I'm linesman and Electrical Fitter in powerline distribution in Australia AMA ?

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The idea is to transition from 240 +/-6% to 230 +/-6%, but you can't do it in one step. You need to allow 40 or 50 years that covers both ranges until everyone gets rid of their old appliances. That's the reason for the odd +5/-10% and +10/-5% for the interim.  Meanwhile, in continental Europe, they are transitioning from 220 V to 230 V.  (With UK, coming from 240 V like us.) The result will be designers can optimise for 230 V instead of covering the wider range of 220 V to 240 V.