Nobody makes a type S 30mA RCD as
@mainline points out it would not meet disconnection times.
You have two issues for selectivity:
- One is when there is a 'hard fault' and so you need some delay to allow the downstream RCD to clear irrespective of the magnitude of the fault current.
- The other is for a 'soft fault' where the imbalance is small and rises slowly, or indeed is simply there all of the time due to accumulation of appliance leak currents, etc. For that you need a difference in trip threshold so again the downstream one always goes first.
So if designed you have no other choice then to go 100mA selective for upstream and then, to meet regulations, it has to be installed so accidental cable penetration is very unlikely. Yes, I know the BBC have some guidance limiting temporary supply RCDs to 100mA / 200ms max as it gives you some protection against a fatal shock, but that is not meeting the BS regs.
If you can clamp the sub-main and check the leakage you might find it is hovering in the 10-15mA region in which case it is not going to be cured by anything other then a change of upstream RCD to a higher current and all of the other implications of that. On the other hand, if it is normally below 10mA and the issue is the odd spike incident (like my oven that trips the RCBO a couple of times a year on switching off) then it
might just be worth looking at ones of these:
Schneider Electric Global. A9R61263 - Residual current circuit breaker (RCCB), Acti9 iID, 2P, 63A, A-SI type, 30mA.
www.se.com
They claim to be less prone to false triggering while still meeting the overall low frequency trip current and times. Here is the FAQ comparing the A-SI type they call it with the normal type A: