Get to Know TWACS

TurnipTruck

TWACS (Two-Way Automated Communication System) is an electronic meter-reading system made by Aclara that is used by a growing number of power companies. In a nutshell, the system works by adding a data signal component to the AC that feeds your home.

The TWACS signals are about two seconds in duration and can happen at intervals deemed necessary by your utility to take full advantage of the system. The short story is that by adding a component to your 60Hz sine wave, the zero crosses of the sine wave are altered in time by the additional signal. Devices that expect the zero cross timing to be consistent can be affected by the TWACS signal presence. Most notable are electronic lighting dimmers and uninterruptable power supplies.

All UPB dimmers, regardless of manufacturer, are affected. The symptom is a syncronized flickering of all lights on dimmers that last for about two seconds.

I have heard that the original Lutron RadioRA dimmers are also affect. I cannot verify though.

APC UPSs are affected. They go to battery backup during the TWACS communication.

Results can vary. The amplitude (strength) of the TWACS signal determines how far from true zero-cross your zero-crossing will be. Devices are affected in varying degrees.

Here is a link to a video I shot from an oscilloscope at my house, showing the presence of the TWACS signal. http://www.youtube.com/watch?v=T2D9DEltOZM