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Class “A” fires

The ignition temperature of wood depends on many factors, such as size, shape, moisture content and type. Generally the ignition temperature of woods is about 204⁰ C (400⁰ F).

A slowly developing fire or a source of radiant heat may gradually transmit enough energy to begin the pyrolysis of wood products. The combustible vapour that is released will mix with the surrounding air. When this mixture is within the flammable range, any source of ignition may ignite mass almost instantly. This condition is called flashover.

Textiles and Fibres. Many variables affect the way in which textile burns. The most important are the chemical composition of the textile fiber, the finish of the fabric, weight and so on.

Vegetable fires ignite easily and burn readily. Wood is difficult to ignite; it tends to smoulder and char rather than to burn. Silk is the least dangerous fiber. It is difficult to ignite and it burns sluggishly. Combustion must be supported by the external source of heat. Once set on fire, silk retains heat longer than any other fiber in wet silk.

The burning characteristics of synthetic fibers vary according to the materials used in the manufacture. The characteristics based on small scale tests may be misleading. Some synthetic fabrics appear to be flame – retardant when tested with a small flame source, such as a match. However, when the same fabrics are subject to a larger flame of a full scale test, they may burst into flames and burn completely while generating quantities of black smoke.

Plastics and rubber. Plastics may be divided roughly into three groups in regard to burning rates:

1). Materials that either will not burn at all or will cease to burn if the source of ignition is removed.

2). Materials that are combustible, burn relatively slowly, but may or may not cease to burn when the source of ignition is removed.

3). Materials that burn without difficulty and can continue to burn after the source of ignition is removed.

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