Cables used for safety circuits and for meeting resistance to fire requirements are expected to continue to operate during a fire. A number of different fire tests may be applied to different cables, but in general, the cables will be expected to operate under load for periods of between 30, 60, 90 or 120 minutes, sometimes with mechanical shock and often with water sprayed onto the burning cable.
These cables are generally used for critical power supply circuits such as smoke extractor fans or for safety circuits such as fire alarms & emergency lighting.
Standard fire resisting cables are required to maintain circuit integrity for 30 or 60 minutes when tested in accordance with BS EN 50200 ‘Method of test for resistance to fire of unprotected small cables for use in emergency circuits, by simultaneously exposing a sample of the cable to a flame at a temperature of 830˚C and to mechanical shock for 15 minutes, then simultaneously exposing the sample to flame, water spray and mechanical shock for a further 15 minutes. There must be no loss of circuit integrity during this period. Annex E of the Standard incudes’ informative guidance on a means of applying a water spray to the cable during the test.
Standard 30 cables are generally used for fire alarm systems and standard 60 cables are used for emergency lighting systems to BS 5266-1.
Enhanced fire resisting cables are required to maintain circuit integrity for 120 minutes when tested in accordance with BS EN 50200. In addition, the cables must conform to the requirements of BS 8434-2 Methods of test for assessment of the fire integrity of electric cables. Test for unprotected small cables for use in emergency circuits. In accordance with BS 8434-2, a sample of the cable is simultaneously exposed to flame at a temperature of 930 °C and mechanical shock for 60 minutes, then simultaneously exposed to flame, water spray and mechanical shock for a further 60 minutes. There must be no loss of circuit integrity during this period.
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