If you can hold the concentration in the room they should put the fire out. There are issues with openings in rooms especially if the rooms have ventilation louvers on the doors, as electrical rooms often seem to have. Design should be in accordance with the British standard (I think the concentration rate for the CO2 would have to be based on deep seated fires for switchrooms, which I think is 60%. Also, you may need extended discharge to cope with any losses as you have to maintain concentration for 20 mins – please check the figures quoted as they are off the top of my head)
The issue regarding H&S would depend on whether the room is unmanned and the facilities the system has to prevent operation when occupied. I’ve seen the provisions range from procedural to using castel key interlocks to prevent the doors being opened unless the CO2 system is physically isolated.
I have seen them designed for swichrooms previously - though they were older oil filled switch gear if I remember correctly.
As for proof, the manufacturers would be able to show you videos of fires being put out, but they may not really count as proof. Alternatively, the appendices of NFPA 12 (co2 systems) or 850 (electric generating plants) may have examples of where the systems have worked.
If I come across anything else, I will email you, if you give me your email address.
Regards
James Goodstadt