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

 

[Deleted member 26818]

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.

 

[malcolmsanford]

^^^^^^^^ seems pretty straight enough that spin

 

[Engineer54]

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

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....

 

[rosewood]

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

 

[malcolmsanford]

I refer you back to our learned friend of post #2 .................is the earth system TN or TT,

As Spin as said TN ..............no TT 99% yes

 

[Richard Burns]

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)

 

[rosewood]

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.

 

[rosewood]

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?

 

[malcolmsanford]

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

 

[SW1970]

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?

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

It's fab.

 

[Engineer54]

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

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!!!

 

[spark 68]

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?

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

 

[Knobhead]

or i'll eat my straw hat!! ...lol!!!

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

 

[Engineer54]

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.

 

[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!