Indeed, and they have to remain within +-1%. They have a realtime graph: http://www2.nationalgrid.com/uk/Industry-information/electricity-transmission-operatio

Indeed, and they have to remain within +-1%. They have a realtime graph: http://www2.nationalgrid.com/uk/Industry-information/electricity-transmission-operational-data/ http://www2.nationalgrid.com/uk/Industry-information/electri... Before quartz clocks were around, synchronous mains clocks were common, and accurate.