Christopher Tidy wrote:

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.

You can download more information than you ever wanted to know about the
National Grid from he

http://www.nationalgrid.com/uk/Elect.../gridcodedocs/

I've pasted two relevant snippets below (NGET stands for National Grid
Electricity Transmission plc).

1. From the definitions:

Target Frequency
----------------
That Frequency determined by NGET, in its reasonable opinion, as the
desired operating Frequency of the Total System. This will normally
be 50.00Hz plus or minus 0.05Hz, except in exceptional circumstances
as determined by NGET, in its reasonable opinion when this may be
49.90 or 50.10Hz. An example of exceptional circumstances may be
difficulties caused in operating the System during disputes affecting
fuel supplies.

2. From document BC3, which deals with the frequency control process:

BC3.4.3 Electric Time
---------------------
NGET will endeavour (in so far as it is able) to control electric
clock time to within plus or minus 10 seconds by specifying changes
to Target Frequency, by accepting bids and offers in the Balancing
Mechanism. Errors greater than plus or minus 10 seconds may be
temporarily accepted at NGET's reasonable discretion.


All very well, but ISTM that the biggest problem with using the mains
alone as a time standard is power cuts, after which you'll always need
some absolute standard such as MSF, GTS, NTP, etc. to reset your clock.

--
Andy