TT sub mains and discrimination..

[Yellowhammer]

Hi,

Just a quick sanity check really around rcbo discrimination.. but also RCDs being used as L-E fault protection.

Scenario being a garage board change. Setup is: garage sub main fed from a main house CU. Initially wired in 10mm t&e with a 10mm earth along side it, run entirely in the ceiling of the house no walls etc., pops out on the external wall rear entry to a box, joints to a 10mm armoured that's running to the garage. Garage will have its own rod and small rcbo board

Given it's TT and realistically this is going to be reliant on the 30ma rcbo at the board to provide L-E protection am I right in thinking it has to be on a 30mA rcbo at the board (not that sourcing a 100mA rcbo would be particularly simple anyway, not to mention then running into discrimination issues with the 100mA unit before the main CU..), obviously having the 30mA RCBOs in the garage means no discrimination, but to me the discrimination issue is 'less bad' given the main CU is rcbos so you're not going to be taking out half the house with a garage fault, just potentially having to check both boards. Fault protection/safety taking priority over convenience here seems the sensible option to me..

 

[timhoward]

So boiling this down, your question is whether the 100ma RCD at the house is adequate fault protection for the distribution circuit to the garage?

 

[Yellowhammer]

Essentially yes, apologies if it's waffle, a real talent of mine . Either I leave it on the 40A/30mA rcbo at the CU without discrimination, or put a 40A MCB on and rely on the 100mA pre CU. In full swa that's exactly what I'd do, it's just the t&e that's throwing me off really.

 

[loz2754]

Presumably the 100mA RCD in the house is a time delay or S type? If not then there will be no selectivity achieved.

 

[Yellowhammer]

Presumably the 100mA RCD in the house is a time delay or S type? If not then there will be no selectivity achieved.

Yes, it's an S type

 

[loz2754]

So if the cable routing and installation methods do not call for additional protection by 30mA RCD, and all circuits at the other end that need 30mA are so protected, then yes, the distribution circuit will be fine on an MCB.

 

[timhoward]

With disclaimer I don't have the regs book here my memory is:
TT Fault protection - distribution circuit, 1 second max disconnection time. So should be met by S-type RCD (500ms max operating time I think)
Provided Ze and CPC total less than 500 ohms (which I'd hope is the case!) you will likely meet TT touch voltage formula 499 ohms X 0.1A < 50V

All fine so far.
RE the twin and earth, Impact Protection regs would need to be met for the bit in then ceiling.

 

[timhoward]

deleted

 

[Yellowhammer]

Many thanks all. Very helpful to be able to bounce these things around. Was fairly happy with it all initially, but once the seed of doubt is there it has a habit of manifesting itself..

Agree that the 100mA will comply with the rcd fault protection requirements in 531.3.5 relating to RA and disconnection of 1s from 411.3.2.4

I've been able to get eyes on about 85% of the run in the ceiling and it's uniformly centre of the joists definitely >50mm from floor and ceiling, and running to the CU it's in trunking so fairly happy from that perspective re. 522.6.201.

Will likely have Napit assessment against this job so just wanted to make sure I wasn't missing anything caveating the above.

Thanks again