Supply to OUTBUILDINGS

[acat]

Hi Folks

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


Thanks


Chris

 

[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