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What Is Fire?

Fire is a chemical reaction known as combustion, it occurs when fuel and oxygen are in the presence of sufficient heat to cause ignition. There are three elements related to the fire triangle, namely; heat, fuel and oxygen.

The burning rate of fuel depends on its chemical composition and physical form, it is the vapour omitted from the fuel which takes part in the combustion process and a fuel will burn if it is mixed with the correct proportion of oxygen and is at a sufficiently high temperature.

The fuel must reach a sufficient temperature before ignition can take place (critical temperatures) heat can be applied from various sources (sources of ignition) and fires spread by the transfer of heat and consequent ignition of other combustible materials.

(mechanisms of heat transfer)

Critical Temperatures

Flash point: Is the lowest temperature of which a particular fuel will ignite momentarily on application of a source of ignition.

Firepoint: Is the lowest temperature at which a particular fuel will ignite and remain alight on application of a source of ignition.

Auto Ignition: Is the lowest temperature at which a particular fuel will ignite and remain alight without the application of a source of ignition.

Mechanisms of heat transfer

Radiation: The transfer of heat through "heat rays" from a source to material directly in its path.

Convection: The transfer of heat from a source through the heating up and movement of surrounding gases and liquids.

Conduction: The transfer of heat from a source through a conducting solid, normally metals.

Sources Of Ignition

Match, lighter, gas burner, cigarettes etc

Sparks (hot metal)The impact between hard metals, electrical short circuit, grinding or welding

Sparks (static electricity)The lighting friction between two non-conductors, plastics, fabrics, dry powders, dry gases

Heat: (surface)The friction between machine parts, car engine/exhaust, electric cooker,electric cables, light bulbs etc

Oxygen:The normal source of oxygen is from within the air and there is typically around 21% oxygen present in the air at sea level, fires typically require a minimum of 16% oxygen to be present for combustion to occur

Extinguishing Fires

The following methods are the ways that fires are extinguished depending on the source of the fire and the type of fire extinguisher used.


By removing the fuel we starve the fire, disconnect the mains supply, disconnect the battery etc


By smothering the fire we remove the oxygen, cover with a fire blanket, foam application covering the surface.


By damping down with water, foam or wet chemical fire extinguisher.

Chemical interference:

By disrupting the combustion process, using a dry powder extinguisher.

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