On Tue, 8 Mar 2011 20:34:49 +0000, Ian Jackson wrote:

I was surprised to see how low the voltage sometimes went (right down to
the minimum limit). When it was really cold over Christmas and New Year,
I expect the guys at the power stations were shovelling coal for all
they were worth, and that the frequency (which is of less interest than
the voltage) was well down on normal.

The frequency is very important, it's maintained at 50Hz +/- 1% ie
49.5 to 50.5 Hz nut National Grid work to an operational limit of +/-
0.4% (+/- 0.2Hz).

Obviously, any type of clock which uses the mains frequency will run
fast or slow if the frequency varies. My understanding is that the
generating people are supposed make sure that, come hell or high water,
during a 24 hour period the correct number of cycles are delivered, so
that the clocks are correct at 8am. I'm not sure that they always
achieve this!

Oh they do! Questions get asked in the house if they don't. IIRC the
last time they had to announce that mains driven clocks had the wrong
time and would need to be corrected was during the winter of 1947...

--
Cheers
Dave.