Protected Escape Route - Cable Enclosure

Hi All,

I’m faced with a question that will probably be faced very frequently in the next few years.

I am looking at a reasonably large student residential building, and the lobby outside the shaft has been defined as a protected escape route to BS7671 by the fire engineer due to the evacuation strategy of the building. The problem is the space is directly outside the riser, where the majority of the electrical services are likely to be running.

A durasteel enclosure is being proposed (link below), to give the required fire separation between the lobby and the cabling. Fortunately, the fire engineer is currently happy with the integrity only option, so fire insulation is not required.

The problem i am left with is working out the correction factor for the cabling. BS 8519 gives an example shows a 50% reduction for HV cables, but this is fully encapsulated within insulation. I was thinking potentially to calculate based on a trunking installation factor, but this again feels a bit onerous, as the durasteel enclosure will be far larger than traditional trunking would be, especially when it has encapsulated data and fire cabling. The run will typically be about 30-50% of the cables lengths so a reasonable amount. 

I have also asked the manufacturer for some guidance, but it feels like the question hasn’t been asked before. I have also asked about the small holes that look to be visible, but I’m told that’s not all the way through the metal, just a thin perforated covering layer.

Thanks in advance if anyone has any guidance

  • Which system of durasteel encasement are you employing? The single board (integrity only) or the double board? The latter may necessitate a correction factor for thermal insulation, or is it more like method A table 4D5 whereas the former may correspond to reference method B for trunking or even method E