New Build house off existing supply, export PME or TT

[grandfortune]

HI all,

customers are building a small house and im coming off thier PME supply to the new build, 25mm 3 core SWA, small two bed house about 40mtrs away

builders have put another MEB box next to the existing one and im fitting a metal clad switch in with 100amp 100ma time delayd RCD to RCBO board in new build.

can i export the earth or should i TT the new house, trouble is ive got metal boards both ends so if i TT it ill have to take the SWA out of the new build board and terminate it into a plastic enclosure first to isolate the PME earth then off new build board to spike, that seem right people?

Cheers
Grand

 

[buzzlightyear]

New house new supply from the DNO.

 

[Baddegg]

New house new supply from the DNO.

I’m guessing he means new build on private land?...

 

[pc1966]

As you have 3-core 25mm the CPC core (in addition to the armour) is plenty good enough for earth bonding to any extraneous parts in the new property so just use that.
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You should also be providing short circuit protection for that cable, presumably you are feeding it with a 80A or similar fused-switch from splitting their meter tails?

 

[Wilko]

Hi - as per Buzz, I like a new supply for a new dwelling. If they’re getting planning for a separate residence, just saying. Perhaps calc the final loop impedance as it may be a bit high after adding 50m of cable to it.

Personally, I like TT for outbuildings probably because it’s easy to get a good result in my area and I’m lazy striving for excellence .

 

[R-fur]

If this is a temporary supply then fine I would advise the builders not to do this as a permanent supply.
A number of years ago I got involved in a job where a farmer built a granny house on his land and supplied it from the farm, several years later granny died and the farmer decided to sell the house, sale was agreed but then the buyers solicitor found that the house was fed from the farm, cost the farmer a fortune getting a DNO supply, in a short time frame.

 

[pc1966]

Perhaps calc the final loop impedance as it may be a bit high after adding 50m of cable to it.

Personally, I like TT for outbuildings probably because it’s easy to get a good result in my area and I’m lazy striving for excellence .

If it is an all-RCBO board and/or has a 100mA incomer then you can use either a rod for TT or the CPC extended from the original PME supply, as either is low enough Zs to meet the disconnection requirements.

Lazy option there is to use the supply earth!

 

[buzzlightyear]

extended from the original PME supply, as either is low enough Zs to meet the disconnection requirement

By doing that lose a neutral then you are buggered it would be better to TT it.

 

[pc1966]

By doing that lose a neutral then you are buggered it would be better to TT it.

I guess it comes down to how likely is a PME fault and you being in contact with true Earth, versus how reliable your TT rod is.

In most cases having the supply earth will lead to disconnection on the over-current trip so the RCD is "additional protection", but on a rod the RCD be comes primary protection. Of course if you have both a incomer delay RCD and RCBO then you don't have that single point of failure and you need not worry about having your extraneous parts touched during a fault...

 

[davesparks]

builders have put another MEB box next to the existing one and im fitting a metal clad switch in with 100amp 100ma time delayd RCD to RCBO board in new build.

You need a fuse in there not an RCD, and yes use the DNO provided earth.
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By doing that lose a neutral then you are buggered it would be better to TT it.

It will be exactly the same as every building that is fed from a PME supply, so by your logic here every installation with a PME supply should be converted to TT.

It is better, in my opinion, to use the PME earth and install supplementary earth rods connected to the MET.

 

[grandfortune]

Thanks for the reply's people, ill put an 80amp fuse in also

Cheers
Grand