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.
A kid knocked himself out in our talent show in high school this way - he was playing guitar, and singing vocals and I think he took his picking hand off the guitar and grabbed the mic stand while still holding his other hand across the guitar strings, and was immediately knocked for a loop - down on the ground and shaking. He was OK but it was quite the scare.
That will only work if the screw on the outlet wall plate is grounded. In modern wiring it is, but if you need the adapter, you can’t count on that being the case, and should test that it’s grounded before relying on it.
In the USA after knob-and-tube was used it became rigid conduit and armored cable, both of which are grounded. Later early nonmetallic cable was two wire and ungrounded and for that period a hazard which lead to a polarized plug for safety.
There were two ungrounded wiring methods that had been used and three (four if you want to include thin walled conduit) grounded wiring methods, depends on your time of installation what you might have.
Grounding connections, both cover plate screw and receptacle prong, should be tested is a good point. They both can be dependent on the receptacle yoke (frame) making a good connection (if a grounded receptacle then wiring the ground is better).