Supply to OUTBUILDINGS

[acat]

Hi Folks

Please post all your ideas and suggestions here so we only have one place to look


Thanks


Chris

 

[Deleted member 9648]

I know the outhouse isn't a Special Location it comes under SPECIAL INSTALLATION there is a difference!!!!!!!!!!!!!!!!!!!!!

Check in the PART P DOCUMENT.Then tell me what you think.

Not sure what the relevance of this is. Part P is about what work is notifiable.....Bs 7671 determines how that work is carried out,not part P. In Bs 7671 a garden outbuilding is not a special location and therefore only the general requirements of Bs 7671 apply.

 

[Jason Smith]

Not sure what the relevance of this is. Part P is about what work is notifiable.....Bs 7671 determines how that work is carried out,not part P. In Bs 7671 a garden outbuilding is not a special location and therefore only the general requirements of Bs 7671 apply.

Right, thats what I am saying, BS 7671 applies to a dwelling as well as an outbuilding. If an outbuiding is obviously part of a dwelling, then 7671 and part P applies, so wheather or not its a special location or Installation, its still 7671. so we can set aside the fact that its either a special installation or location, as it has no bearing on the technical aspects. Whats next.

 

[pushrod]

I was asking a lecturer today about this today - he teaches the 17th wiring regs (and really knows his stuff) and it was his opinion that any shed/outbuilding outside the main building's equipotential zone should have its own earthing system, ie an earth rod.
The swa doesn't need an rcd because it has mechanical protection.
Earth the swa at the supply only. There must be an rcd at the outbuilding

I did ask about extending the house's equipotential zone to include the outbuilding ... but didn't really get the chance to explore that question.

 

[martysparky]

Hi again,


I'll probably post some more info from it when I get a minute Hope it helps.

Good call nice one

 

[WayneL]

I was asking a lecturer today about this today - he teaches the 17th wiring regs (and really knows his stuff) and it was his opinion that any shed/outbuilding outside the main building's equipotential zone should have its own earthing system, ie an earth rod.

Hi mate,

You should get him to have a read of GN8 then mate, coz he's got it wrong

I'm not criticising him, by the way, a lot of people have got this idea that you have to TT an outbuilding, but it's all rumour.
There may be some circumstances where it would make more sense to TT it, but that's a judgement call by the sparky on the job.

The other thing worth mentioning is that, if there are no extraneous conductive parts in the outbuilding, and therefore no bonding requirements, then you can simply run your SWA supply to the outbuilding, utilising either the armour, or a third core, as the CPC - once again, regardless of the type of earthing system of the main installation.

 

[WayneL]

Just wanted to add -

The one time I would seriously consider installing an earth rod, and seperating the outbuilding earthing system, is if the outbuilding was steel framed and the main earthing system was TN-C-S.

The requirement to bond to the frame of the outbuilding would mean that if there did happen to be a loss of the supply neutral, the whole of the frame of the outbuilding could rise to 230v.

This would obviously cause a shock hazard to anyone standing outside the outbuilding and touching the frame. TT'ing the outbuilding, in this instance, would be safer.
With a TN-S / TT system this problem wouldn't arise.

 

[martysparky]

Hi mate,

a lot of people have got this idea that you have to TT an outbuilding, but it's all rumour.

hence the expression 'drove a rod in for good measure'. When I hear that it just sounds like that person is unsure.

 

[WayneL]

hence the expression 'drove a rod in for good measure'. When I hear that it just sounds like that person is unsure.

I agree, mate.

The funny thing is, by rodding the outbuilding, you are probably giving it a worse earthing system (i.e. Ze/Ra) than you would be by using the supply earth

 

[chris gray]

nice picture.
where would you terminate the cpc going to building 2?
just says insulated from building 2 earthing arrangement.
cheers

 

[Graeme Harrold]

I agree, mate.

The funny thing is, by rodding the outbuilding, you are probably giving it a worse earthing system (i.e. Ze/Ra) than you would be by using the supply earth

Just done a CU change for my ELECSA Assessment in outbuildings. Fed by SWA from a 32A MCB, no RCD. I was asked loads of questions inc. disconnection times and R1,R2 tests etc. I broached the subject of spiking as its away from the main property, and he simply said "why make the earthing potentially worse". ALso had to prove the armour was adequate for the job and explain why RCD (front end) was not required.

