Exporting TN-C-S

[Engineer54]

thinking out out of the box here, wouldn't the 16mm Earth core plus the swa armouring combined be equivalent to 25mm??

In a single word ....NO!!

One or the other needs to be capable of carrying the full expected fault current, you can't combine two different metals with two vastly different conductivity values for obvious reasons....

Oh and if as you say this garage is steel framed and the sprinkler system has been fixed to the steel anywhere, it will still need main bonding. Always test all metal pipework services whether they enter a building in plastic or not....

So what is this installation all about then?? How about giving us a bit more information??

 

[Leesparkykent]

If the SWA cable to be used as a combined CPC and main bonding conductor it must meet the requirements of both 544.1.1 and 543.1.1, In addition with a PME system there may be network circulating currents flowing normally in the protective conductors which may raise the temperature of the SWA cable. In these circumstances the cable will need to be de-rated to accommodate this temperature rise. The resistivity of copper at 20C is 17.2 x10-9. The resistivity of steel varies with carbon content from 100 to 1000 x 10-9 that's why GN8 suggests using a factor of 8 for the ratio of the resistivity of copper to steel.

To put it another away your 3core 16mm swa won't cut the mustard to attempt it this way.

It's simple pull a 25mm bond in or TT the outbuilding, that's the only two options you have.