Penetration of compartment walls with electrical trunking

[lyledunn]

BS7671 2008 requires that any wiring system, such as trunking or conduit, which has an internal cross sectional area of 710mm2 or more and which penetrates a compartment wall, should be internally sealed. That is pretty small trunking! Great emphasis is placed on the external seal and certainly on internal seals of larger ducts but I haven't seen any one nosey at the arrangement inside even 50mmx50mm trunking. To save you rushing for your calculator 32mm dia conduit would need to be so sealed.
Would any one know why a figure so precise as 710mm2 was specified?
Regards,
Lyle Dunn

[kurnal]

The requirement for the protection of pipes penetrating a compartment boundary is similar, unless non combustible pipes exceeding 40mm must be protected in one of several ways. I guess a factor requiring internal seals in cable ducts is that the pipe will contain empty space but a duct will contain cables with combustible insulation that is more likely to propagate fire from one side to another?

To be honest no I have never looked in small cable ducts either but do sometimes check big ones, of course the seal will last only as long as the first alteration to the wiring. Usually ones attention is more drawn to the inevitable missing bricks in walls and dampers in air con ducts etc  rather than small cavities in ducts.

[John Webb]

I take it this is section 527.2.6 of BS 7671?

It may be based on work carried out at the Fire Research Station many years ago when PVC drainage and other piping came into regular use. (And before my time there!) But I regret I can't give an explanation of why that particular cross-sectional area was chosen.