Fair point. I guess there is a band where someone could get a lethal shock and the rcd not operate.
It's really more than a 'band' - there is never any guarantee that an RCD will prevent a shock proving lethal, no matter what the current through the person. Limiting the duration of currents >30mA through a person to a maximum of 300ms will prevent the shock proving fatal in most healthy people, but that's not a certainty - people vary, as do the routes current takes through their body. Once one considers 'unhealthy' people, all bets are off. There are people walking about (some seemingly totally healthy, with no knowledge of any problem) with hearts which are so electrically unstable that they are constantly at risk of
spontaneously developing fatal disturbances of heart rhythm; in such people, just a mA or two of electrical shock can be the 'final straw' which results in their death.
However, that's not really much to do with the point I was making. In practice, it is all-but-impossible for enough current to flow through a person to trip an RCD (whether the shock proves fatal or not) if they are indoors, standing on floor or ladder and
only touching an L conductor. The current through them would only be high enough to trip an RCD if there were simultaneously touching L and something (other than neutral) close to earth potential - the risk of which is obviously considerably
reduced if there is no earth connection to, or in the vicinity of, a light fitting. Sure, they theoretically could touch an earthed pipe or radiator etc. at the same time as touching the live L of a lighting circuit, but that's incredibly unlikely. Perhaps most important, that very unlikely scenario is just as possible when the lighting circuit
does have a CPC, so I don't really see it as a valid argument for fitting an RCD 'to compensate for' the absence of the CPC.
I'd rather it be fitted though.
Having an RCD in any situation is obviously theoretically better than not having it - but, as above, I think that the argument for including one to compensate for the absence of a CPC on a lighting circuit is, in itself, pretty (very?) weak. That's how I see it, anyway.
Kind Regards, John