Safety cables and circuit integrity

Determination of cable cross sections under fire conditions

Manfred Kirschvink, Karl Audenaerde, Kabelwerk Eupen
Stefan Fassbinder, Deutsches Kupferinstitut

Cables for a safety installation should continue to function in case of a fire incident. This is not only a question of an appropriate insulation. For guaranteeing the integrity of the electrical circuit, a good calculation of the cross section is just as important. When heating a cable, its electrical resistance rises. For a proper functioning of electrical devices, the voltage drop should not exceed 10%. The attached paper shows how to make the necessary calculations. A factor k indicates which proportion of the cable is exposed to the fire. If the complete cable can be exposed, the cross section should be made no less than five times larger. In Belgium, the national electrical code includes this factor 5 – in contrary to most European and international standards.