Jinx,

On Mon, 09 Jan 2006 17:58:53 +1300, Jinx wrote:

>...
> According to Transpower, NZ's power generator, measuring a
> 24hr period from 3:30am to 3:30am should be accurate to one
> or two cycles, +/- 1/25th second.

That sounds a lot more tolerance than I expected - having spoken to generator people, I understood that they 
try to get the 24hr error as low as possible - zero in relation to their reference is their target, and 
certainly less than one cycle, because of the number of systems that rely on it.

> This is an average, (which varies
> because the nominal 50Hz goes up and down during the day, and
> that pattern in turn varies with seasons and special events), so the
> longer you sample (a year ? "Oi, where's that clock I ordered ?"),
> the closer you'd get to 50Hz.

I'm not sure it would help - do you know that they carry errors over from day to day, rather than just working 
to a 24hr timeframe?

> But he reckoned 3:30am is about
> when they'd put out the 4,320,000th and final cycle for the day.

Yes, I suppose they need to tweak it at a time when there is the smallest variation in load, so they probably 
start working towards the correct cycle count at, say, midnight, with the appropriate delta to get it right by 
03:30.  Since the Cook Strait link is DC, do the two islands have their own controls for their respective 
grids, I wonder?

> You could, I suppose, measure a few days and average that

As I say, I'm not sure it would help - I don't think they, for example, let it slip a bit during the week and 
catch up at weekends, I think they treat each day independantly.  I presume you don't have any sort of radio 
time standard there, like MSF, DC77, and whatever the Colorado one is called, there?

Cheers,


Howard Winter
St.Albans, England


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