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Why does my mains electric digital clock not keep time with the Greenwich Time Signal?

Why does my mains electric digital clock not keep time with the Greenwich Time Signal? Today it is three seconds slow, yesterday it was exactly right, and over a period of weeks it can vary by as much as twenty seconds either way. I vaguely remember hearing sometime that it depends on the frequency of the power supply – is this correct and if so why?
Cec Neale from Norfolk (Age 55+)


6 Responses

  1. The speed at which your mains clock goes depends on the frequency of the mains electricity (faster the frequency, the faster your clock goes). During the day the mains system frequency varies depending on the balance between electrical load and electrical generation. As load increases the generating units try to work harder and slow down a bit and vice versa. National Grid tries to even out the faster frequency and the slower frequency over the day, so that ON AVERAGE it is 50 Hz.

  2. I can’t be sure but i would imagine that your alarm clock uses an oscillator which sets the time, this may be governed by the frequency of the mains which has an allowable tolerance (it’s not always 50hz) but is likely to have a dc supply that drives it.

    The oscillator may also have a tolerance on it so for example it may oscillate at 10Mhz + or – 5% giving it the few seconds difference each day.

    A more accurate system is either gps based or through the MSF time signal which are both accurate through atomic clocks. Radio controlled clocks are fairly common now whilst gps ones are generally for specialist applications.

  3. Many clocks are “Quartz”. This means they use a small wafer of quartz that vibrates at a certain frequency. This frequency is quite high and quite constant, but it is still affected by temperature so will not stay perfectly correct over a long time period and if the quartz sliver is made incorrectly it will vibrate at the wrong frequency and your clock will be slightly slow or fast. The accuracy of good quartz clocks is quite good and you will only have to reset them by a few seconds every now and then but poor ones can become very bad.

    The radio controlled clocks mentioned above still run in the same way, but they are reset every day automatically using a time signal transmitted by radio stations. This means that they will only drift by one days worth of error.

  4. Hi there.

    What we need to establish here is what is meant by a Clock”

    In this context, a clock is something which records the passing of time in regular intervals, and of course we are all used to talking about “time” in seconds, minutes, hours, and days etc..

    Now – what is to say that YOUR clock is any more accurate, or less accurate than someone elses ?? –

    Well, if the “someone elses” clock is the greenwich time signal – then that is a REFERENCE and will be more accurate than yours! – and so by defintion – you will always require “calibration” to “syncronise”

    In times gone by, the ONLY reliable way of keeping track of time was a pendulmn, or a coiled spring,
    (pendulmns wouldn’t work properly on ships (ships is the connection with “greenwich” – the 0 degree longtitude etc – look it up on google etc !) becuase the movement of the ship would interfere with the “swing” of the pendulumn – and so you couldn’t navigate at sea properly bceause you couldn’t accurately tell the time. …. until the “escapement” mechanism was developed, which was used in most clocks and watches),


    Coming up to date, more accurate means of keeping time are available, these being more far more accurate.tha mechanical means

    HOWEVER, as far as a mains digital clock is concerned, it will either:

    1: USE a “crystal or ceramic resonatort oscillator” to provide a stable time reference ( a crystal / ceramic resonator is like an electronic tuning fork usually vibrating at a high “multiple of 2” , so it can be divided electronically down to 1second intervals or ..

    2. USE the 50Hz “mains supply” as a reference.

    Now … NEITHER 1 nor 2 is entirely accurate.

    The 1st solution will “drift” one way or the other, and so will be fast or slow when compared to an ACCURATE time reference such as the Greenwich time signal – and so will need occasional “resetting”

    The second solution will VARY about the time of day, that is the mains electricity frequency is NOT ALWAYS exactly 50 Hz, BUT, what the electricity providers TRY to do is MAKE UP the loss or gain over a 24 hour period – that is, EVERY second SHOULD have exactly 50 “cycles” of alrernating current, BUT say if there were only 49, then we have to make up the 1 later on …. etc etc

    and so … if your clock uses the 50Hz “mains” as a reference, it will probably be either fast or slow at times during the day , BUT overall over days and weeks, will keep reaonably good time overall

  5. Mechanical electric clocks that run on household alternating current can run for years without losing a second, provided the power source is maintained at exactly the set frequency, being corrected by a “cycle count” which either adds or subtracts cycles over time by slightly varying the speed of the generator dynamos at the power station.

    Since the time when such clocks were very common, they have been largely replaced by digital clocks which keep time internally by a vibrating quartz crystal. These tend to precess over time, as they can be affected by temperature and small variations in the crystal itself. Usually, no means to correct the speed of such a clock is available.

    Even wind-up or battery operated mechanical clocks have often had some means for adjusting the speed of the clock. Either an adjustment dial (usually internal) or by automatically advancing or retarding the clock based on the amount of adjustment over a 24 hour period, a user could often expect accuracies of less than a few minutes precession per month, or even better.

    Rarely have digital clocks had any such adjustment, and users are forced to live with a clock that may precess several minutes per month (or a week) if the owner is so unlucky.

    It seems a relatively simple engineering task to build in a correction feature to digital clocks which could either add or subtract clock cycles over a given period, based on the amount of adjustment required by the user over the time elapsed since the last setting.

  6. The town I live in eveyones digital clocks speed up;(approx. 5 to 7 minutes a week. My question, pushing a higher freq, will this cause us to appear to consume more power (watts). At the higher Freq, will the KW over a month be higher?

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