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Using the electric utility frequency for timekeeping — How accurate is it?

The Analog-Style LED Clock uses the 50 or 60Hz from the electric utility as a time base to keep accurate time without the use of a crystal oscillator. The clock automatically detects whether it is connected to 50 or 60Hz and adjusts accordingly.  A question I’m often asked is “How accurate is the electric utility frequency?” The short answer is that it is surprisingly accurate.

In the 1920’s Laurens Hammond invented an electric clock driven by a synchronous AC motor. It kept time using a motor that was in sync with the 60Hz frequency generated by the utility companies. He gave away hundreds of these clocks to power stations as an incentive to maintain a steady frequency which allowed his inexpensive clock to be used anywhere in North America. Hammond went on to help create the Hammond Organ which used a synchronous AC motor to drive a “tone wheel” generator which created perfect pitches for the organ.

Just about everything that has a clock and plugs into the wall, uses the 60Hz for synchronization. I have an inexpensive alarm clock on my nightstand, that if power is lost, the display goes out, but it keeps time on battery backup. While plugged in, it keeps excellent time. If I unplug it and take it on a trip, when I get to my destination a few hours later, the clock has invariably lost or gained several minutes.

In 2010, I made a countdown timer that utilized the 60Hz from the electric utility as a time base. I used a clock that kept in sync with the atomic clock in Boulder, CO to test my timer’s accuracy. At that time, I noticed that the timer would drift 5 or 10 seconds throughout the day. I never saw it more than 10 seconds off, and it always came back to the correct time around the same time each day.

Since then, the utility companies have gotten more lax concerning the accuracy of the 60Hz generated. In order to maintain accuracy, an adjustment is made to the frequency, known as a Time Error Correction (TEC). The more often TECs are made, the more accurate clocks basing their time on it will be. Power utilities used to ensure that the total number of cycles in 24 hours was correct using daily TECs. Now a TEC is made when the cumulative error exceeds a certain threshold. As a result, the frequency is not as accurate as it used to be, but it is still very accurate.

I’ve been running my countdown timer as well as an Analog-Style LED Clock, periodically checking them against each other and the atomic clock. As I write this, over a year later, the countdown timer and the Analog-Style LED Clock are in perfect sync with each other (as would be expected) and both are 24 seconds ahead of the atomic clock. The most my timer and clock were ever off from atomic time, was 52 seconds. I think that any clock that is within a minute over the course of a year is very accurate.

To get a better idea of how accurate your utility frequency is, check out FNET/GridEye. They monitor real-time measurements of electric grid frequency from around the world.

Unfortunately, there are efforts to remove the requirements for TECs in North America. It was estimated that if TECs has not been implemented in 2016, clocks using the frequency for their time base would have lost about seven minutes over the course of the year. Still not too bad; much better than the clock in my car!