FOCUS
Transmission risks
air at 328 amps, but in a real fire of 1,000°C the current rating of this same cable (where the conductor temperature is to be limited to say 1,040°C) is only 136 amps. This reduction in ampacity of the cable where exposed to real fi re temperatures is of course true for any fi re rated cables from any manufacturer, as it is a fundamental property of copper. Going forward then, if you really need the 328 amps during a fi re, you would need to use a cable with a conductor significantly larger (see Figure 1), or to thermally protect the cable by encapsulation. As mentioned earlier, it is also important to consider the effect of fi re temperatures on the voltage drop of the circuit, including the part of the circuit exposed to the fi re. This is easier to calculate and is effectively simply adding the series’ resistances (cable length in fi re and cable lengths not in fi re). With current ratings, however, the critical point is not the length exposed to fire but the temperature at any given point in the fire. This is because even if only one or two metres are exposed to these fire temperatures, the conductor can fail at that point if the fire temperature and cumulative current induced temperature rise exceed the melting point of the conductor, or if the temperature exceeds the dielectric ability of the mica tape or insulation to provide adequate voltage insulation, at which point the protective device may trip and thus kill the circuit.
40 OCTOBER 2019
www.frmjournal.com
In the UK, the maximum temperature to
which we test power cables is 842°C and, with consideration of the mechanical strength of copper conductors above this temperature together with the known reduction in insulation resistance of the mica/insulation fl ame barrier or any other insulation materials used, any fi re exposure temperature would need to be below 842°C for safe calculation of any current rating. So for the example cited of three 95mm2
single
core 90°C rated cables in a trefoil group in free air, if a fi nal operating temperature of 842°C is defi ned and/or limited by our current test method (BS 8491) and 328 amps is required by the connected load during fi re, the maximum fi re temperature to which the cable can be exposed – including the current induced temperature rise – must be limited to about 790°C. Given that a standard BIC cigarette lighter burns at about 800°C, this is unrepresentative of many ‘real’ building fi res. Effectively this means that, even with current standards and codes, where fi re rated power cables are carrying any signifi cant current, they should be additionally protected by thermal barriers, encapsulation or routing in areas where exposure to fire temperatures is not possible.
Forward proposals
The published depiction of a real fire profile1 and two other publications2,3
indicate that real
fires in modern buildings can reach 1,000°C within ten minutes. This tends to vindicate
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