Smallest Sub-Main board with selectivity

Msitekkie · May 7, 2022

I need to fit a small sub-main board by my service head, to feed a garage unit in an outbuilding.
I am thinking of using a Chint shower unit (Screwfix), but replacing the internals with a S Type Time Delayed RCD.
Is this a valid approach for a sub-main, as opposed to using a switch/fuse unit from the likes of Wylex? I am aware consumer units used to be available with an RCD main switch, but have the impression the S type RCD approach would normally be used where the sub-main is being fed as an extra circuit from the existing consumer unit (not an option here).
Space is at a premium & I am aiming at the smallest possible solution - actually I see Chint even do a 2 module enclosure I could use in this scenario.

westward10 · May 7, 2022

Does the submain need rcd protection?

pc1966 · May 7, 2022

There are a number of factors to consider.

The sub-main need protection against faults, and usually protection against overload. Depending on the supply characteristics you might be able to meet all of those with a fuse or MCB, but if you can't due to high supply Ze or similar then using a delay RCD to meet fault to earth disconnection times is a valid approach.

But even with the delay RCD you probably still would need overload protection and you might also want to know that is selective with the supply protection (so a major L-N short on the sub-main won't take out the main DNO fuse). This is where a supply fused-switch is handy as normally BS88 fuses are totally selective at a 1.6:1 ratio, so 100A supply and 63A sub-main, or 80A/50A and so on.

Msitekkie · May 7, 2022

westward10 said:
Does the submain need rcd protection?

Good question. I hadn't considered the possibility I could get away without it (& thus without selectivity issues). It is SWA cable, part surface, part buried.
The reply from pc1966 below appears to suggest, protection is required, although he specifically mentions overcurrent protection for selectivity with the main fuse rather than S type RCD applying selectivity in the opposite direction. That would appear to suggest Type S RCD + MCB.
Time delayed RCBOs don't appear to exist.

westward10 · May 7, 2022

Assuming this is a TN earthing system although it isn't an absolute requirement for TT why does it need rcd protection if ADS is maintained without it.

Msitekkie · May 7, 2022

westward10 said:
Assuming this is a TN earthing system although it isn't an absolute requirement for TT why does it need rcd protection if ADS is maintained without it.

It is TN. 100A main fuse at the service head. There is TT at the outbuilding end, but the sub main cable itself is on TN earthing & separated from the TT.
If overcurrent protection is required in the manner pc1966 suggests, that means fitting an MCB. As I want to be able to isolate this circuit on it's own, that then requires either a double pole main switch or a (double pole) RCD (assuming I can't use a double pole MCB - as mentioned). If I have space to use an RCD as well as an MCB, that would appear to be safer in some scenarios, so I am thinking why omit it, to save a few pounds? Is there any other benefit?
I do plan to add an earth electrode later as suggested in amendment 2 (hopefully a Condudisc), partly for broken PEN conductor mitigation & partly for a future Island Mode scenario, although currently I am not expecting to make this sub-main circuit live if I am off-grid.