A number of fire disasters happened in the 1980s and after that, there had been a strong demand for cables which behave more safely in a fire. Cables have been developed to provide the following key areas of improvement:
- Improved resistance to ignition.
- Reduced acid gas or toxic fume emission.
- Reduced flame spread and fire propagation.
- Reduced smoke emission.
After that, an optimized combination of these properties is achieved in LSF cables. These LSF cables provide all of the above-mentioned characteristics. The original concept of LSF cables was identified through the requirements of underground railways in the 1970s. At that time, the main concern was to maintain sufficient visibility such that orderly evacuation of passengers through a tunnel could be managed if the power to their train were interrupted by a fire involving the supply cables.
This led to the development of a smoke test known as the ‘3-metre cube’, this being based on the cross-section of a London Underground tunnel. This test is now defined in BS EN 50268. The reduced emissions of the toxic fumes also ensured passengers escape was not impaired. PVC sheathed cables can, by suitable use of highly flame retarded PVC materials, be designed to provide good resistance to ignition and flame spread. But they produce significant volumes of smoke and toxic fumes when burning. On this basis, the LSF materials become specified for underground applications.
The tests for reduced flame propagation are defined in BS EN 50265 (single cable) and BS EN 50266 (grouped cables), with tests for acid gas emission being defined in BS EN 50267. The demand for LSF performance has since spread to a wide range of products and applications and LSF now represents a generic family of cables. Each LSF cable will meet the 3-meter cube smoke emission, ignition resistance and acid gas emission tests, but fire propagation performance is specified as appropriate to a particular product and application. For instance, a power cable used in large arrays in a power station has very severe fire propagation requirements, while a cable used in individual short links to equipment would have only modest propagation requirements.
Hence BS EN 50265 would be appropriate to assess the single cable, but BS EN 50266 comprises a number of categories to cater for varying numbers of cables grouped together. Additionally, there has been significant growth in demand for cables that are expected to continue to function for a period in a fire situation, enabling essential services to continue operation during the evacuation of buildings or during the initial firefighting stages. In these cables, in addition to these LSF properties, the insulation is expected to maintain its performance in a fire. This insulation may be achieved by use of a compacted mineral layer, mica/glass tapes or a ceramifiable silicone layer.
Source: Newnes – Electrical Power Engineers