OK, having Kirchoff's this idea on paper with some current flow in a consumer side fault to earth, I think I've convinced myself that an RCD would work, but I've still got a nagging thought I may have missed something.
During a fault in which one live of the 3 phases is earthed, this is what happens to the other wires. In effect the phase triangle of voltages stays the same size, or only flexes a bit, but you are now grounding the corner, rather than the star centre.
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