Another possibility exists regarding Keith Relf’s electrocution. Amplifiers of that era (all of mine anyway) were equipped with a “polarity” switch, intended to fight 60 cycle hum. If you got a lot of 60 cycle buzz with the switch in one position you tried the opposite position. That switch referenced the chassis to one side of the 120 volt AC power or the other. In theory, if your guitar amp was switched to one side and the P.A. amp was switched to the other there could be a 240 volt potential (voltage) difference, and assuming the ground post had been removed on one of the amps, you are the path to ground.
I have been bit on the lip by that 240 volt potential more than once. It is natural for a guitarist to be touching their strings and if their lip touches the mic under these circumstances bad things happen.
There are other factors that figure into an electrocution. The relative locations of power source and ground, the path the current takes through the body, just how sweaty, hence conductive, the subject is. A relatively low voltage like 120 volts AC is just as dangerous as higher voltages if there is adequate current. One hundreth of an amp (10 milliamps) is enough to kill someone in the wrong circumstances.