If you mean the false back  inside an enclosure, that carries DIN rails, terminals and so on, that will be whatever the enclosure makers recommend - it could well be steel and in a steel box it usually is, but it could also be thick fibreglass or even Tufnol for certain situations where there are terminal bars to attach.

If you mean the back panel that switch gear or similar is screwed to, that historically has been plywood or chipboard in the UK, but can be almost anything - in  a sense the whole point is that the switchgear enclosure contains all the bits that might go bang or glow re in a fault. In really old installations you may encounter asbestos fibre boards in which case some care is needed.

As far as I know, no-one has made 'backless' boxes or fittings that are in effect an open square  that relies on the wall behind to complete the enclosure since the early 1980s - that would not be accepted today. (though if done well it can be fine, sadly it was not always done well... slightly live damp masonry anyone ?)

Mike.

If you mean the false back  inside an enclosure, that carries DIN rails, terminals and so on, that will be whatever the enclosure makers recommend - it could well be steel and in a steel box it usually is, but it could also be thick fibreglass or even Tufnol for certain situations where there are terminal bars to attach.

If you mean the back panel that switch gear or similar is screwed to, that historically has been plywood or chipboard in the UK, but can be almost anything - in  a sense the whole point is that the switchgear enclosure contains all the bits that might go bang or glow re in a fault. In really old installations you may encounter asbestos fibre boards in which case some care is needed.

As far as I know, no-one has made 'backless' boxes or fittings that are in effect an open square  that relies on the wall behind to complete the enclosure since the early 1980s - that would not be accepted today. (though if done well it can be fine, sadly it was not always done well... slightly live damp masonry anyone ?)

Mike.