In an earlier contribution to this discussion,
Christopher Tidy wrote:

Hi all,

I'm thinking of building an electronic clock control circuit which
uses the 50 Hz mains frequency for time keeping. The reason for this
is that the clock dial is rather large, so probably wouldn't run for
long on battery power, and I don't fancy spending £40 buying a
programmable quartz oscillator chip.

Despite doing some research online and in electrical engineering
books, I can't find a figure for the accuracy of the time keeping of
the UK power grid. Perhaps this is because there aren't official
bounds set for the time error - one of my electrical engineering
books says it is a legal requirement that the frequency be kept "as
close as possible to 50 Hz" - but even if this is the case it should
be possible to determine a typical error figure.

From my point of view I'd regard an acceptable error as 5 minutes
every 6 months. Does anyone know the typical time error seen on the
UK grid, or where I might find this information? Any suggestions
would be much appreciated.

Best wishes,

Chris Tidy

My understanding is that the frequency is allowed to fluctuate by a small
amount (don't know how much) but that, over a 24 hour period, exactly the
right number of cycles will be delivered. So your clock should at least be
exactly right once per day.
--
Cheers,
Roger
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