It is a common question why Herschel states 240v as the rated voltage limit of its single-phase heaters, and whether this is adequate (high enough) given total supply voltage in the UK can legally reach 253 volts.

The regulation on the matter is clear enough: that the input power of a heater must not exceed 5% of its rated power (as it has implications on product longevity, reliability and safety, quite apart from unnecessary extra running cost).

Because power of resistive loads increases with Voltage^{2}, this +5% limit is reached within only a 2.5% increase in voltage above rated. This automatically implies that the preferred operating voltage of a heater (and many other appliances) must be less than total possible voltage.

There is an additional regulation requiring all single phase heaters stating a rated voltage range, that this range must also pass through 230 volts. (So it is not permissible to market a heater of 240 - 250 volts, for example). So this restriction further anchors rated voltage around 230V and not around higher levels.

The question whether a heater must be able to operate at 253 volts because that is the legal maximum in the UK is not a correct interpretation of the Voltage quality and continuity regulations.

Total permissible voltage is 230v -6% + 10% (i.e. 16% variance) which gives 216 - 253 total possible volts with the central tendency being 220 - 240V (i.e. -4% + 4%) in which most items on the market are rated to operate safely.

Voltage supply when measured over time should demonstrate a bell-curve around 230V, with the extreme voltages being no less than 1% of total operating time. This is the correct way to interpret the regulation around total possible voltage. The extremes should never be the norm.

A fuller explanation may be downloaded here.