Until 1987, mains voltage in large parts of Europe, including Germany, Austria and Switzerland, was 220 ± 22 V while the UK used 240 ± 24 V. Standard ISO IEC 60038:1983 defined the new standard European voltage to be 230 ± 23 V.
From 1987 onwards, a step-wise shift towards 230 − 23 + 13.8 V was implemented. From 2009 on, the voltage is permitted to be 230 ± 23 V.
No change in voltage was required by either the Central European or the UK system, as both 220 V and 240 V fall within the lower 230 V tolerance bands (230 V ±6%). Some areas of the UK still have 250 volts for legacy reasons, but these also fall within the 10% tolerance band of 230 volts.