For any type of accuracy, depending on the power grid is not a dependable
option! There can be noise, interference, and power dips. On the short
term, the power grid will be accurate, but over the long term, it can be out
by a fair amount.

I have an electric clock that is dependent on the power grid for its timing.
This clock can be a few minutes or a few seconds out at times.This is not
from the power company being off frequency. It is most likely caused by
power dips, or noise in the power delivery.

The accuracy of the power companies timing is very accuracy. It has to be in
order to keep synchronized to other systems that they are working with. They
have many generating facilities that are working together, therefore all
their systems have to be synchronized to within fractions of a degree on the
line frequency. From what I am told, here in North America, their margin of
error is within about
1 second per month.

The problem when connecting a device at home to the
power line, such as a clock, the clock can jump off time very easily. It is
susceptible to interference, and power disturbance through their
distribution. I was also told this by an engineer from our local power
company.

Normally, during peak conditions, they allow the frequency to drop very
slightly. It may be as much as 0.5 to 1 Hertz or so. During peak conditions,
they will give it back. This means that all the power generation systems
working together must also drop, and increase by the same amount.

The clocks that I have that are referenced to the AC line, are not
dependable for accurate time. Over a few weeks, I found the quartz clocks to
be more accurate. I can check them with the NIST broadcast.

Here in North America many stores are selling quartz clocks with built in
time receivers. These are getting the time reference from the NIST. The user
only has to put in the approximate time to within about 30 minutes. After 12
hours, the clock will be exactly on time, as long as it can receive the NIST
signal. The clock automatically checks in with the NIST about every 12
hours. If the NIST signal is unavailable, the clock will keep time to the
spec of any consumer type quartz clock. This is about 5 to 10 seconds per
month.

Another thought would be to get a low cost GPS that can accept an external
antenna and external power supply. This will give you the most accurate
possible time for home use.

You can also go to the NIST site with your computer, and get very accurate
time. It should be within about 100 to 200 ms. Going from memory, I believe
it will tell you its error to your computer. By hitting the reset key, you
can sometimes get it down to within a few milliseconds.

--

JANA
_____


"Christopher Tidy" wrote in message
...
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