Fair enough. [in the interests of avoiding ambiguity, I've added a few brackets (in red) to the above].Yes I used Ct = [230 + Tp - {(Ca² x Cg² - {Ib²/It²}) x (tp - 30)}] / (230 + Tp) in my Java Script program to get the correction factor.
It's actually quite complex. What you have done (assuming a {method 100} It of 42A for the two legs of the ring together) is fair enough for loads at the midpoint of the ring, such that current will be shared equally between the two legs and will flow along the full length of both legs. However, since the 'guts' of the temperature correction (Ib²/It²) is not a linear function of current, the calculation will presumably not hold exactly for other arrangements of the loading.
Hmmm. That might be a good one for a debating society or in a pub with a glass in one's hand but, in the real world, it is only likely to cause total confusion! An RCD is not an "over-current device" in the sense that the term is nearly always used, or usually defined. There is clearly a need to distinguish between devices which operate in response to the current in L (and/or N) and those which operate in response to the current difference between L and N - and we do that at least partially by calling the former, but not the latter, "over-current devices".We can argue about the technical merits but questions like is a RCD an over current device may be good fun in a collage class room but in real terms does the DIY man need to know or care. To me it is an over current device can't see how it can be classed as anything else. The fact that the current measured is the differential rather than current used does not change the fact it measures current.
Kind Regards, John