Curiousity got the better of me ...

Safest is a battery-powered, pulsing (as opposed to continuous) electric
fence controller bearing Underwriters Laboratories label ANSI/UL69 (Electric
Rence Contollers) One town's regulations specified that the current pulse
a maximum of 60 times a minute with the duration of each pulse a max of
1/10 second and be 25 milliamps or less. This is on the high side; the
newer controllers have pulse duration around 1/1000 second. Voltages seem
to run in the 5K-7K range. The shorter pulse duration also has the
advantage of not heating dried vegetation to combustion temperature.