Supply to OUTBUILDINGS

[Deleted member 9648]

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)

The purpose of bonding as I and others have pointed out has nothing to do with 'earthing pipes so they cant become live'. This is a common misunderstanding which results in an awful lot of incorrect advise on bonding.

 

[acvc]

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.

Hi Geller

What is the earthing system in the house? And is there any connection between the main earthing conductor in your house and the one in your garage?

 

[pushrod]

Thanks for the detailed reply Wayne. I appreciate that bonding is to maintain an equipotential zone eg so that in the event of a fault, and before disconnection, if a person touches a live exposed conductor and at the same time an extraneous conductor eg cold watertap there will be no difference in potential ie they will have both risen to the same level and so the person will not get a shock. What i suppose i was getting my head around was that some of the pipe work that can possibly be both acting as an exposed conductor eg continuous with hot water pump body or an extraneous conductor capable of introducing an earth potential. My main concern is with large sections of pipework becoming isolated from there bonding conductor by the handywork of a plumber sticking in a bit of plastic pipe. Should the plumber then take responsibility and ensure that there is equipotential bonding if the plastic then goes back to copper? Or if the electrician comes across it should they do the same?

The purpose of bonding as I and others have pointed out has nothing to do with 'earthing pipes so they cant become live'. This is a common misunderstanding which results in an awful lot of incorrect advise on bonding.

This not what i was originally asking and the "quote" above is not mine and you are incorrect if you inferred it was meant.
Quote:
Originally Posted by wirepuller
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?"

The above is the main question that i am asking.

 

[Davey101]

Although I think Pushrod may misunderstand the definition of 'extraneuos' (I also misunderstood the exact definition before someone on this forum corrected me), I agree with him on his point about earthing pipework. If I saw a house with a copper heating system, which had been electrically dissconected from the boiler by a short section of plastic pipe, I would want the remaining copper pipework to be bonded.
There are far too many ways a pipe (and all the radiators and taps connected to it)could become live. I should think that there are enough sparks who have seen live unearthed metalwork to agree

It may not be covered by the regs, but it's good practice at the very least.

 

[martysparky]

I was told that the copper pipes should be tested for continuity.

 

[WayneL]

I was told that the copper pipes should be tested for continuity.

This is true, although it's irrelevant if all extraneous metalwork is bonded at its point of entry to the building - i.e. if it's continuous, it's already bonded - if its not continuous, it doesn't need bonding

If you have a section of pipe, and there's no continuity back to the MET (not bonded), you can always test it to see if it's extraneous - put your mind at rest

 

[Deleted member 9648]

I was told that the copper pipes should be tested for continuity.

I think you may have heard this when referring to supplementary bonding,it is permissible to use a copper pipe as a supplementary bonding conductor ,(NOT a main bonding conductor),in which case the electrical continuity of the pipe would have to be confirmed.

Pushrod......apologies if I misunderstood the posts.

 

[Deleted member 9648]

Although I think Pushrod may misunderstand the definition of 'extraneuos' (I also misunderstood the exact definition before someone on this forum corrected me), I agree with him on his point about earthing pipework. If I saw a house with a copper heating system, which had been electrically dissconected from the boiler by a short section of plastic pipe, I would want the remaining copper pipework to be bonded.
There are far too many ways a pipe (and all the radiators and taps connected to it)could become live. I should think that there are enough sparks who have seen live unearthed metalwork to agree

It may not be covered by the regs, but it's good practice at the very least.

I see what you are saying here....but you cant stop at just plastic sections,compression joints are electrically unreliable,are you going to do a continuity test on every single pipe in the building between each electrically suspect joint and perhaps link across compression joints?
A sledgehammer to crack a nut IMHO....just stick to the requirements of BS7671.

 

[pushrod]

Although I think Pushrod may misunderstand the definition of 'extraneuos' (I also misunderstood the exact definition before someone on this forum corrected me), I agree with him on his point about earthing pipework. If I saw a house with a copper heating system, which had been electrically dissconected from the boiler by a short section of plastic pipe, I would want the remaining copper pipework to be bonded.
.

I would be interested to know which of my posts make you think i don't understand the term "extraneous" conductive part, at least i can spell it

By the way in the OSG section 4.7 page 31 it says "supplementary bonding is not required to metallic pipes supplied by plastic pipes"

apologies as well if i have been a bit off with anyone!

 

[geller]

Hi Geller

What is the earthing system in the house? And is there any connection between the main earthing conductor in your house and the one in your garage?

I believe mine is a standard earth system (TNC-S !), and the SWA is connected to the garage c/u from my house c/u. Connected to a separate breaker on the rcd side of the unit and i presume the earth in the unit too.

