TT for Detached outbuilding

[Traineeboy]

Hi all we were looking at a job today where the customer wants to have a garage consumer unit from sockets and lights. The main house board has a spare way before the RCD with a convenient 32a MCB unused  I suggested to chap I was working with to have an SWA from the board at the 32a breaker ( no need for RCD protection there as its mechanically protected) the run 20 meters to the garage where there could be a garage board ( 40 a main switch is a 30ma RCD). Then used earth rod straight from garage CU. I was told that this shouldn't be done and we have to put in a S type RCD at the main house and the RCD at the garage needs to be 100ma. Am I going mad or was not the way I suggested OK ? The earthing arrangement is a PME. 

Your thoughts chaps 

 

[Fleeting]

Why does it need to be TT.

 

[Sharpend]

Not sure why you’re thinking time delay RCD, or 100mA, or TT but other than that………

 

[Traineeboy]

Not sure why you’re thinking time delay RCD, or 100mA, or TT but other than that………

Why does it need to be TT.

As it's PME we can't export the earth. I'm not saying 100mA or time delay the chap I was with said that. I thought it would be OK with a 30mA RCD. My understanding was you have to have an RCD upfront for fault protection as a TT systems Zs for it's a circuits would be over the 80% rule so RCD needed. 

 

[Fleeting]

"Exporting the earth" is a bit of a misnomer. What are you exporting exactly, the PME stops at the origin after that it becomes separate cpcs and neutral conductors, it is not possible for you to export PME you are employing a cpc derived from a PME earthing system.

 

[Traineeboy]

"Exporting the earth" is a bit of a misnomer. What are you exporting exactly, the PME stops at the origin after that it becomes separate cpcs and neutral conductors, it is not possible for you to export PME you are employing a cpc derived from a PME earthing system.

Ok I get it. If there was extraneous conductive part in the outbuilding would you then need an earth rod? Or in what case there the main house incomer is either a TN-S or TN-C-S would you need an earth rod in an outbuilding? 

 

[Fleeting]

If you have extraneous conductive parts which require main protective bonding then the cpc serving this area must be able to support it. For example if main protective bonds require a csa of 10.0 in the outbuilding then your cpc must be a minimum of 10.0. If this cannot be achieved then yes you can resort to a TT earthing system.

 

[binky]

in those circumstances I would run your armored cable to garage - not connecting Armour or CPC of cable to garage board, then rod and 30mA RCD main switch or 30mA RCBOs.

Fella you're working with is getting confused. 

 

[Traineeboy]

Great thought so Thanks for the clarity.

Locked to prevent being resurrected (again)