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]

I'll start it off, then.

Contrary to some popular urban myths, there is no requirement for a seperate earthing system (e.g. TT/Earth Rod) at the outbuilding, regardless of the earthing system of the main installation.

The only thing that needs to be taken into consideration is the presence of extraneous conductive parts in the outbuilding and, if required, the CSA of the' Protective Bonding Conductor' that will need to be run along with the supply to the outbuilding.

The option is there to install an earth rod, should this be an easier and more cost effective option.
You then need to seperate your outbuilding earthing system from the main installation earthing system.
The feed to the outbuilding would be earthed at the supply end only.

That should get things rolling

 

[Mr.Black]

As the cable(buried) is supplying a special installation the supply cable therefore should be RCD protected.

If the supply cable is visible for its entire length does it still need RCD protection?

Anyone agree

 

[WayneL]

As the cable(buried) is supplying a special installation the supply cable therefore should be RCD protected.

If the supply cable is visible for its entire length does it still need RCD protection?

Anyone agree

I would have to disagree, Mr Black, no requirement for RCD protection of the cable if mechanically protected. (e.g. SWA), unless an RCD was required for earth fault protection to meet EFLI maximums.

 

[Mr.Black]

What about it being a special installation?

 

[Deleted member 9648]

Bs 7671....the BRB section 7, lists special locations. A normal garden outbuilding is not a special location. Where such outbuildings come under special locations such as construction sites,swimming pools etc there are additional requirements and it is important to refer to the BRB for more information.

 

[Sav]

Superb post and really helpful !! I am writing down my notes as we gom along !!
Keep up the good work and any other such subjects or ideas others may have, would be great to put them on, so that we ( really meaning me ) , can write down helpful notes and links etc.

Best regards and have a great weekend everybody,

Sav

 

[SirKit Breaker]

I would have to disagree, Mr Black, no requirement for RCD protection of the cable if mechanically protected. (e.g. SWA), unless an RCD was required for earth fault protection to meet EFLI maximums.

I agree and I always try to split supply with a Henley block and switch fuse to out buildings, just so any problems with it doesnt affect the house. The supply to your house doesnt have RCD protection and its basically the same set up.

 

[Jason Smith]

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 Smith]

I'll start it off, then.

Contrary to some popular urban myths, there is no requirement for a seperate earthing system (e.g. TT/Earth Rod) at the outbuilding, regardless of the earthing system of the main installation.

The only thing that needs to be taken into consideration is the presence of extraneous conductive parts in the outbuilding and, if required, the CSA of the' Protective Bonding Conductor' that will need to be run along with the supply to the outbuilding.

The option is there to install an earth rod, should this be an easier and more cost effective option.
You then need to seperate your outbuilding earthing system from the main installation earthing system.
The feed to the outbuilding would be earthed at the supply end only.

That should get things rolling

Wayne, sounds a brilliant explanation and I bet it couldnt be put into fewer words.watching this thread closely.

 

[Mr.Black]

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.

 

[Jason Smith]

Right then, a special installation, in this context, is a outdoor power installation, and is therfore covered by Part P.(leckys guide to the building regs,green book, page 71). Therfore, its to be treated as a dwelling,additional info within this page give advice on outside sockets etc, so I cant see it effects anytechnical decisions . Do you agree

 

[Mr.Black]

If you mean that the supply cable should be protected by an RCD then absolutey my good man.Very important as some spark may come along at a later date and decide the outhouse needs a earth spike,fires it into the ground and ouch ouch what happened there..

 

[Jason Smith]

If you mean that the supply cable should be protected by an RCD then absolutey my good man.Very important as some spark may come along at a later date and decide the outhouse needs a earth spike,fires it into the ground and ouch ouch what happened there..

WOT - I take it you infer that a sparky would do such a thing as to drive an earth rod through a cable ? never, surely ?

 

[WayneL]

Hi again,

I don't think many people have got a copy of 'Guidance Note 8', so I though I'd post this scanned picture from it.

Quite informative, I thought.

The sizes given are much higher than you'd see in a domestic - but the principal is the same, just scale it down.

It shows the two options for additional buildings using a TN-C-S supply. Building B3 is using the existing earth system and Building B2 is using a rod.

You'll notice that the combined CPC and Protective Bonding Conductor running to building B3 is 35mm, as are the bonding conductors in B3, even though the Line and Neutral conductors feeding B3 are only 25mm - this is because their selection is based on the supply neutral (120mm), and not the neutral feeding B3 itself (25mm).
This wouldn't matter to a domestic, as the size of protective bonding conductor remains at 10mm for a supply neutral up to 35mm

The regs quoted are the 16th reg numbers, as the IEE haven't thought it necessary to update GN8 to the 17th yet - I think a lot of the info didn't change.

This was the 2nd best £25.00 I've ever spent, by the way, so if anyone is thinking of buying it - go right ahead.

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

 

[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.

 

[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?

 

[Des 56]

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

I think comparing Tnc-s and TT and saying better and worse earth is meandering away from the safety reasons
A Tnc-s system although having lower Ze than TT is a potentially poorer earth system than the higher TT

The outbuilding earthing choice,ought to be about whether danger exists by using the potentially dangerous Tnc-s or not, rather than" its better earthing than a rod"

Assessing the particular outbuilding and taking any bonding factors into account and deciding from there whether to use TT or Tnc-s

 

[WayneL]

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

I wasn't comparing them directly, Des, which is why I said 'i.e. Ze/Ra'.

If your outbuilding was 2 meters from your house, and you could get a Zs at the outbuilding DB of, say, 0.5 ohms using the supply earth (TN-C-S), why on earth would you want to put a rod in and get an 'unpredictable' 50 ohms.

