100ma rcd is not protection against fatal electric shock, 30ma is! most gennys have an earthing connection built in to them for installing a ground earthrod unless they are fully insulated type such as a kipour suitcase type where the potential for electric shock via the genny is not possible unlike the open frame type. as stated already an rcd operates via dectcting varying potential difference between the live and neutral conductors, so has nothing to do with the earthing. The earthing system is not there to back up an RCD, a (30mA or less)RCD is used to provide additional protection against electric shock and the earthing system is there to keep all exposed/extraneous surfaces at or about the same potential in the event of a fault and to clear the fault quickly by operating the protective device. Where the earth fault loop impedance is too high for a fuse/mcb to disconnect the supply quickly in the event of a fault, an RCD is used (such as in a TT installation in a house)
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