TT earth rod cable

[marcuswareham]

Hello,

A friend of mine has a garage which is powered from an FCU on a ring final from his house, the garage is a little way from the house and is a TT earthing arrangement (the house PME earth is terminated in a plastic wiska box (where the SWA cable is terminated) and isolated from the garage earth), it then runs around the garage to a few socket outlets the earth rod is underneath the furthest outlet and has 10mm earth cable running back to the wiska box where there is a small MET on the wall just above it.

The 10mm earth runs a different way than the conduit for the sockets and he is having some brickwork repaired and a new door fitted so needs to remove the cable temporarily although he has asked me if he would be able to take the earth cable into the last socket directly above it instead of going back to the MET, this would then connect the earth rod to the CPC for the sockets, although terminating a 10mm in a socket may prove tricky, as the sockets CPC is 1.5mm then can the earth rod cable be a smaller size.

Currently, the earth rod is connected to the MET with 10m which runs around the garage then there is 1.5mm cable also terminated in the MET from the T&E from the sockets, so the MET is just taking the 10mm down to 1.5mm effectively

I am not quite sure if this is alright or not etc, I know its not the best way to get power to a garage but that was already there.

Thanks Marcus

 

[telectrix]

sounds a right lash-up.why did he not use the perfectly good earth from the house?

 

[marcuswareham]

sounds a right lash-up.why did he not use the perfectly good earth from the house?

He didn't do it,

 

[pc1966]

I guess the garage has its own RCD if it is TT'd? Or is this depending on the RFC's supply RCD?

Just heading to bed but there is a table of minimum earth conductor sizes, not just based on potential fault current but also on physical protection, etc. So that is your start point for checking this.

 

[Dartlec]

Is there also bonding to metal frame, parts of the building or anything else running back to the MET?

If so, then the 10mm may need to remain with it to the MET if it's the minimum size suitable for bonding in the regs (Table 54.8). It may be the reason why the cpc from the house was not used.

If it's a wooden shed with no extraneous parts, then you should be able to reduce the cable to the rod (allowing for earth fault current and minimum sizing) and connect it to the sockets.

This article has some more information

As mentioned above, hopefully there is an RCD in the garage, or RCD sockets used?

 

[marcuswareham]

Dont think there is any metal parts, maybe parts of the door frame which is being replaced.

There is an RCD on the ring final and it is feed from an RCD FCU so no discrimination

 

[marcuswareham]

I guess the garage has its own RCD if it is TT'd? Or is this depending on the RFC's supply RCD?

Just heading to bed but there is a table of minimum earth conductor sizes, not just based on potential fault current but also on physical protection, etc. So that is your start point for checking this.

I will have a look, still unsure if it is ok to have the earth connect at that point though, so currently the 10mm connects to the sockets cpc where the house earth is isolated, but could it be connected at the last socket instead regardless of the size etc