The outbuilding earthing choice,ought to be about whether danger exists by using the potentially dangerous Tnc-s or not, rather than" its better earthing than a rod"

Assessing the particular outbuilding and taking any bonding factors into account and deciding from there whether to use TT or Tnc-s

Which is probably why I said this, earlier:

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.

 

[Deleted member 9648]

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? It shouldnt because within the equipotential zone you should not be in contact with any other earth potentials and so not subject to a potential difference

Didn't Part P come about because a politicians daughter got electrocuted to death in her kitchen via an extraneous conductive part?I thought it was a cooker cable drlled while fixing something on the wall

It is the introduced earth which gives rise to a danger,as when an earth fault is in progress there will be a rise in voltage on conductive parts (earthed parts of the electrical installation) relative to the general mass of earth untill the protective device clears the fault.
A person touching an earthed appliance and an unbonded extraneous part may recieve a shock while the fault is in progress, as they are effectively touching a live part while being in contact with the ground. As soon as that extraneous part is bonded there can be no potential difference between conductive and extraneous parts and the shock risk is reduced.
It doesnt matter about ensuring continuity of pipes to sink etc beyond th position of the bond as once inside the building they can no longer introduce an earth potential.

 

[pushrod]

It doesnt matter about ensuring continuity of pipes to sink etc beyond th position of the bond as once inside the building they can no longer introduce an earth potential.

So if copper water pipe comes into the house and then there is a section of plastic pipe which is effectively separating other sections of copper pipe you are saying that they do not need to be bonded? If those unearthed pipes became live, wouldn't they just stay live and not trip an mcb because the fault current would have no where to go? (assuming no rcd)

 

[Des 56]

Which is probably why I said this, earlier:[/QUOTE]


Originally Posted by WayneL


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.



Lowering my headwith a sorry Waynel

 

[WayneL]

So if copper water pipe comes into the house and then there is a section of plastic pipe which is effectively separating other sections of copper pipe you are saying that they do not need to be bonded? If those unearthed pipes became live, wouldn't they just stay live and not trip an mcb because the fault current would have no where to go? (assuming no rcd)

Sorry to butt in, but how would they become live?

That's a bit like saying - if the metal legs on my glass coffee table became live............ - should I bond them?

 

[pushrod]

Sorry to butt in, but how would they become live?

That's a bit like saying - if the metal legs on my glass coffee table became live............ - should I bond them?

Sorry but nothing like that at all! Surely electricians are always trying to pre-empt unlikely accidents, but as an example - we had a loft conversion done which required the mains fed water tank being moved and the plumber used speedfit? plastic pipe the feed from this rejoins copper pipes to the hot water tank. the cupboard containing
this has cabling for immersion heater , the pump and various other surface mounted cabling! I don't think it is much of an argument to stand up in court and say i never thought they would become live!

Can you answer my question or not, don't mind if i am wrong or not - would just like to know!

 

[martysparky]

We know if there was a fault current it would most likely disconnect.

How would a sink become live?

I cant see a sink being fed via plastic except push fit that most plumbers I know avoid due to future leak potential.

If you can get a back current from the neutral bar why could this not happen with the earth bar?

The hot water pipe comes from the boiler where it is bonded.

We know if we do an IR test and wrongly put the live tester probe on the earth bar it makes everything live so if the line conductor came into contact with a cpc or conductive part and lets just say it didn't disconnect why would it not be possible for the sink to become live. Can we assume that the current will take the path straight to the main earthing terminal without any flowing away through conductive pipe.


I know this is not very likely but just raising this thought.

Also maybe this should now be a different thread? its become outbuilding/sink

 

[geller]

Hello guys.

After reading your topic i have a query.

My garage (not attached to the house but only about 10 feet behind it) is wired from the consumer unit to a smaller C/u in the garage running 4 sockets and two strip lights.
Earth wise, it is connected with SWA from the house to the Smaller unit.

Q1 - Do i need to add an Earth electrode to this ?
Q2 - Is this set up regarded as a TT system ?

Thank you.

 

[WayneL]

Sorry but nothing like that at all! Surely electricians are always trying to pre-empt unlikely accidents, but as an example - we had a loft conversion done which required the mains fed water tank being moved and the plumber used speedfit? plastic pipe the feed from this rejoins copper pipes to the hot water tank. the cupboard containing
this has cabling for immersion heater , the pump and various other surface mounted cabling! I don't think it is much of an argument to stand up in court and say i never thought they would become live!

Can you answer my question or not, don't mind if i am wrong or not - would just like to know!

Hi Pushrod,

I'll try answer it the way I see it

Any metalwork entering your house (or outbuilding-to stay on topic), could introduce earth potential into the property, especially if originating underground.

In this case you would bond the metalwork as near to it's entry point as possible.

If this is done to all metalwork on entry (gas, water, oil, structural), then you create your equipotential zone.

If you touch two metal parts simultaneously, within this zone, you won't get a shock, as all metal will be at the same potential - whether that be 0 volts or 100 volts.

Supplementary bonding, which isn't always required now, is just additional links between metalwork to keep that equipotential zone going - e.g. between hot and cold taps

No other metalwork in the house will require bonding, as it can't possibly introduce a potential.

What will be required is earthing.

Exposed conductive parts need to be earthed - e.g. cooker casings, washing machine casings, boiler casing and pipework, metal wall lights - the list could go on and on - anything that could become live, under fault conditions, needs to be earthed - not, though, my coffee table

This ensures that the fault will be cleared by the protective device, rather than the exposed conductive part remaining live.

This is the best I can do - maybe someone can be clearer