Power Grid Change May Disrupt Clocks

hawguy writes with an AP story about upcoming tests of greater allowed variation in the frequency of the current carried on the U.S. electric grid: "A yearlong experiment with the nation's electric grid could mess up traffic lights, security systems and some computers — and make plug-in clocks and appliances like programmable coffeemakers run up to 20 minutes fast."

Comments

Re:"Clocks"

[jpmorgan]

You're telling me that an Internet denizen who's only been exposed to this idea for 5 minutes doesn't know more than the engineers who have been running the system for decades?

That goes against everything /. stands for!

Using the mains as a timebase

[Anonymous Coward]

In the 70's I developed a system to control the of the light sensitive coating onto 35mm rolls of film. This ran on a PDP-11 that used the mains cycle to keep time (20ms interupts with the UK's 50Hz supply) and measured the coating by the amount of x-rays reflected by the silver halide in the coating each second.... there were coninual errors in the accuracy of the coating as the time approached midnight.

It turns out that the National Grid was legally required to maintain a 50Hz average from midnight to midnight and would add or subtract cycles in the last minutes of the day in order to meet this requirement.

Five or so years later I was working in the National Grid Control Centre and saw the 2 clocks, one with an independent time source and one running from the mains frequency. The aim of the controllers each night was to adjust the mains frequency to bring the two clocks in sync at midnight.