 

[Davey101]

I would be interested to know which of my posts make you think i don't understand the term "extraneous" conductive part, at least i can spell it

By the way in the OSG section 4.7 page 31 it says "supplementary bonding is not required to metallic pipes supplied by plastic pipes"

apologies as well if i have been a bit off with anyone!

Apologies, I seem to have merged your post with someone else's in my head, after re-reading, I can see nothing to back up my previous post
I'm fully aware of what the OSG says.

FYI, it was a faulty keyboard, rather than operator error

I see what you are saying here....but you cant stop at just plastic sections,compression joints are electrically unreliable,are you going to do a continuity test on every single pipe in the building between each electrically suspect joint and perhaps link across compression joints?
A sledgehammer to crack a nut IMHO....just stick to the requirements of BS7671.

I understand your point entirely. There was a similar thread about this a few weeks ago, someone wanting to know if they needed to bond radiators, and short sections of pipe which were supplied by plastic pipe. Whilst I would agree that there is no need to bond individual radiators, I would still expect a metallic heating system to be bonded.

Lets face it, the IEE cant make their mind up about earthing, so I doubt we are all going to agree with each other

This is rather off-topic, I'd be happy to leave it where it is.

 

[Crackedwolf]

In domestic situations,the minimum cable size to an external building attached to a TNC-s system would be 10mm ( the earth size must be equal to the main protective bonding conductor)
The other way of doing this if your cable to an outbuilding is under sized would be to segregate from the building cpc and install and RCD/RCBO to protect it .

 

[WayneL]

In domestic situations,the minimum cable size to an external building attached to a TNC-s system would be 10mm ( the earth size must be equal to the main protective bonding conductor)
The other way of doing this if your cable to an outbuilding is under sized would be to segregate from the building cpc and install and RCD/RCBO to protect it .

Unless there are no bonding requirements in the outbuilding - then a normal size CPC will do (i.e. 6mm SWA...2.5mm cpc)

 

[Deleted member 9648]

In domestic situations,the minimum cable size to an external building attached to a TNC-s system would be 10mm ( the earth size must be equal to the main protective bonding conductor)
The other way of doing this if your cable to an outbuilding is under sized would be to segregate from the building cpc and install and RCD/RCBO to protect it .

Can you please give the reg which states this requirement as I dont believe it is right.....happy to be corrected though.

 

[acat]

Closest I can find is 544.1.1 Where an installation has more than one supply source to which PME conditions apply, a main protective bonding conductor shall be selected according to the largest neutral conductor of the supply.


Cant find anything else hopefully someone else will


Chris

 

[Crackedwolf]

Wayne is right....if there is no gas, water or structural steel to bond.

Table 4.1 will give you main protective bonding conductor sizes if there are

Some authorities differ with regards to taking the pme system out to external buildings (ie southern & central networks, eastern don't )

In domestic new build situations providing calculations and networks supplier is ok you wil normally cover every base installing a cable with 10mm cpc. (obviously depending on neutral conductor size)

Forums are a minefield!!!!! contact the NICEIC helpline !

 

[Deleted member 9648]

I'm still confused as to whether you are stating the minimum size of the bonding conductor to the outbuilding or the size of the cpc.
The size of the main bonding conductor is related to the size of the neutral and earthing conductor at the origin of the installation, and has no relation to the size of the condutors feeding an outbuilding which is either a distribution circuit or a final circuit.

 

[Deleted member 9648]

Further to my post above I am certain crackedwolf is not correct.
The main bonding to extraneous conductive parts in an outbuilding must be continuous back to the MET at the origin of the installation where TNCS conditions apply. Accordingly it must be sized as required by bs7671 in relation to the neutral and earthing conductor at the origin of the supply.
The supply to the outbuilding itself is either a distribution circiut or final circuit and can be sized in the normal way with reference to load and protective device rating.
If crackedwolf was correct then all circuits on a TNCS system where main bonding is required would have to be a minimum of 10mm.

 

[Spudnik]

Indeed, main bonding in out buildings, if TNCS supply, should be 10mm min.

One of the reasons for using a rod and divorcing the earth, is to do away with the need to run a separate bond from the "shed" back to the MET in the house.

 

[Deleted member 9648]

Indeed, main bonding in out buildings, if TNCS supply, should be 10mm min.

One of the reasons for using a rod and divorcing the earth, is to do away with the need to run a separate bond from the "shed" back to the MET in the house.

Yes but as far as I can see crackedwolf has stated that the cable size (as well as the main bond) would need to be 10mm......that is not correct.

 

[sirocco]

So can anyone clarify, when specifying a distribution circuit from the main consumer unit in a house to an outbuilding with its own circuits.
If the main house supply is TN-C-S do you need 2core SWA using the sheath as earth or 3core SWA?
Confused!

