I don't really get that. Let's face it, at least in a TN-C-S installation the neutral of other (intact) circuits would be connected through an extremely low impedance path to 'earth' very close to the CU - so I would expect that the proportion of 'return current' that would travel through the circuitous 'parallel path' via the Rn+R2 of the circuit with a fault (and hence back to the MET) would usually be minuscule.Many circuits may now be using the fault as a return path.
Am I missing something?
Kind Regards, John