OP
radiohead
The whole plant room is already on one RCD, a wylex 100a four pole 30ma (4293) on the incomer in the plant room. It's separated to a TT at that end. I'm informed it rarely trips. A 100ma T/D at source will protect the SWA conductive parts and meet selectivity requirements, AND drop all 3 phases in the event of an earth fault on the sub.If fuses comply from a Zs point of view then they will provide a better solution. To put the whole plant room onto one RCD seems like a recipe for trouble with regards nuisance tripping. Have you measured the earth leakage when the plant is operational and/or when it is starting up?
Will the upfront RCD comply from a selectivity point of view?