TT install, re -wire with power to out house not attached

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Hi Chaps, currently re-wiring a house with an overhead supply and TT (well now there is).

DNO are coming tomorrow to hopefully supply PME.

Now would like peoples opinions on this part.

There is a 16mm SWA to the out house 45m away, what I am trying to decide is whether to:

1) place this on an unprotected way, with a 40A RCBO to a board with 100A main switch

or

2) Henley block to fused switch to RCD garage CU.

Like your thoughts on this as it is the ONLY TT system I have had to work on thus far and I know some of you have a lot of experience on them

Thanks in advance
 
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Personally I'd wait until you know whether you have PME or not.

If you are stuck with TT then you will have no choice but to provide some form of RCD protection for the cable at the origin. If you have a TN system it's generally considered better to put the RCD protection in the outbuilding.

Further if you have PME you will have to decide whether to take the PME earth to the outbuilding or whether to make the outbuilding a seperate TT system. How many core is the SWA? what is the outbuilding made of? are there any metallic services coming into the outbuilding?
 
In the same way that the tails from the meter to the CU are RCD protected? ;)
Different installation methods require different levels of protection!

Meter tails are generally visible and therefore unlikely to be penatrated by a tool. So all they need in the way of protection is double insulation (the double insulation so that any damage becomes obvious before it becomes dangerous).

Underground cable runs OTOH are considered to be at risk of being dug through. Protection against this is provided by the earthed armour which will short to the live conductors if the cable is dug through. However this only provides protection if the earth fault loop impedance is low enough to trip a protective device located before the cable. The reliable earth fault loop impedance* on a TT is (at least on every TT i'm aware of) too high to trip a MCB. Therefore RCD protection is needed at the origin of the SWA run.

*that is not counting earthing paths through the main bonding which may dissapear at any time)
 
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There is a 16mm SWA to the out house 45m away, what I am trying to decide is whether to:

1) place this on an unprotected way, with a 40A RCBO to a board with 100A main switch

or

2) Henley block to fused switch to RCD garage CU.

You cannot connect the SWA direct to Henleys unless you provide fusing. If you don't then the only thing between your SWA and the sub station would be your main (DNO's) supply fuse.:eek:

Go with 1 or use a suitable contact breaker in an unprotected way and terminate the SWA onto an RCD protected board located in the out house.
 
You cannot connect the SWA direct to Henleys unless you provide fusing.
Henley block to fused switch to RCD garage CU.
;)


If you don't then the only thing between your SWA and the sub station would be your main (DNO's) supply fuse.:eek:
Which could be safe with 16mm², but DNOs don't like it...


Go with 1 or use a suitable contact breaker in an unprotected way and terminate the SWA onto an RCD protected board located in the out house.
(2) is fine, but as plug pointed out an RCD might be needed as well.

In fact, if the supply does remain TT then separating the outbuilding in that way would be beneficial - a 100mA Type-S could be used at the source, and DP RCBOs used in the outbuilding to keep any action local.

If the outbuilding is to be its own TT don't forget to isolate the SWA earth from it.
 
cheers guys, don't worry TT, we all misread. Any way up date; DNO came and installed PME ( I could have easily done it!!). So going with 40A RCBO to main board and exporting the earth
 

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