Metal CU in garage T.T system

[rattlehead85]

Would like a bit of advice on how others have tackled this.
Got a situation where we have a TN-C-S supply to a house with a 6mm 2core swa feeding a detached garage using a T.T. Earthing arrangement with a service water pipe within the garage which has a 10mm bond to it. My question is the client has provided a new Metal clad Rcd 30mA front line board to install in the garage, what would be the best and most effective way of separating the earthed armour of the swa feeding the garage from the earthing system employed to the garage itself T.T. [emoji106]

 

[uksparks]

I must be wrong but was under the impression you could use the steel?

 

[rattlehead85]

Why do you say it doesn't? It's 6mm 3core is it?

Because earthing and bonding are 2 different concepts. The resistivity of steel is 8x that of copper therefore you'd need the armour of the 6mm to be 80mm2 for it to comply as 10mm2 is required for a copper cable on a TN-C-S

 

[spark 68]

Why do you say it doesn't? It's 6mm 3core is it?

On the other epic thread it turned out that the copper equivalent of 3C 6mm SWA was about 7.odd mm, this is 2C so even less.

 

[Leesparkykent]

No the water is bonded back to the earthing within the garage. Cant see how you could drop the T.T. and connect the swa at both ends the water bond would then not comply size wise as the supply to the garage is only 2 core the armour on the 6mm aint big enough to carry the water bond back to source.

Sorry bud read it wrong and took it as the bond was connected to the MET and the garage sub board had been made a TT system.

IMO the easiest way would be to get the customer to go and exchange the board for a plastic one.

 

[uksparks]

Because earthing and bonding are 2 different concepts. The resistivity of steel is 8x that of copper therefore you'd need the armour of the 6mm to be 80mm2 for it to comply as 10mm2 is required for a copper cable on a TN-C-S

Ah ok so you can if its a big enough cable? I've got a pdf here somewhere with the CSA of the steel on.

 

[spark 68]

Ah ok so you can if its a big enough cable? I've got a pdf here somewhere with the CSA of the steel on.

Be careful UK, I had a dubious table that gave over optimistic copper equivalent values lol

 

[uksparks]

Ill go and dig it out, give me a min

 

[rattlehead85]

Sorry bud read it wrong and took it as the bond was connected to the MET and the garage sub board had been made a TT system.

IMO the easiest way would be to get the customer to go and exchange the board for a plastic one.

So what happens next year then if all this non combustible malarki means we end up going down the ferrous route??

 

[uksparks]

So what happens next year then if all this non combustible malarki means we end up going down the ferrous route??

I was just going to say that!

 

[uksparks]

Be careful UK, I had a dubious table that gave over optimistic copper equivalent values lol

This is the file I have, admittedly I have only ever looked at it a couple of times. What do you think? I'm sure I got it off this forum somewhere.

View attachment Reference Sheet - Actual cross-sectional areas of conduit, trunking.pdf

 

[rattlehead85]

I was just going to say that!

Wait for it one of the know alls will rock up shortly quoting all the technicalities of glow wire tests and why we should be questioning the interpretation of the byb.[emoji42][emoji42]

 

[Leesparkykent]

6mm 2core the swa has a CSA of 22mm and the equivalent in copper is 7.08

 

[spark 68]

So what happens next year then if all this non combustible malarki means we end up going down the ferrous route??

Hagar for one do a class 2 conversion kit for their metal clad boards for TT, Niccy say you can use an insulated gland and careful routing of the tails, or an upfront 100mA RCD or whatever where the EFLI is too high to disconnect the supply OCPD on TT.

 

[uksparks]

6mm 2core the swa has a CSA of 22mm and the equivalent in copper is 7.08

Can you tell me how you converted that please, I can find the resistivity values etc from somewhere no doubt but wondered if you have them all to hand on a PDF or something?

 

[ipf]

Sorry bud read it wrong and took it as the bond was connected to the MET and the garage sub board had been made a TT system.

IMO the easiest way would be to get the customer to go and exchange the board for a plastic one.

What!!!!!.....about the fire hazard. Are you not working to Am3 mg_smile:... :icon12: