Unless the key is immediately available (e.g. in a 'break glass' enclosure) I can't see that is any 'safer' than locking an MCB 'on' if there is no other (accessible) way of killing the circuit - and, as I've just written, the existence of 'another way' really makes a nonsense of the whole exercise.
If he did want that (which I doubt) then, for pretty obvious reasons, he would be out of luck!
The Crabtree C50 range could be held on, one of the theatres I worked in had a 30A 3ph supply for stage lighting and full lights took it out in about 20 seconds. When full up was needed it was the sound engineers job to hold the breaker in and to watch the clock for 2 minutes afterwards before releasing.
I have to take your word for that but, if so, from considerations of safety I would describe that as a 'design fault' - the electrical equivalent of having an overflow pipe or pressure release valve with a built-in way of 'disabling its intended safety function' by manual action!
Main isolator would still be usable I guess
Indeed, although that would obviously sometimes be less than 'convenient' - although, I suppose, adequate for 'emergencies'.
Crabtree used to sell a red cover with a sort of leg on it that slipped over the C50 lever, they were quite common on Fire alarm circuits to stop people turning them off.
I want this 'red thing'
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