in front of you is main cut out fuses and meter and isolator it on the right where there is not a door so you cant open the isolator. will get pics tonite!

The Dno's don't like cu's being fitted more then 2-3m from the meter without a fuse in place to protect the long feed cables, normally a switch fuse will be installed.
The mem box should be a switch fuse unit to comply and any down stream fuses/mcb's should be selected to provide discrimination with these fuses or you risk one of these fuses going before/same time as one in your cu dose if a fault occurs.

 

[anthanth]

OK great info.

I realise that this is a bit of an old blog but can i just clarify that #

If the SWA is more than 600mm deep in the ground then you do not need RCD protection unless the system is TT??

So any armoured cable on a TT system would need to be on an rcd ??

Have i got that right??

Any help would be appreciated.........

 

[malcolmsanford]

OK great info.

I realise that this is a bit of an old blog but can i just clarify that #

If the SWA is more than 600mm deep in the ground then you do not need RCD protection unless the system is TT??

So any armoured cable on a TT system would need to be on an rcd ??

Have i got that right??

Any help would be appreciated.........

Basically yes. Though the RCD is not to protect the cable due to it's depth, you could literally bury it 100mm underground and still not need RCD protection. The depth, and 600mm is more to do with protecting the cable from damage with a spade or such.

The RCD protection on a TT system is mainly due th the fact that there is very little chance of you being able to achieve a low enough Zs value to trip a fuse/MCB. So often in a domestic situation where you have a SAW feeding an out building on a TT the SWA will be protected by a 100mA RCD

 

[anthanth]

Basically yes. Though the RCD is not to protect the cable due to it's depth, you could literally bury it 100mm underground and still not need RCD protection. The depth, and 600mm is more to do with protecting the cable from damage with a spade or such.

The RCD protection on a TT system is mainly due th the fact that there is very little chance of you being able to achieve a low enough Zs value to trip a fuse/MCB. So often in a domestic situation where you have a SAW feeding an out building on a TT the SWA will be protected by a 100mA RCD

Ah ha yes of course. So the rcd just makes sure it trips out in time as Zs may be too high.

What value Zs would you need in order to make it so you don't have to have the rcd ?? I am at about 60 Ohms at the moment

thanks

 

[derek preece]

Zs (or RA) on a TT system can fluctuate which is why an RCD is a requirement in pretty much every case imho, check 411.5.2 particularly note 1. You should still try and get the value as low as possible though, try banging the rod deeper.

 

[High Tower]

63a c curve mcb, I haven't got the regs to hand but a wild guess would be 0.30-0.40 ohms ish so your 60ohms is miles off, that's why you almost always need RCD's on TT

 

[GLENNSPARK]

I would suggest you recalculate your sub-main loads From what your implying, if you have a 100A DNO main supply, your leaving less than 40A to cover the house loads??

I don't think your going to need anywhere near 63A for this sub-main's distribution loads. Think ''diversity'' and i reckon that a good 50% of that badly calculated load will just disappear, or i'll eat my straw hat!! ...lol!!!

yep....give em 32A on a 4.0mm 3 core SWA (length of run considered of course).....that will give em a 20 A A3 circuit....and 6A for lighting.....with a bit spare...