if any 'critical' systems use the power lines for timekeeping, I wonder what the designers were thinking...
Many of these systems were first designed and installed a long time ago. Before there was GPS, before there was an internet and NTP time servers. Perhaps they exist far away from any reliable radio time signal. The only thing avaialable was the mains power and it's frequency came with a gurantee of stability. So it was a good choice, possibly the only one.
Taking the case of traffic lights in Scandinavia, they are mostly out of range of any time signal. Until very recently they had no networking for NTP servers. Many still don't. So if you want to keep a street full of intersections in sync all day the mains frequency was a good choice.
Traffic lights aren't that 'time critical' usually, unless they're pattern-synched with others, and even then it's synch between the lights that matters, not to a defined 'correct time'
All the lights in an intersection is handled by the same controller, so frequency drift on the power won't matter. The worst it can do is to change the intervals with a few milliseconds.
I was thinking more of alarm and access-control, mostly.
Traffic lights aren't that 'time critical' usually,...
True, not as critical as many other things.
..unless they're pattern-synched with others, and even then it's synch between the lights that matters, not to a defined 'correct time'
Exactly, often they are. As you say no matter if their time drifts off a bit as long as they all do it together. Hence the mains synch.
Also it's nice to have them synched with other systems like time stamps on videos, temporary vehicle counter installations etc, when one is monitoring how well the control works.
Comments
Many of these systems were first designed and installed a long time ago. Before there was GPS, before there was an internet and NTP time servers. Perhaps they exist far away from any reliable radio time signal. The only thing avaialable was the mains power and it's frequency came with a gurantee of stability. So it was a good choice, possibly the only one.
Taking the case of traffic lights in Scandinavia, they are mostly out of range of any time signal. Until very recently they had no networking for NTP servers. Many still don't. So if you want to keep a street full of intersections in sync all day the mains frequency was a good choice.
All the lights in an intersection is handled by the same controller, so frequency drift on the power won't matter. The worst it can do is to change the intervals with a few milliseconds.
I was thinking more of alarm and access-control, mostly.
True, not as critical as many other things.
Exactly, often they are. As you say no matter if their time drifts off a bit as long as they all do it together. Hence the mains synch.
Also it's nice to have them synched with other systems like time stamps on videos, temporary vehicle counter installations etc, when one is monitoring how well the control works.