safetybod,
Try ESQCR available on line from gov website.
Otherwise manuf. data really as this is outside the scope of the general wiring regulations.
It is covered by EAWR, but this will just require suitable and adequate maintenance.
I don't believe that there is a statutory maintenance period.
You could try your local DNO for help as they have hundreds of these!
I guess these are true high voltage, not what is commonly called by lay persons high voltage at 400V, 3 phase.
They are 1.1, 3.3, 11, or 33 kV items on the input side?
If so and you are responsible for them then you should have an "Authorised Person" for working on high voltages at your facility if you are responsible for the maintenance.
These persons would be responsible for the safe working, switching etc. along with really the maintenance schedules.
I would suggest that they would be a valuable resource.
if you are stuck I'll as the Q at the IET (IEE as was).
or one of my colleagues on the local IET committee is responsible for the high voltage network at his facility.


Paul