The thing is, if you can contact it with a ladder, you may well be able to touch it by hand.
Therefore, other suitable precautions are required.
It is better to use insulated ladders, however if the switch room has been built since the 70's it is unlikely to have any accessible live parts.
I've worked on a few.
Now if you need to use an insulated ladder, because there are exposed live parts you can contact with the ladder, then once you have ascended said ladder then there is every chance these parts are even more accessible.
Plus you need to be very wary of seperation distances with HV.
At 400V 3ph, you will not get any gap jumping arcing, once you move to HV, then you can easily get gaps jumped by an arc.
Now an insulated ladder may just put the user within the distance an arc can jump, are the ladders "guaranteed" to provide adequate insulation against HV?
Could the arc jump from source to person, to ground, e.g. adjacent earthed electrical enclosure?
I don't think that you will find a law in the UK that states insulated ladders are required.
As Safety Man 1 says, you, need, to understand, these environments.
It will be down to you RAMS.
Or, simply shut it all down before working, not so easy, & not always necessary.
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