RCD Types for EV Chargers on TT systems

gkenyon: 
 

722.531.3.101 doesn't seem to care whether the EVSE has a Type B RCD or RDC-DD within it, except that you might be able to get away without the Type A RCD upstream if there's a Type B within it.

So, is the wording in 722.531.3.101 wrong?

722.531.3.101 seems reasonable for its scope of protecting the EV side of the RCD.

It ignores possible effects on upstream RCDs that could be protecting other parallel circuits as well as the cables to the EV installation. If the regulations are to be taken as a whole, it's reasonable to leave other parts of the regulations to deal with ensuring that upstream RCDs are suitable types to handle whatever the EV installation (with its local RCDs or built-in detection) might throw at them. The general one seems to be 531.3.3, which wants the 'appropriate RCD' to be selected for each case. If a new load is connected downstream, that appropriate choice may have to be reassessed for upstream RCDs.

Realistically, it might be good to have a note near 722.531.3.101 to remind that the installation of a new piece of power electronics could require upstream RCDs to be checked for suitability. After all, the 531.3.3 is vague, and not very helpful by giving short descriptions and then referring to IEC/TR 62350 [which would cost about two copies of BS7671].

Practically, of course, we come down to the usual arguments about how worthwhile the effort and cost is to try to avoid reduced protection in a rather special combination of circumstances - e.g. significant dc from EV system, but not enough to trip type-B, and also a pulsating dc fault in the same direction elsewhere in the system. But it seems that the intention of the regulations is for RCD protection not to be lost for any within-spec behaviour of equipment in the system.  (Mild changes of threshold are a different matter: e.g. an RCD in a 3-phase system could theoretically have its threshold pushed up to nearly twice its rated operation level if there's near-threshold leakage on the two other phases. No one seems to worry about this mild change that's unlikely actually to happen to the full extent.)
 

gkenyon: 
 

722.531.3.101 doesn't seem to care whether the EVSE has a Type B RCD or RDC-DD within it, except that you might be able to get away without the Type A RCD upstream if there's a Type B within it.

So, is the wording in 722.531.3.101 wrong?

722.531.3.101 seems reasonable for its scope of protecting the EV side of the RCD.

It ignores possible effects on upstream RCDs that could be protecting other parallel circuits as well as the cables to the EV installation. If the regulations are to be taken as a whole, it's reasonable to leave other parts of the regulations to deal with ensuring that upstream RCDs are suitable types to handle whatever the EV installation (with its local RCDs or built-in detection) might throw at them. The general one seems to be 531.3.3, which wants the 'appropriate RCD' to be selected for each case. If a new load is connected downstream, that appropriate choice may have to be reassessed for upstream RCDs.

Realistically, it might be good to have a note near 722.531.3.101 to remind that the installation of a new piece of power electronics could require upstream RCDs to be checked for suitability. After all, the 531.3.3 is vague, and not very helpful by giving short descriptions and then referring to IEC/TR 62350 [which would cost about two copies of BS7671].

Practically, of course, we come down to the usual arguments about how worthwhile the effort and cost is to try to avoid reduced protection in a rather special combination of circumstances - e.g. significant dc from EV system, but not enough to trip type-B, and also a pulsating dc fault in the same direction elsewhere in the system. But it seems that the intention of the regulations is for RCD protection not to be lost for any within-spec behaviour of equipment in the system.  (Mild changes of threshold are a different matter: e.g. an RCD in a 3-phase system could theoretically have its threshold pushed up to nearly twice its rated operation level if there's near-threshold leakage on the two other phases. No one seems to worry about this mild change that's unlikely actually to happen to the full extent.)