Supply to OUTBUILDINGS

[acat]

Hi Folks

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


Thanks


Chris

 

[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.

 

[WayneL]

- we are constantly being told different things- one lecturer says one thing and another says something different with regard to all sort of things .
cheers.

Exactly the same at my college, mate, when doing the 2330 - you could literally go from one lesson to the next and be told two totally different things regarding the same topic.

I think it's a combination of some misinterpretation of the 'regs' and some passing on of wrong information.

That's why I think it's important to give sources of information (e.g. Reg numbers) when making statements on topics like this - it's all to easy to repeat totally wrong information

 

[Deleted member 9648]

That's why I think it's important to give sources of information (e.g. Reg numbers) when making statements on topics like this - it's all to easy to repeat totally wrong information

But bear in mind the fact that there isnt a reg stating that outbuildings need to be on a TT.What the regs dont say is just as relevant as what they do

 

[WayneL]

But bear in mind the fact that there isnt a reg stating that outbuildings need to be on a TT.What the regs dont say is just as relevant as what they do

That's exactly what I mean, mate

It's no good coming on here stating that an outbuildings has to be TT'd, without giving a reference to back a statement like that up.

The statements that I have made, I have backed up with reference to GN8.

 

[dixon9]

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.

Err... So are you saying the metal sink is unlikely to be piped in from the outside?

Edit: If the pipes are plastic then fair enough - if not then the sink is extraneous....

 

[dixon9]

As an aside, this gives a pretty good explanation of the situation and also provides references to regs:

wiring matters supply to outbuilding - Google Search

 

[Deleted member 9648]

Err... So are you saying the metal sink is unlikely to be piped in from the outside?

Edit: If the pipes are plastic then fair enough - if not then the sink is extraneous....

No,it is the pipes coming into the building which introduce an earth potential from outside,not the sink.
The pipes should be bonded at the point of entry,if they are plastic then no bonding is required.
The sink itself cannot introduce an earth potential therefore it is not an extraneous conductive part.

 

[dixon9]

Fine ....but if the metal sink is joined to the metal pipes then the sink IS extraneous!

 

[Sintra]

An Extraneous Conductive Part is a metal part that is not part of, but in proximity to the electrical installation and is liable to introduce earth potential. p24
All extraneous conductive parts in an installation must be connected to the main earthing terminal by main protective bonding conductors. This applies to the metallic sheath of a telecommunications cable where permission from the owner of the cable must be obtained. 411.3.1.2


It is not generally required to supplementary bond the following :
kitchen pipes, sinks, draining boards, metallic kitchen furniture, boiler pipes, metallic parts supplied by plastic pipes or metal pipes to hand basins or wc's ( excluding metal waste pipes in contact with earth ). OSG p31

 

[Deleted member 9648]

Fine ....but if the metal sink is joined to the metal pipes then the sink IS extraneous!

Er....um....only if the incoming pipes (the real extraneous conductive parts) were not bonded at the point of entry would the sink be an extraneous conductive part by virtue of being connected to the pipes. Once the pipes are bonded ,everything on the building side of the bond cannot introduce an earth so is not an extraneous conductive part

 

[martysparky]

All the pipes are bonded to earth if there is continuity???

If an earth fault was to arise when you had your hands in the water in a sink, wouldn't that continuity give rise to danger?

Didn't Part P come about because a politicians daughter got electrocuted to death in her kitchen via an extraneous conductive part?