TNCS Outbuilding earthing arrangement

[swifters]

I have a large shed (affectionately known as the warehouse) that is fed by a 16amp rcbo. The main earthing arrangement at the distribution boards is tncs and now a more experienced electrician than myself has stated that the earth in the 3 core 2.5mm armoured cable should be connected to a separate earth rod and make its own tt earthing arrangement rather than be fed back to the main supply, stating parallel paths are the reason. Is he right? Could anyone point to it the new regs book?

many thanks, Jon

 

[davesparks]

There is no Regulation requiring out buildings to be TT.
The main reason for considering making an out building TT is simply cost.
Is it cheaper to run a 10mm bonding conductor from the house MET to the out building, or to install an earth rod at the out building?

For an installation with 10mm bonding then I doubt there is much cost difference.
However if the installation needs 50mm bonding and an outbuilding is to have a 32A submain the TT option may be considerably cheaper

 

[Deleted member 26818]

Yes, and it may be that the difference in cost is negligible when compared to the greater safety afforded by PME over TT.

 

[Monkeyblaine]

Sorry for making you spell it out. But thank you very much for clearing my thinking.

 

[Had8Lives]

Its a Di Log 9083p I think. It is meant to be non tripping but always does.

Are you putting the neutral and earth probes on the earth connection together? I was taught at college to do it, but not all testers need it and the DC bias that is meant to 'hold' the RCD on, will trip it.