Garage Supply - 2 way metal consumer unit in a seperate garage

[Dunc88]

Hi,
I'm installing a 2 way metal consumer unit in a seperate garage, the earthing arrangement in the property is TNCS. I am planning to install a new 4mm2 ciruit from the main consumer unit in the house, in 4mm2 swa to a garage consumer unit (5M run of cable).
There are no extraneous-conductive parts in the garage, so can I gland this swa directly into the garage consumer unit and have an RCD main switch feeding 1x 16a & 1x6a mcb? If so, how do I prevent both RCD's tripping (garage & main consumer unit)?
Would RCBO's still trip the RCD back in the main consumer unit?
Could I turn the garage into its own TT system? I could gland the swa into an adaptable box next to the garage consumer unit, & join the earth from the 4mm core to the swa inside that box, carrying the Live & Neutral through into the garage consumer unit, from there I would take a seperate earth down to an earth spike.
Thoughts/advice please!
Cheers

 

[Had8Lives]

I'd rather export a decent earth than rely on a twig ....

Tin hat on..

Ah, but, if you happened to be touching a metal clad socket and lightning struck a power line and it burned your arm you'd wish you'd put a twig in lol

 

[Midwest]

The only thing that's not been mentioned and I'm sure someone will correct me, I was told by an ELECSA assesor that you're not allowed to export a TNCS earth in less than 10mm cable. So, as you can't get an mcb unprotected in the house consumer unit, have a main switch in your garage consumer unit and put an earth rod in to earth your garage consumer unit.

Here goes then; in a typical domestic property, the 10mm bit only relates to the minimum size cable if you have extraneous conductive parts to bond. Or run a separate 10mm protective bonding conductor to the extraneous conductive part in the garage, and run an appropriate sized swa for load/vd only. Or as you suggested turn supply into TT, bonding locally as necessary.

Don't think there is an extraneous conductive part in the situation described by the OP.