I agree 1SC,
I can see both sides of this argument, and neither side are wrong per se.
E54's argument is correct in that stability is the main factor (IMO), I would rather have a rod that gives an Ra of a 100 ohms but was deep enough to be stable and not fluctuate than a shallow rod that read 50 ohms that is likely to increase in the dry or frozen seasons.
The other posters are correct too, in that it does not really matter whether the rod measured 20 ohms or 200ohms, in the sense that neither will operate the OCPD device directly, and both will rely on the RCD to achieve disconnection.
As I said stability is the key here.
Incidentally, the 16th ed amd.1 OSG gives 220 ohms as the max recommended Ra, this OSG uses 230V as the Uo for the calcs.
So which edition Regs gave the lower 10 ohms figure E54 ?, I have seen this figure given for generators and the like, but not for general TT systems that used RCDs to achieve the disconnection times.