 

[WayneL]

So can anyone clarify, when specifying a distribution circuit from the main consumer unit in a house to an outbuilding with its own circuits.
If the main house supply is TN-C-S do you need 2core SWA using the sheath as earth or 3core SWA?
Confused!

If there are no extraneous conductive parts in the outbuilding, then you will be able to use either as your CPC for the outbuilding.

If there are extraneous conductive parts in the outbuilding, then you will require a 'protective bonding conductor' to be run to the outbuilding and, in this case, the 'third core' or 'armouring' may not be sufficient.

Hope that's clearer

 

[sirocco]

If there are no extraneous conductive parts in the outbuilding, then you will be able to use either as your CPC for the outbuilding.

If there are extraneous conductive parts in the outbuilding, then you will require a 'protective bonding conductor' to be run to the outbuilding and, in this case, the 'third core' or 'armouring' may not be sufficient.

Hope that's clearer

Thanks Wayne,
Does that mean 2 core + sep CPC single core running alongside or 3 core + sheath?
I had thought that 3 core + sheath would be the best option to provide for future flexibilty /upgrade.
There are no extraneous parts at present, but if eg boiler fed heating was to be installed fed from the main house etc this could become an issue and providing for this now would be a relatively small investment.
Also if I wanted to split this supply in a 900mm trench to serve two outside locations what is the preferred method of T ing the cable at that depth?
Sorry to throw another problem into the pot.

 

[pushrod]

Also if I wanted to split this supply in a 900mm trench to serve two outside locations what is the preferred method of T ing the cable at that depth?
Sorry to throw another problem into the pot.

You could use something like one of THESE

 

[WayneL]

I think the best way to describe it would be to give an example as each installation will have different requirements.

Lets say you want to supply a brick shed, 5 metres from the house/CU, with a lighting radial and a socket radial.

If no Extraneous Conductive Parts:

You could sellect a 4mm, two-core, SWA cable and use the armour as your CPC/Earth.

You could sellect a 4mm, three-core, SWA cable and use the third core as your CPC/Earth.

Either of these should be appropriate to feed your outbuilding, protected at the house CU by a 32A MCB.

At the outbuilding you would likely have a 30mA RCD main switch and a 6A and 20A MCB respectively.

If there were Extraneous Conductive Parts in the shed:

This changes things completely.

The installation now requires that the extraneous conductive part/s are bonded to the MET (This can be via an 'Earth Marshalling Terminal' - basically a 'remote' MET - as shown in the diagram in post 15)

The bonding of these 'Extraneous Conductive Parts' follows similar rules to the bonding carried out in the house/main installation, so the armour/third core probably wouldn't do.

You would probably be looking at minimum 10mm bonding conductor, so you either run 10mm SWA 3- Core - or you run a seperate bonding conductor alongside your SWA.

The good news is you just connect this conductor to the Earth Bar (EMT) in your garage CU, and use it as a combined CPC/Bonding.

You then run any final circuit CPCs and bonding conductors from this EMT.

That's about as good as I can explain it.

As I said earlier, each installation is different so you have to decide what is best.

E.G. If the outbuilding was 50 metres from the house, you might decide it's easier to rod it.
It's your decision at the end of the day

 

[sirocco]

I think the best way to describe it would be to give an example as each installation will have different requirements.

Lets say you want to supply a brick shed, 5 metres from the house/CU, with a lighting radial and a socket radial.

If no Extraneous Conductive Parts:

You could sellect a 4mm, two-core, SWA cable and use the armour as your CPC/Earth.

You could sellect a 4mm, three-core, SWA cable and use the third core as your CPC/Earth.

Either of these should be appropriate to feed your outbuilding, protected at the house CU by a 32A MCB.

At the outbuilding you would likely have a 30mA RCD main switch and a 6A and 20A MCB respectively.

If there were Extraneous Conductive Parts in the shed:

This changes things completely.

The installation now requires that the extraneous conductive part/s are bonded to the MET (This can be via an 'Earth Marshalling Terminal' - basically a 'remote' MET - as shown in the diagram in post 15)

The bonding of these 'Extraneous Conductive Parts' follows similar rules to the bonding carried out in the house/main installation, so the armour/third core probably wouldn't do.

You would probably be looking at minimum 10mm bonding conductor, so you either run 10mm SWA 3- Core - or you run a seperate bonding conductor alongside your SWA.

The good news is you just connect this conductor to the Earth Bar (EMT) in your garage CU, and use it as a combined CPC/Bonding.

You then run any final circuit CPCs and bonding conductors from this EMT.

That's about as good as I can explain it.

As I said earlier, each installation is different so you have to decide what is best.

