[Ed_Dinning]

Hi Gents, carrier current protection was and still is used on overhead HV power lines.
The carrier is typically about 1MHz or less and various signals, both analogue and digital are modulated on to it. Pioneered by Reyrolle and later adopted by most other switchgear manufacturers.
It was the cause of an aircrash (near Geneva I think) 30 or so years ago, when the local HV line used a frequency very close the airport NDB beacon.
The aircraft locked on to the wrong signal and flew into a mountain.

Ed