Originally posted by Shallow Blue
That's what I was thinking. There are so many things that can influence the exact specs of the mains one way or another (tea kettles in the world cup final break!) that anyone relying on precision from that source is a gambler.
For the most part it is very accurate if the power company ensures the accuracy of the sine wave sent to the house or business. That's what they have been doing for decades in the US.
I know for a fact you get a lot of pulses and such in industrial apps where, for instance, a megawatt motor (about 1200 horsepower) turns on, it can make power spikes in the line that can effect other users a mile away or more. But that does not effect the overall accuracy of the sine wave power, instead putting out some kind of voltage spike that lasts for a brief time, say 1/10th of a second (just sticking out a number).
Since you get in the US 60 cycles per second and in the UK 50, then in 1/10th of a second 6 cycles can go by or 5 in the UK. So for that time period the sine wave regularity is interrupted and you maybe lose 5 or 6 cycles of continuity. But since the waves are very well synced when the sine wave is reinstated the phase of the wave has not been lost.
That kind of thing does not happen on such a large scale in home use, someone turns on a microwave and some smaller pulses can get into the house wiring but not at a level that would effect an old fashioned electric clock.
BTW, computer power supplies don't have to have extreme precision in converting from ac to dc. If so those UPS systems (uninterruptable power supplies) would make a computer go bonkers because it puts out a wave closer to a square wave than the nice regular sine wave coming in from the mains.
That does not present that much of a problem to computer power supplies which are designed to filter out nasty square wave components of a sine wave but I know also for a fact (having seen the outcome) of some of the cheapo power strip surge protectors when presented with power from these UPS supplies, they tend to blow up.
That is because in a capacitor, for instance, if it is designed to suppress ripple at 100 or 120 hertz, all is well when it is given voltage from a nice sine wave mains supply but with a UPS, the output is anything but sine like, lots of high frequency components are now presented in the current coming out of the UPS.
The problem there is the capacitor allows higher frequency components of the wave to pass right through since capacitors tend to block DC but pass higher frequencies. So cheapo surge protectors typically have a combination of toroid coils, capacitors and MOV (Metal Oxide Varistors, which which shunts high voltage spikes to ground). The capacitors however, just pass higher frequency components of the power spike right on through.
If an audio mixer for instance, is designed to have a nice low noise DC supply at 50 Hz, when used at 60 Hz, will have somewhat more hum than it had when plugged into a 50 Hz mains supply. The slightly (about 10 Percent) higher frequency means that much more ripple protection will be needed at 60 hertz.
I know older reel to reel tape decks designed to run at 50 Hz, taken to the US and run there on 60 Hz, runs about 10 percent faster, a 15 IPS deck would run about 17 IPS in the US. They used motors designed to be directly driven by AC and intimately responded to the mains frequency.
Newer recorders went to DC motors which get timing accuracy from quartz synced supplies.