It seems that it was probably forced on them' from outside. However, it doesn't really need to be evidential. They decided what they regarded as the maximum safe disconnection times and, on that basis, specified a way of determining (and provided tabulations of) the maximum Zs figures which would achieve those 'maximum safe disconnection times'. The problem was that the calculation (and their tables) only achieved those 'maximum safe times' for a proportion (albeit probably a fairly high proportion) of electricity users. They have now got close (but not quite achieved) a situation in which the 'maximum safe disconnection times) will be achieved even in installations with a supply voltage at the permitted minimum.
Kind Regards, John