E.G. If the outbuilding was 50 metres from the house, you might decide it's easier to rod it.
It's your decision at the end of the day

Thanks Wayne,
Great response, its getting clearer all the time.
Couple of further queries
The water supply to my outbuilding is 32mm Polyethylene Blue pipe fed directly from a street main with my house tee'd off it also in polyethene.
The only metal connections to this pipe in the outbuilding is a tap screwed to a plastic sink and a toilet cistern.
The drainage is similarly plastic connected to the street sewer.
Does the fact that, although the supply pipe is clearly non conductive, water flows in it make it in anyway an extraneous conductive part?
There are no metal gas/oil or water supply pipes coming into the outbuilding ,only plastic.
Also does the diagram at Post 15 hold good for both TNS and TNCS sytems?
Thanks for your help
Graham

 

[pushrod]

Does the fact that, although the supply pipe is clearly non conductive, water flows in it make it in anyway an extraneous conductive part?

Graham

Not an extraneous conductive part

 

[Hundleton1]

Got a question on this, looked at a job today, needs a shed at top of garden on supply, was going to run 3 core SWA up to it via the garden wall however the problem is i cant get the supply back to the CU, its a terraces house and the board rite at the front, i would have to rip the house in half to get a cable to it, there is just no way to get there with out them moving out.

the best option is off the utility room ring main as thats where the back door is, the main CU is split load twin RCD


whats the best way of doing this as its really classed as a spur ?

i was going to put a new CU in the shed with 6 and 16A MCB for lights and sockets with a another one in the utility room with the 20A, however the regs state that a fused spur must be limited to a rating of 13A

i was also not going to fit a RCD as per the RCD in series i keep reading about

 

[jase]

feeding off main house C.U would also be dependent on size of the submain to outbuilding too right? say if using a 16mm 3 core swa would be hard to terminate the earth and neutral in the C.U s so best option would be to split tails into fused switch, earth the swa banjo into an MET and then at outbuilding C.U bring in the 16 mm swa tails and earth with a rod and 10mm? earth

 

[Deleted member 9648]

Got a question on this, looked at a job today, needs a shed at top of garden on supply, was going to run 3 core SWA up to it via the garden wall however the problem is i cant get the supply back to the CU, its a terraces house and the board rite at the front, i would have to rip the house in half to get a cable to it, there is just no way to get there with out them moving out.

the best option is off the utility room ring main as thats where the back door is, the main CU is split load twin RCD


whats the best way of doing this as its really classed as a spur ?

i was going to put a new CU in the shed with 6 and 16A MCB for lights and sockets with a another one in the utility room with the 20A, however the regs state that a fused spur must be limited to a rating of 13A

i was also not going to fit a RCD as per the RCD in series i keep reading about

How much load is going to be used in the shed?....I would of thought a 13a FCU would cover most sheds.

 

[pushrod]

whats the best way of doing this as its really classed as a spur ?

i was going to put a new CU in the shed with 6 and 16A MCB for lights and sockets with a another one in the utility room with the 20A, however the regs state that a fused spur must be limited to a rating of 13A

i was also not going to fit a RCD as per the RCD in series i keep reading about

run it to a 13A metal clad FCU in the shed and wire it in either1.5mm T&E clipped direct, or 1.5mm singles in plastic conduit. Can't really see the point in a submain, just alot of expense.

 

[sirocco]

Waye,
Again, great post
Would the layout shown the diagram hold good for a TN-S system as well as PME?
Graham

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.

 

[jase]

Waye,
Again, great post
Would the layout shown the diagram hold good for a TN-S system as well as PME?
Graham

what about if the main house was a TT?

 

[burt]

A definitive answer to this question (without it being immediately corrected) would be really nice. Do we rod the outbuilding and separate the earthing system from the house when supplying an outbuilding?

 

[telectrix]

A definitive answer to this question (without it being immediately corrected) would be really nice. Do we rod the outbuilding and separate the earthing system from the house when supplying an outbuilding?

read post#2 and others that explain it clearly.

 

[Deleted member 9648]

A definitive answer to this question (without it being immediately corrected) would be really nice. Do we rod the outbuilding and separate the earthing system from the house when supplying an outbuilding?

Definately maybe or not at all.

 

[Joe V.]

Although I think Pushrod may misunderstand the definition of 'extraneuos' (I also misunderstood the exact definition before someone on this forum corrected me), I agree with him on his point about earthing pipework. If I saw a house with a copper heating system, which had been electrically dissconected from the boiler by a short section of plastic pipe, I would want the remaining copper pipework to be bonded.
There are far too many ways a pipe (and all the radiators and taps connected to it)could become live. I should think that there are enough sparks who have seen live unearthed metalwork to agree

It may not be covered by the regs, but it's good practice at the very least.

This reminds me of a house where the CH radiators/pipework repeatedly attracted lighting strikes through the wall (breeze block & timber frame). Does / how does the presence of a lightning protection system on the outbuilding influence the choice of local rod or extending Earth from the main building ?

 

[GMES]

The thread is 3 years old so time to close it.