Water Bond size on outbuilding

[The General]

Evening all.

Bit of an odd one this and I just wanted to run it past bigger brains than mine to make sure I'm not missing something obvious.

A brand new PME supply is being brought across to a field which has got 3 outbuildings, all of which need lights and a couple of sockets.

The supply comes to the first building, and is going to run in armoured out to the next and then onto the 3rd building. Each building will have it's own mini 'garage' C/Unit with RCD etc. In the third building there is a solitary sink which is the only water supply for the whole place and the only metal incoming service pipe.

My initial thought was to bond from the MET at the 3rd C/U in the building with the sink, but I've suddenly just had a panic that I ought to be taking the 10mm cable all the way back to the original MET at the 1st board where the supply comes in.

Then I started doubting myself even more - the incoming supply to the 3rd building is 6mm 3-core armoured on a 32A breaker - it seems mental to have a water bond in 10mm when the main earthing conductor for that building is only 6mm?


So, my question is:
10mm water bond back to main MET on 1st board?
10mm water bond to MET on 3rd board?
6mm water bond to MET on 3rd board?
any other options!?!?

Hope that makes sense - it's been a long day and my head hurts - the beer hasn't kicked in yet either ;-)

Thanks for your thoughts!

 

[Deleted member 26818]

You either need a 10mm Main earth and 10mm bonding conductors, or no earth and make the outbuildings TT.

 

[Richard Burns]

Main bonding of services should go back to the MET at origin.
You are trying to ensure that the services have the same potential as the MET in fault conditions with no significant voltage drop.
The Main bonding (for PME) is sized according to the supply neutral conductor size and can be larger than the earthing for any individual sub main.


For your situation, assuming your supply neutral does not exceed 35mm², I would tend to supply the buildings with 4 core 6mm² and double up two cores to provide an earthing and bonding conductor throughout. Then you can link the EMT at each building back to the MET without problem.


I can't quite make out if the cables have been laid already or if this is in the planning stage.

 

[Leesparkykent]

I would just use 10mm 3 core SWA if a 10mm bond is required.

 

[Deleted member 26818]

Does the sink require bonding?

 

[Dave OCD]

Can't they do the water supply in alkathene ?

 

[The General]

Main bonding of services should go back to the MET at origin.
You are trying to ensure that the services have the same potential as the MET in fault conditions with no significant voltage drop.
The Main bonding (for PME) is sized according to the supply neutral conductor size and can be larger than the earthing for any individual sub main.


For your situation, assuming your supply neutral does not exceed 35mm², I would tend to supply the buildings with 4 core 6mm² and double up two cores to provide an earthing and bonding conductor throughout. Then you can link the EMT at each building back to the MET without problem.


I can't quite make out if the cables have been laid already or if this is in the planning stage.

sort of mid-way through! I thought I had it all sorted and then discovered this water pipe in the final building.
it's too late to change the main armoured cable, so I'll just have to run an extra 10mm single back to the MET.
Poo bum. I'll remember your idea for next time though!

 

[The General]

Does the sink require bonding?

Sorry - pipe supplying the tap supplying the sink needs bonding.....

 

[Deleted member 26818]

Don't have to run another conductor, can make the outbuilding with the sink TT.
Depends what's cheaper?

 

[telectrix]

as long as it has 30mA RCD for all circuits, divorce the 3rd building from the earthing system and make it TT, cheapest option.

 

[The General]

Don't have to run another conductor, can make the outbuilding with the sink TT.
Depends what's cheaper?

Good idea. Definitely cheaper/quicker.
so what do I do with the 'spare' Earth on the incoming SWA? It has to remain connected at building 2 as it's the cpc for the sub main, so I just snip short of the connection at building 3 and tape it up?

 

[telectrix]

Good idea. Definitely cheaper/quicker.
so what do I do with the 'spare' Earth on the incoming SWA? It has to remain connected at building 2 as it's the cpc for the sub main, so I just snip short of the connection at building 3 and tape it up?

no. you gland it into a plastic ad. box or mnini CU, but don't banjo it or take the cpc anywhere into the installation of the 3rd building.

 

[Simonslimline]

You can even get insulated glands for this situation i believe.


Edit...

http://www.cmp-products.com/uploads/documents/768da3b3b0734472a7ed7c0ae57301f9-1591.pdf

 

[The General]

as long as it has 30mA RCD for all circuits, divorce the 3rd building from the earthing system and make it TT, cheapest option.

Each circuit has 30mA except for the main armoured supplying building 2 which has 100mA type S

 

[bigspark17]

I would just use 10mm 3 core SWA if a 10mm bond is required.

As above^^^.! Poor design on your part, bit of a schoolboy error, extraneous parts in outbuildings supplied off tn-c-s should be top of priority at design stage!!

 

[Deleted member 9648]

Evening all.

Bit of an odd one this and I just wanted to run it past bigger brains than mine to make sure I'm not missing something obvious.

A brand new PME supply is being brought across to a field which has got 3 outbuildings, all of which need lights and a couple of sockets.

The supply comes to the first building, and is going to run in armoured out to the next and then onto the 3rd building. Each building will have it's own mini 'garage' C/Unit with RCD etc. In the third building there is a solitary sink which is the only water supply for the whole place and the only metal incoming service pipe.

My initial thought was to bond from the MET at the 3rd C/U in the building with the sink, but I've suddenly just had a panic that I ought to be taking the 10mm cable all the way back to the original MET at the 1st board where the supply comes in.

Then I started doubting myself even more - the incoming supply to the 3rd building is 6mm 3-core armoured on a 32A breaker - it seems mental to have a water bond in 10mm when the main earthing conductor for that building is only 6mm?


So, my question is:
10mm water bond back to main MET on 1st board?
10mm water bond to MET on 3rd board?
6mm water bond to MET on 3rd board?
any other options!?!?

Hope that makes sense - it's been a long day and my head hurts - the beer hasn't kicked in yet either ;-)

Thanks for your thoughts!

The reason the main bond has a larger cross sectional area requirement for PME/TNCS installations is because you are effectively creating a parallel path for return currents by connecting the neutral to a conducting service at earth potential. Therefore a significant current can flow through the bond when then installation is loaded.