Julie, hope you had a good Christmas.. ive been having more thoughts on my TT earthing arrangement at my pond. The pond supply is protected by a single 30ma RCD back at the solar/battery backed up supply DB in the garage.. which trips at 66.2 ms when rcd test carried out at pond fuse box. This rcd is the only shock protection. Should the rcd fail to trip with fault introduced I'll have circa 7 amps of earth fault current flowing through my earth rod at 36ohms which will not trip the pond supply breaker which is 16amps at the pond end 20amp at the garage supply end. I guess I do have the individual fuses on the pond equipment themselves for example pond uv light which on a 2amp fuse so this would blow within 400ms? How can I make this more robust if needed? Another rcd at the pond end?
I kind of share the same concerns, when an RCD is present as additional protection then I am quite comfortable, but like in this case - the rcd is fault protection (411.4.204/411.5.2 sort of thing) just having this one device just doesn't feel enough.
By the regulations of course it is sufficient, the imbalance would be detected by the RCD, so providing it works all is good.
However, in general for ADS you have the MCB/fuse on the circuit itself, and if that doesn't operate, then the upstream main fuse will eventually operate, this along with the inherent reliably of fuses (and MCBs to an extent), plus the RCD for additional protection gives a really comfortable feeling that a fault will be cleared by at least one of the three things there.
I just don't get that when you have just the one device, especially as RCDs just haven't the same inherent reliably as other devices.
The standard solution is as used for TT installations, you have a higher current 300ms RCD upstream, and a 30mA instantaneous RCD on the circuit itself. In your case as they would be both on the same circuit, they could both be standard 30mA inst RCDs , either, or both could operate and the use of two would just provide redundancy in the event one failed to operate correctly.
But as indicated above it is actually compliant with the one RCD for fault protection