[cnvogel]

In Europe (also including UK ;-) ), the joint Europe electrical grid is maintained according to the mechanism outlined in this technical document.

https://www.entsoe.eu/fileadmin/user_upload/_library/publica...

    Page P1-29, Definition D-D1, Tolerated Range of Discrepancy

    A discrepancy between SYNCHRONOUS TIME and UTC is tolerated
    within a range of ±20 seconds (without need for time control
    actions).

And corrective action is to change the global-grid setpoint for frequency (of 50Hz) up or down by 0,01 Hz (i.e. 1sec difference in time shown on a clock / 5000 seconds of real time). So the system allows for ±20s·50Hz=1000 cycles of deviation until corrective action starts, and allowing for some time until the system reacts, that means that the clock might be off by ±30s in reality (the last one is a guess by me).

But it's part of a much more elaborate scheme with several control-loops taking care of different things, the timing is only the global, outermost, regulation. All this makes much more sense, once one has realized that phase between two parts in the grid is the main controlled variable to determine flow of energy, so this document may appear to be quite obscure and strange to most people.

EDIT: corrected my math.

ADD: And, obviously, "Time Nut" Tim van Baak has a plot on the topic. For the US ;-). http://leapsecond.com/pages/mains/

[danellis]

I always wondered what the importance of maintaining such an accurate frequency is. Are there reasons other than keeping parts of the system in phase? (From what you're saying it sounds like they take additional measures to ensure that too.)

[teh_klev]

Thanks for the links.