Transmission risks
As mentioned, small control cables of similar design are tested to 920°C according to BS 8434-2, but these small cables do not often have a significant load current or the mass of larger cables, which mechanically stresses the dielectric, especially in vertical installations. It is noted that cables of similar design to the common fire rated cables sold in the UK today (using glass mica tapes) have been successfully tested in other countries to the same time temperature protocol as used in BS 476 Parts 20 to 24, being 945°C at 60 minutes; 1,006°C at 90 minutes; and 1049°C at 120 minutes. Whether the fire rated cables made and sold in the UK today would also pass this standard fire test used for all other fire rated elements of building construction would of course need to be tested. According to BS 7671: 2018, a trefoil group of
single core 90°C rated, insulated and sheathed 95mm2
copper cables on a perforated cable
tray or cable ladder in free air has a current rating of 328 amps (see Figure 1). Calculation to IEC 60287 (the basis calculation for all cable current ratings in BS 7671) indicates that, in a fire of 842°C where the load current is 328 amps, the conductor will attain a temperature of 1,000°C, which is well above any temperature the cable or its now degraded insulation is tested to.
Making the situation worse is the fact that
the resistance of copper at 20°C will increase about four times at this temperature, and almost five times at 1,000°C, so the additional
ohmic heating caused by a current flow is even greater. We must remember copper has a melting point of 1,083°C (or a few degrees less where abundant air/oxygen is present), so there is not much headroom for such additional ohmic heating due to the required current load.
Informed choice
What this means for designers and contractors is that for any essential service drawing significant load during fire, they need to choose cables which will carry the required load current even during the fire (and have volt drop parameters designed for these ‘in-fire’ conditions). In practice, copper also loses about 80% to 90% of its tensile strength at real fire temperatures, posing more issues for vertical installations, and of course it loses 100% of its tensile strength near 1,083°C when it melts. Trying to operate a fire rated power cable at
or above the maximum tested temperature of 842°C (as tested in BS 8491) is simply entering the unknown and likely to be on a rapid path to failure. Fire rated cable systems are tested to remain operational at or below this maximum temperature with only a nominal test current of a few amperes, so the additional current temperature rise induced by ‘actual’ load currents is today simply not tested. BS 7671: 2018 gives the maximum current
rating for a 90°C rated single core of 95mm2 insulated and sheathed cable in trefoil in free
FOCUS
www.frmjournal.com OCTOBER 2019
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