This could be the answer to your question of "why a percentage?" Appendix 12 quotes - for a lighting circuit, for example - 3% of nominal voltage rather than 6.9V. The 3% or 230V can be amended in ONE alteration and would apply throughout the book.
Good point - not that nominal voltage changes that often; I suspect it had been the same for decades prior to the 'harmonising' 1995 change. However, in practical terms, I think the 3% figure only appears on one page (Appendix 12, p358), so it would not take a lot of searching to make any changes!
However, noone has definitively stated why it is 3% and 5%.
4% of 240 is close enough to 5% of 230, although a slight reduction, without the need for a figure to decimal places but 4% of 240 is reduced to 70% (approx.) of its value with 3% of 230. Do we know why?
I obviously don't - that's why I started this thread to ask 'why?'! The suggestion that this may be due to the increasingly widespread use of non-incandescent lighting, some of which is more voltage-sensitive, seems pretty credible.
However, I don't quite get the figures you're quoting. 4% of 240 is 9.6 but 5% of 230 is 11.5 - those are pretty different figures, not as close as you suggest. Furthermore, as you go on to imply, even if the percentage figures had been revised such that the absolute permissible voltage drops (in volts) were the same as there were in 14th Ed., that would still mean that the 17th Ed was allowing (in worst case supply voltage situation) about 10V less to reach the current-using equipment - since the worst case voltage became 216.2V, rather than 225.6, following the 'harmonisation'.
This Another teaser for you with regard to varying voltage: It states in Appendix 12 that the voltage drop should (should again, not shall ???) not be greater than, for lighting, 3% of nominal voltage i.e. 6,9V.
Eureka!! You seem to have modestly missed the point that you have just found what we have all be unsuccessfully hunting for!! The first sentence of Appendix 12 says that the (percentage) figures in Table 12A are "expressed with respect to the value of the nominal voltage of the installation". So, the figures definitely are (currently) 'percentages of 230V' - How did we all miss this?!
This Therefore, I read this to mean that the figure of 6.9V would be applied whether the voltage was at its maximum or minimum. Another quirk in the wording?
Quite - but that's one of the major issues we've been discussing in this thread.
This would only apply when you were testing a circuit that had been wrongly designed in the first place (heaven forbid there should be such a thing) if you were to measure the voltages rather than work it out from R1 etc. but, obviously, does not affect the design of new circuits which would be the primary reason for the regulation.
As you say, the issue only arises if one measures, rather than calculates, the voltage drop - but what you're saying is really one of the points I've been making. However, now that we know (first sentence of Appendix 12) that the maximum permissible drop is 3% of 230V (i.e. 6.9V), regardless of the actual supply voltage, I don't any more see any problem or ambiguity.
One has to assume that the authors of the regs were satisfied that (per the 'worst case' scenario) providing a lamp with only 209.3V (216.2 - 6.9) would be acceptable - or, similarly, that providing any other load with 204.7V (216.2 - 11.5) would also be acceptable.
Kind Regards, John