It is possible to alarm the trolley wire (or any pole line) without it being energized. The use of TDR technology (time domain reflectometry) has been successfully used in the past to thwart copper thieves.

In simplified terms; sharp electrical pulses are continually sent down the line. At each tight bend in the wire, tap-off, splice, or other mechanical amomaly; a small reflection is produced. Measuring the time between the pulse transmittion and its reflections tells you the distance to each reflecting point.

It just so happens that cutting the wire produces a huge sudden change in the reflection pattern, which can be automatically detected - setting off an alarm. Since the distance to any reflection can be quickly calculated, sherriff deputies can be directed very quickly to the precise spot where the thieves are working. They have been caught in the act this way - still on the pole, sitting ducks as it were. And will be again and again, because the change in reflection patterns cannot be hidden. Even the slightest alteration down the line changes the pattern and is easily and instantly detectable.

OPB