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]

 

[spark 68]

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

that is over optimistic steel is 8.5 X less conductive than copper according to GN8, 22 /8.5 gives around a copper equivalent of 2.5mm

 

[Leesparkykent]

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?

SWA, armour equivalent copper - Ted

Edit but as spark 68 has just pointed out this chart is over optimistic lol

 

[uksparks]

Lee, can you just confirm in reading that correctly, a 6mm 3 core has s copper equivalent of 7.40mm but its 23in steel yes?

 

[spark 68]

SWA, armour equivalent copper - Ted

I gave up with most of those types of tables after that last thread lol , we used to use the division of 8, but after looking in GN8 where it suggests 8.5 I might have to revise this again, luckily I used an extra core where there was doubt, erring on the side of caution.

 

[uksparks]

I gave up with most of those types of tables after that last thread lol , we used to use the division of 8, but after looking in GN8 where it suggests 8.5 I might have to revise this again, luckily I used an extra core where there was doubt, erring on the side of caution.

Spark68, sorry I don't know your name, but are you suggesting then just divide it by 8.5 as per GN8

 

[rattlehead85]

Just bare in mind sizing earthing and bonding are 2 different concepts to chew over.

 

[rattlehead85]

Going back to the original question,..so if i source an insulated gland (which will be interesting at the wholesalers) i will comply?

 

[uksparks]

Yes very true, when you think about it, why is it you can have a 6mm cpc but need a 10mm bond eh? Seems silly doesn't it.

 

[spark 68]

Spark68, sorry I don't know your name, but are you suggesting then just divide it by 8.5 as per GN8

I am going to re-think and revise this, as I think probably GN8 might be on the over cautious side, I am going to have a closer look at some cable data where the SWA can be used as the CPC, and try to find some sensible calculations, but yes in GN8 it says that steel is 8.5 times less conductive than copper, more research is needed I think.

 

[Leesparkykent]

Uksparks I know it's a bit of topic but if using the swa as the cpc ( not for bonding) use table 54.7 in the BYB page 166 Then you just have to compare it to the manufactures data for the typical CSA of the swa.

 

[Engineer54]

What you're all reading here is for CPC compliance only, NOT when (in this case steel wire armouring) is to also comply for MPB, especially where the supply is PME. In this case the CSA of the steel wire needs to be around 80mm to be equivalent to a 10mm copper cable.... It's that simple!!

 

[Leesparkykent]

That's hope Geoff doesn't see this thread other wise we are in this for the long haul haha :icon12:

 

[spark 68]

That's hope Geoff doesn't see this thread other wise we are in this for the long haul haha :icon12:

Oddly enough lol OT for this subject , I was looking through GN8 at the hi-integrity earthing section, and you can use the armourings for the second CPC, conditions permitting of course it actually states this is allowed for that application. Bloody regs! they just make it up as they go along lol :ack2:

 

[rattlehead85]

Yes as we have identified some posts back the armouring can be used as a cpc upto about 95mm2 for swa however not for bonding purposes unless you can ascertain the armouring is correctly sized using the equation provided in gn8

 

[davesparks]

Yes very true, when you think about it, why is it you can have a 6mm cpc but need a 10mm bond eh? Seems silly doesn't it.

Because main bonds are sized according to the incoming service wheras a CPC is sized accordingly to each circuit.