In message , n cook
writes
Some future use for the dips in mains frequency use to reduce National Grid
load
http://www.publications.parliament.u...n/lds06/text/6
0324-07.htm
starting at heading
Dynamic Demand Appliances Bill [HL]

" Dynamic demand appliances contain a low-cost electronic microcontroller.
This listens to the mains hum, which runs at a frequency of around 50 hertz.
The signal can be detected through every plug socket connected to the
national electricity supply. Through this signal, the dynamic demand
appliances can sense whether the National Grid is under stress and adjust
the time at which they use electricity. The technology is suitable for
appliances that already switch on and off during the day on a "duty cycle",
such as domestic and industrial fridges, freezers and water heaters.

Millions of such appliances acting together would smooth out demand for
electricity. "

Have a look here - an on-line frequency monitor.

http://www.dynamicdemand.co.uk/grid.htm

The rest of the website covers the topic of dynamic demand control which
is quite interesting (well it is if you work, as I do, for the company
that owns 70% of Dinorwig).

Andy
--
Andrew Sinclair http://www.smellycat.org