In message , David Hansen
writes

An occasional poster to uk.railway has written of the time he spent
testing the various designs of such bars on live railway supplies
(as they have been used since the earliest days). IIRC one of the
parts of these tests was to see how the designs minimised the
chances of injuries.

While ideally they should be used just to ensure that supplies
cannot be re-energised I think that in an emergency they are still
used first. When radio communications become rapid, as they are in
some places, this may be changed/has been changed relatively
recently as it will then be at least as rapid to make a radio call
compared to getting hold of the bar, clambering out of the cab and
applying the bar.

On LU we are trained to lay SCDs (Short Circuit Devices) live as an
absolute last resort although if we have to that's what we do. We are
also trained to look away though

The ones on LU are designed to clamp themselves to the rails and carry
the fault current required to trip out the breakers in the switchrooms.
--
Steve Fitzgerald has now left the building.
You will find him in London's Docklands, E16, UK
(please use the reply to address for email)