Letter: Keeping time on the national grid

From David Clarke, Seaford, East Sussex, UK

I am not sure why Steve Swift is having trouble with his electricity tariff timings ( Letters, 29 September). He says his meter takes its timing from the mains frequency; I understood that most Economy 7 meters switch tariffs as a result of a signal broadcast on BBC Radio 4 longwave.

I remember working with the UK National Grid in its London control centre in the 1980s. An important control it had was to compare, during the day, the time shown on a mains electric clock – “electric time” – with the time shown on an independent master clock, “clock time”.

A task of the control engineer was to adjust system frequency so that sometime around midnight, the clocks agreed. I assume that an equivalent system operates today.