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Reading various docs I find it can be not so cut and dried. There is the idea of a fireproof wall needing fire pads in the socket that I can see. There is also regs which require any connection to be fireproof. I think 421 and 625 off the top of my head. Although thin metal galv can be seen as fireproof, when a hole is left in it then the fireproof nature of the containment is compromised. It is clear, among other items, that penetration of any stud wall (or ceiling/floor) with electrical connections/accessories does increase risk of fire where there is not fireproof containment. Just to distinguish the two ideas, one is the fireproof nature of the walls the other the fireproof nature of electrical accessories/connections in stud walls. I am undecided as to what is the final fact on the matter it is open to interpretation as usual.
The only part of 421 which could be siad to apply is 421.1.6 which requires that enclosures comply with a relevant standard, you'd need to look up the product standard for backbones to find out what that is.
Chapter 62 doesn't exist so there isn't a 625, did you mean another section?
Backboxes are not fireproof, they are full of gaps for a start, plus they are thin metal.
I suspect that if exposed to fire then all unused knockouts will just pop out as the metal distorts as it gets hot.
The construction of the boxes has gaps in it, thorugh design and/or poor fitup.
Plus they often have grease left on them from manufacture which is likely flammable.
Then there are plastic plasterboard boxes, full of holes and liable to melt when exposed to fire, so also not fireproof to the point that they wouldn't even be on sale if any fireproofing was required.