As folk have said above, you need an up-front RCD to disconnect on any cable faults as the TT earth wont do so one the OCPD.
For selectivity that has to be x3 above the final circuit RCDs and to have some delay. Most common for domestic would be 100mA S-type (just to add confusion on AC/A/B/F/etc nomenclature!) and they ought to be fairly readily available.
With your rod impedance you could use a 300mA (Ra < 167 ohm) or 500mA (Ra < 100 ohm) delay RCDs. But it is unlikely to be needed as:
A final concern for selectivity is the downstream RCD or RCBO must also disconnect the faulted circuit's neutral, otherwise a N-E fault (or hard L-E fault on a low impedance load) will still trip the main RCD as they won't stop current imbalance due to the spurious neutral path.
All standard RCD (e.g. dual RCD bard) do this, but many RCBO do not. In the UK it seems you choice for RCBO would be best from the new compact Fusebox, Wylex, or Crabtree RCBO that are neutral-switching DP style.
TL;DR Use 100mA delay up-front, then DP RCBO board (or dual RCD board if you must).
For selectivity that has to be x3 above the final circuit RCDs and to have some delay. Most common for domestic would be 100mA S-type (just to add confusion on AC/A/B/F/etc nomenclature!) and they ought to be fairly readily available.
With your rod impedance you could use a 300mA (Ra < 167 ohm) or 500mA (Ra < 100 ohm) delay RCDs. But it is unlikely to be needed as:
- A 100mA S-type copes will cope with up to 50mA normal leakage, which is unlikely in any sort of system (short of a massive amount of computers on single phase)
- You are probably not to cascading further RCDs for sub-DBs where you would need to have again a x3 sensitivity factor and longer delays for hard-fault selectivity.
A final concern for selectivity is the downstream RCD or RCBO must also disconnect the faulted circuit's neutral, otherwise a N-E fault (or hard L-E fault on a low impedance load) will still trip the main RCD as they won't stop current imbalance due to the spurious neutral path.
All standard RCD (e.g. dual RCD bard) do this, but many RCBO do not. In the UK it seems you choice for RCBO would be best from the new compact Fusebox, Wylex, or Crabtree RCBO that are neutral-switching DP style.
TL;DR Use 100mA delay up-front, then DP RCBO board (or dual RCD board if you must).