Fire Properties Of Cables

Standards relating to fire properties of cables

IEC, BS standards

This is an area of increasing public and legislative concern, and therefore of increasing interest to engineers. There have been major advances in the fire performance of cables in recent years, and table below lists some of the relevant standards.

Standard	Description
IEC 60331	Fire resisting characteristics of electric cables.
IEC 60332	Tests on electric cables under fire conditions. Test methods and flame propagation of power and control/communication cables.Note the identical EN60332 and equivalent national standard BSEN60332 supersede EN50265 and BS 4066.
IEC 60754	Test of gases evolved during combustion of electric cables.
IEC 61034	Measurement of smoke density of cables burning under defined conditions. Identical EN61034 and national equivalent BSEN 61034 supersede EN50268 and BS7622.
BS 6387	Performance requirements for cable required to maintain circuit integrity under fire conditions.
BS 6724	Electric cables. Thermosetting insulated, armoured cables for voltages of 600/1000 V and 1900/3300 V, having low emission of smoke and corrosive gases when affected by fire.
BS 7211	Electric cables. Thermosetting insulated, non-armoured cables for voltages up to and including 450/750 V, for electric power, lighting and internal wiring, and having low emission of smoke and corrosive gases when affected by fire.
BS 7835	Specification for cables with cross-linked polyethylene or ethylene propylene rubber insulation for rated voltages from 3800/6600 V up to 19 000/33 000 V having low emission of smoke and corrosive gases when affected by fire.
EN 50267	Common test methods for cables under fire conditions. Tests on gases evolved during combustion of materials from cables. Apparatus. BSEN50267 is identical and supersedes BS6425. Similarly French standard NF C 20-454 is superseded.

Toxic and corrosive gases

It is recognized that conventional flame retardant cables having sheathing based upon PVC type materials evolve considerable quantities of halon acid gases such as hydrogen chloride upon burning.

Such materials are not therefore suitable for use in confined spaces where the public are likely to travel, and moreover the fire in the ENEL power station at La Spezia in 1967 showed that in certain circumstances PVC cables will burn completely and contribute to the spread of a fire.

Materials have now been developed for cable oversheaths and bedding which are normally free of halogen based compounds. They consist of a mixture of inorganic filler such as aluminium hydroxide and polymers such as ethylene vinyl acetate, acrylates and ethylene propylene rubbers.

 

Cables manufactured with such materials are known as ‘Low Smoke and Fume’ (LSF) and have acid gas evolution less than 0.5% in comparison to 25–30% for PVC compounds.

IEC 60754-1 specifies a method of determining the amount of halogen acid gas, other than hydrofluoric acid, evolved during combustion of halogen based compounds. The method essentially measures the existence of halogen acid greater than 0.5%, the accuracy limit for the test.

Therefore cables tested having less than the 0.5% limit are generally termed ‘zero halogen’ or ‘low smoke zero halogen’ (LS0H).

Smoke emission

Normal cable sheathing compounds also give off dense smoke when burned and this is of particular concern in underground transport system installations. The generation of large amounts of smoke obscures vision and reduces the ease with which the fire brigade is able to bring members of the public to safety in the event of a fire. LSF cables therefore play an important part in reducing this danger to a minimum.

London Underground Limited (LUL) have developed a test of practical significance which has been designed to measure the density of smoke emission from cables and it has now been adopted by British and IEC Standards. This defines the standard absorbance produced across the opposite faces of a test cubicle and is popularly known as the 3 m cube test.

Paris Metro (RATP) adopts the French Standard UTE C20-452 on smoke emission which determines under experimental conditions the specific optical density of smoke produced by burning material. This slightly different approach is generally known as the NBS smoke chamber test.

Oxygen index and temperature index

‘Oxygen index’ is the minimum concentration of oxygen in an oxygen/nitrogen mixture in which the material will burn. As air contains approximately 21% oxygen it is stated that a material with an oxygen index greater than about 26% will be self extinguishing. In general, a particular oxygen index value offers no guarantee of resistance to the spread of flames.
In practice materials having identical oxygen indices may have widely different burning properties especially if base polymers or additives are of different types.

The ‘temperature index’ of a material is the minimum temperature at which the material supports combustion in air containing 21% oxygen when tested under controlled conditions. The test is useful for the comparison of similar materials but no correlation with flammability under other fire conditions is implied.
Oxygen and temperature indices are to some extent inter-related.

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