Do you have to protect submain if wired in SWA?

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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
 
Last edited by a moderator:
No, as long as the installation is TN, and not TT.
SWA is one of the cable types (in fact all bar one of them) listed in 522.6.6 (522.6.100 in new money) as being one of the acceptable methods for additional protection.
 
Last edited by a moderator:
rosewood said:
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
Click to expand...


I have to ask, ....why do you need a 16mm cable to supply a garage??

The other thing is you can't have a 30mA RCD protecting the sub-main at the main CU and another 30mA RCD at the garage. there will be no discrimination between the two. Supply the sub-main with a non RCD'd MCB....
 
Thanks guys,

Engineer54, the garage board will be a 10way split board feeding 4 ring mains, 3 lighting and a radial (and allowed a bit more for extras they might have in the future)

That was my way of thinking but then the sub main is not protected by an RCD? or is this acceptable?

Thanks in advance
 
The SWA sub main does not need an RCD because it has an earthed covering as in 522.6.6.
63A is a very high load for a barn and stables. Have the circuit coming from the house as a non RCD protected way and have a 30mA RCD at the termination in the barn.

(oops too slow)
 
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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.
 
Richard burns, tahst what i cant get my head around...all the circuits in the garage add up to more than 63a (obviously thats not what is going to be pulling) but what determines the size of the breaker protecting the sub main then?
 
So now you will need to do a Ze enable you to calculate your Zs for this cable.

The reason that a TT system would need RCD protection for you sub main is that is is virtually impossible to obtain a low enough Zs value to operate a fuse/mcb in the allowed disconnection time of 1 second for a circuit exceeding 32amps
 
rosewood said:
Richard burns, tahst what i cant get my head around...all the circuits in the garage add up to more than 63a (obviously thats not what is going to be pulling) but what determines the size of the breaker protecting the sub main then?
Click to expand...

This book...Electrical Installation Design Guide: Calculations for Electricians and Designers - IET Electrical

It's fab.
 
rosewood said:
Thanks guys,

Engineer54, the garage board will be a 10way split board feeding 4 ring mains, 3 lighting and a radial (and allowed a bit more for extras they might have in the future)

That was my way of thinking but then the sub main is not protected by an RCD? or is this acceptable?

Thanks in advance
Click to expand...

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!!!
 
the circuits in the garage add up to more than 63a (obviously thats not what is going to be pulling) but what determines the size of the breaker protecting the sub main then?
Click to expand...


That would be the design current (Ib) after diversity is taken into account for the garage in this case, as your OCPD 63A (In) => Ib

too slow
 
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I think i'm pretty dammed safe this time...lol!! I don't normally come out with such statements, but i'm sure that a garage a small stables and a barn won't draw anywhere near 63A amps. Not unless there are some pretty big KW guzzling equipments that he hasn't mentioned.
 
rosewood said:
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.
Click to expand...

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!!
 
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?
 
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.
 
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.
 
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!
 
rosewood said:
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!
Click to expand...


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.
 
rosewood said:
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!
Click to expand...

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.
 
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.........
 
anthanth said:
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.........
Click to expand...

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
 
malcolmsanford said:
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
Click to expand...



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
 
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.
 
Engineer54 said:
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!!!
Click to expand...
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...
 
GLENNSPARK said:
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...
Click to expand...

Granted 63A is a fair lick , but the way things are going now days it is far from being unlikely .

It seems to be all the rage at the moment ( Well with the ones we are involved in at least .) for even the small lower end stable yards / couple of stables at the bottom of the garden , to have showers for the horse then they want a solarium and not forgetting rug driers and even the odd American style top loading washing machine to wash the rugs with if they have more money than sense and then some form of background heating to keep the tack from going mouldy , all this before any flood lighting and the obligatory kettle !

Don't get me wrong some still do just want just a couple of lights and socket , but these types seem rarer than the other type we tend to find now .
 
Hehehe was reading the posts and about all I can think about now is the 20m run between the buildings I think instead of the run I would walk it and enjoy the view while working out how best to do the job but I have seen a meter for a caravan over half a mile away fixed to the hydro pole. I had the cu changed before the owner arrived to tell me where to isolate it talk about confused looks when he saw I was done and he had the only key to the isolator. Good luck looking forward to the pics.
 
Quinnysparks said:
I'm not 100% sure, but won't you need an earth rod at the garage if it's a PME system and being outside the equipotential zone from the main installation?
Click to expand...

There is nothing in either the BS 7671-2008 or the ESQCR-2002 to stop you extending the equipotential zone on a TNC-S system. This is another myth within our industry with really no sound basis.

Often if there are extraneous conductive parts in the out building, such as metallic construction or water services, it is financially better to TT the installation rather than trying to take a bond back to the MET
 
malcolmsanford said:
There is nothing in either the BS 7671-2008 or the ESQCR-2002 to stop you extending the equipotential zone on a TNC-S system. This is another myth within our industry with really no sound basis.

Often if there are extraneous conductive parts in the out building, such as metallic construction or water services, it is financially better to TT the installation rather than trying to take a bond back to the MET
Click to expand...

Don't forget though Malcolm that there is a stable involved in this one as well , i think ?
So unless it has required measures in place for such the op has no choice but to TT .
I know you would realise this , but others following this thread in the future might not .
 
Thanks DP I was just looking at the garage part

Though a TN-C system is not allowed in a Agi , or any other installation come to that, you can use a PME with the use of a metal grid in one.

But in the majority of cases I think your right TT systems are used mainly in Agi
 
Sorry to drag up an old thread but I'm in a similar situation... I have a 3p 25mm swa running underground that was installed during 17ed. It is a pme system it is protected by an mccb but does anyone know does it by 17th ed regs require rcd protection? As far as I'm aware it doesnt but I cant seem to find confirmation. Thanks in advance
 
Marko10010 said:
... I have a 3p 25mm swa running underground that was installed during 17ed. It is a pme system it is protected by an mccb but does anyone know does it by 17th ed regs require rcd protection? As far as I'm aware it doesnt but I cant seem to find confirmation. Thanks in advance
Click to expand...
Hi - with its mechanical protection SWA cables can be exempted from the need for RCDs due to impacts. Perhaps confirm your cable is included in the list of BS types in Reg 522.6.204(i).
 
You won't find confirmation it doesn't need it only instances where it is required. Why do you need to know if it complied to the 17th Ed.
 
westward10 said:
You won't find confirmation it doesn't need it only instances where it is required. Why do you need to know if it complied to the 17th Ed.
Click to expand...

Thanks Westward

I was doing a lot of remedial works at a place 2 yrs ago after a guy failed a lot of tests on it. We finished work just before 18th edition officially started.

Basically there was a plant room outside a big house and business we replaced some of the control gear and boards as the original contractor had specced. There were half a dozen sub mains going to different buildings all SWA and underground. We fitted mccbs as that was what the previous contractor had put forward. It was a pme system so that seemed fine to me.

There was one of the 3p sub mains that went out to a mini pillar in the garden then split to go to 2 other locations. A new spark has come in now complaining that there is no rcd protection on said ccts and they have now been on the phone with me now to resolve. I'm just trying to clarify before forcing my opinion that there is no regulatory requirement for it...

Does that make sense?
 
If they are submains and fault protection is provided by the mccb there is no reason for rcd protection on a TN earthing system. I assume the split from the pillar just feeds distribution boards.
 
Best thing to do is for the other spark to confirm where in the 17th Ed it would be a requirement.
 

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