The whole point of my earlier posting seems to have been sadly entirely missed
the two protective measures are entirely different, RCDs are great for Earth referenced supplies ie, public mains supplies, generally different protective measures should not really be mixed, unless there are good reasons for doing so, which granted there are times where this is warranted (as always there are exceptions) sometimes this is unavoidable such as in the case of an ELV (extra low voltage usually up to 55 volts) transformer or psu also that happens to be an isolation transformer or isolated psu at the same time
, yellow 110V site transformers that are also centre tapped Earth also fit into this category,
RCDs are not the panacea that they are made out to be, to be sure they have a valuable contribution and can indeed be life savers when used for their intended application ie. Earth referenced mains supplies.
more is not always better
ie. trying to mix RCDs with isolation transformers, as the isolation transformer is its own protective measure in its own right and does a good enough job for all intents and purposes.
Fully floating supplies are often used in conjunction with Earth free environments which is yet another protective measure, however these are usually strictly monitored/supervised and are out of bounds as far as a domestic workshop is concerned.
The point still stands that if you simultaneously happen to come into contact with two current carrying conductors (supply and return, or some variation of) unless it is of Extra Low Voltage (ELV yet another protective measure) then you will almost certainly get a belt, and no protective measure can save you from this.
Ps, there are other protective measures available, one is double or reinforced insulation, another two not available/applicable in a domestic environment is placing out of reach or with barriers, again usually supervised