So, no spike required if earthing conductors are suitable, No RCD required on SWA provided disconnection times are met.

 

[WayneL]

nice picture.
where would you terminate the cpc going to building 2?
just says insulated from building 2 earthing arrangement.
cheers

Hi mate,

The CPC in this instance would most likely be the armouring, so I think you would normally bring your SWA into an adaptable box or the CU itself (if big enough) - but obviously needs to be PVC - then just terminate gland as normal, but no flylead or connection to the outbuilding earth bar.

If that makes sense - I know what I mean ha ha

 

[pushrod]

Hi mate,

You should get him to have a read of GN8 then mate, coz he's got it wrong

I'm not criticising him, by the way, a lot of people have got this idea that you have to TT an outbuilding, but it's all rumour.
There may be some circumstances where it would make more sense to TT it, but that's a judgement call by the sparky on the job.

The other thing worth mentioning is that, if there are no extraneous conductive parts in the outbuilding, and therefore no bonding requirements, then you can simply run your SWA supply to the outbuilding, utilising either the armour, or a third core, as the CPC - once again, regardless of the type of earthing system of the main installation.

Hi Wayne glad you didn't shoot the messenger Story of my life electrical life is this - we are constantly being told different things- one lecturer says one thing and another says something different with regard to all sort of things . Think i'll see if there is a copy of GN8 in the library, read it and then see what he has to say about it
cheers.

 

[dixon9]

Hi mate,

You should get him to have a read of GN8 then mate, coz he's got it wrong

I'm not criticising him, by the way, a lot of people have got this idea that you have to TT an outbuilding, but it's all rumour.
There may be some circumstances where it would make more sense to TT it, but that's a judgement call by the sparky on the job.

The other thing worth mentioning is that, if there are no extraneous conductive parts in the outbuilding, and therefore no bonding requirements, then you can simply run your SWA supply to the outbuilding, utilising either the armour, or a third core, as the CPC - once again, regardless of the type of earthing system of the main installation.

Well said that man!

It is also worth noting that you should never introduce two different potentials into the location (in this case the shed/outbuilding). So NO TT earth rod combined with the TNCS earth run from the main building (this is often annoyingly referred as "belt and braces" - which could not be further from the truth).

If the outbuilding has extraneous metal parts such as a metal sink - then TT it and don´t use the TNCS earth from the main building. If the outbuilding doesn´t have extraneous metal parts then use the TNCS earth.

 

[SirKit Breaker]

On a subject like this full of Grey areas, we need to define what 7671 says, as thats the law. After that, we need to find out what our individual governing bodies state.Best practices etc. As its for those reasons that opinions differ so greatly. So if we can define what we say by quoting a reg, or say I do this, etc, because I believe its best practice etc, that would help.
I personally have followed my" master" and will rod the lot if I get a chance to prevent transporting a fault. It is time to move on from that as I have learnt from this forum, to develop my own methods.

Jason,

Fortunately BS7671 is not the law. However Building regulations are, and as Part P insists on BS7671 being the minimum standard required for compliance then i suppose in a grey sort of way it could be perceived as being law.

In my opinion there are far too many grey areas and impractical and unworkable regulations in both the Wiring and building regs, not to mention the contradictions. I view both as a guideline only as i know from experience that i can do it better in some cases.

My preferred way of doing things is to do it as safe as possible, and note it in departures if required.

You have started a good thread, and maybe opened a can of worms as well, time will tell but i will also watch it closely as well.

Cheers...............Howard



Cheers.........Howard

 

[Deleted member 9648]

If the outbuilding has extraneous metal parts such as a metal sink - then TT it and don´t use the TNCS earth from the main building. If the outbuilding doesn´t have extraneous metal parts then use the TNCS earth.

I think i'ts worth pointing out here just what an extraneous part is as an awful lot of people are confused on this.
In bs 7671 an extraneous part is defined as-
"A conductive part,liable to introduce a potential,generally earth potential,and not forming part of the electrical installation"

The key part of this is underlined. Therefore a metal sink wholly within the building is not generally an extraneous conductive part. Pipes coming into the building from outside and metallic structure are likely to be extraneous conductive parts. The only time a metal sink,radiator etc is likely to be an extraneous conductive part is if it is fixed to the metallic structure of the building.