BASEC News

Fire Resistant Cable Standards

01/04/10

Dr Jeremy Hodge, chief executive of the British Approvals Service for Cables (BASEC)  
Focus on low smoke cables

There are three types of cable with this classification to talk about:
• Large power cables BS 6724. Used anywhere where, in case of fire damage, loss of human life and material assets must be prevented, e.g. in industrial plants, airports, underground railways and tunnels.
• Small wiring cables BS7211. Ideal for use where smoke and toxic emissions would pose a major hazard in the event of a fire in domestic and commercial environments. They are also suitable for laying in trunking or conduit where mechanical protection is required.
• Fire protection cables BS 5839-1. These are used in the design and installation of fire alarm systems.
There are also specialist low smoke flexes available.
The testing process
To describe a cable as ‘LSHF’, it must pass two tests: (a) a smoke cube test, which sets a maximum accumulated smoke density produced when burning the cable with a standard fire, and (b) a corrosive and acid gas test where the cable materials are roasted and must give off a maximum of 0.5% acid gas. These tests are both specified in cable standards which include the description ‘having low emission of smoke and corrosive gases when affected by fire’. Cables made from PVC or similar halogenated materials are unlikely to pass these tests. A common misunderstanding is that a ‘LSF’ cable means it has only passed the smoke test and not the acid gas test.  We prefer to use the ‘LSHF’ designation as it reinforces both requirements.
Ensure you have the right cable for the job
In terms of market practice more people are specifying and using LSHF cables.  However this does not mean that these cables are fire resistant. Fire safety engineering techniques permit a more open building structure, and ensures safe evacuation in the event of a fire by a combination of traditional and modern, active fire systems.  Many of these systems require electrical power supplies and control circuitry to remain functional throughout a potentially serious fire.  Examples include fire-fighting lifts and smoke extraction systems.  Robust fire resistant cables are needed here to continue to work in a fire. For this you need special fire resistant cables such as BS 7846 armoured cable or mineral cable to IEC 60702-1.  These fire resistant cables can also be described as LSHF as they use the appropriate materials (optional oversheath for mineral cables), but they have additional construction features to provide circuit integrity throughout a fire.
If you would like more information about low smoke halogen free or fire resistant cable standards please visit www.basec.org.uk for more information.

 

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