A network nominal voltage Ue=420Vac is not a Std value according to IEC 60664-1 Annex B nor IEC IEC 60038 Std whatever the type of network.
For such kind of specific nominal voltage you can use MCBs breaking capacity declared under Ue(Ph/Ph)=415Vac only if the network voltage tolerance it's less than ±8.7% (415*1.1/420) instead of ±10% which is the normal and maximum Std tolerance taken into account into MCBs Std in accordance with IEC 60038 Std §4.1.
If you have higher network voltage tolerance, you have to use the breaking capacity at the upper declared voltage level (in this case at 440V).
