The Fire Triangle Theory and General Causes of Fire

The fire triangle theory graphically illustrates the 3 (three) factors essential to start and sustain a fire.

  • There must be a flammable gas or vapour or, there must be a solid or liquid capable under certain temperature conditions of giving off flammable gas or vapour.
  • There must be oxygen present. Generally, this will be supplied by the surrounding air.
  • There must be present a source of heat of sufficient intensity to initiate the chemical reaction.

In other words, to start a fire, there must be fuel and oxygen, together with sufficient heat to bring the fuel to the temperature at which it will combine chemically with the oxygen and burn. No two of the three factors will produce a fire in the absence of the third factor.

A simple illustration of the process of starting a fire is provided by striking a match on a rough surface. Before the striking, the match head is at room temperature and is surrounded by air (which contains 20% by volume of oxygen). The act of striking the match head against a rough surface generates heat in the tiny area which is abraded by the rough surface. The amount of heat generated is very small. You can check this yourself by running your finger for 1-1.5 cm along a rough surface at a similar speed and pressure to those involved in striking the match. This example shows how easily a fire can be created. This is why whenever fuel, oxygen and heat are close proximity, they must be kept apart to avoid accidental fire.

The fire created by striking the match can usually be extinguished by holding it upright. The fuel source is then below the hot area in which the flame sits. Heat travels upwards and the match usually goes out because the unburnt match is not heated enough to sustain burning.

As a second illustration, what happens when you light a candle, stand it upright on a flat surface and cover it with glass tall enough to clear the flame but having a flat rim which will rest on the flat surface on which the candle stands? The flame shrinks and goes out.

As a final illustration, if you ignite a small screwed up piece of paper placed on a flat incombustible floor in a still atmosphere, the paper will burn away and the fire will go out.

By general causes of fire, we mean causes of accidental or fortuitous fire which damages insured property. Such accidental fires result from the bringing together of something which will burn, a source of heat and a supply of oxygen. When heat brings the thing which will burn to a temperature at which it ignites, fire results.

Example of the principal sources of such accidental fires are as follows:

  • People
    Carelessly discarded smoking materials.
    Unsupervised cooking of fats or other flammable foods.
    Accidentally knocking overheating appliances.
    Unrepaired machines which start to run hot and overheat.
    Lack of care in grinding where showers of sparks are produced.
    Wilful disregard of warning signals on the heat processing plant.
    Use of wire or oversize fuses to allow an electric appliance whose fuse has blown to be re-used.
  • Processes
    Lack of maintenance of the control areas of the heat processing plant, resulting in overheating.
    Accidental ignition of flammable material by sparks, torches, etc.
    Overheating and eventual ignition of combustible materials going through a drying process when the conveyor breaks down.
    Foreign bodies dropping into fast-moving machinery.
  • Heating systems
    Unsuitable heating systems: a heater using a naked flame or electric radiants must not be used in rooms where flammable vapours may be present (for example, from solvents used for cleaning or paint spraying) or where flammable dust may be present (for example, flour mills and woodworking mills).
    Unsafe heating systems: a portable heater must not be used in flammable atmospheres; not sited where it may be knocked over or within 1 metre of combustible material; not left working while unattended, and not refilled when still hot (if it uses liquid fuel).
    Overall, no heating system should be used unless it has been serviced by a qualified engineer within the last, say, 12 months. The service examines the operation of the burners, air intakes, fume exhausts, the fuel tank, feedlines and connections and any ancillary controls, whether manual or automatically operated.
  • Lighting and power systems (usually by mains electricity)
    Fuses and circuit breakers should be rated as specified by the manufacturer of the equipment they protect; otherwise, a larger current may reach the equipment than it can handle with consequent overheating and a risk of fire.
    Flexible cables should be eliminated wherever possible. If elimination is not possible, they should be clipped out the way of any traffic likely to crush or stretch them. Damaged insulation can lead to heat where short circuit sparks occur or heat where a connection is broken by a jerk on a cable and produces sparking where the contact is intermittently restored.
    Water must be kept away from electric equipment since it may cause short circuits.
  • Other occupiers
    It is well to be informed of the occupation of other tenants in nearby rooms or premises. It would be futile to have a spray painter operating in a safe spray room with flameproof lights and extraction systems in a multiple tenure building where the next room was used by a metal welder and cutter. On a calm day, the flammable vapours from the paint shop may drift into the welder’s workshop.
  • Waste
    Dry waste: cloth clippings, wood off-cuts and other combustible material may help the rapid spread of fire if not cleared frequently.
    Oily and greasy waste: used cleaning cloths or cotton wool type cleaning pads, if soaked in oil or grease and left in a heap for a considerable time, will degrade. The oil starts to oxidise very slowly, but this process gradually speeds up and the pile starts to warm up. The process accelerates until the centre becomes charred. The charred part loses strength so that the weight of material on top causes a small collapse over the charred area. This causes a slight disturbance of the air which bring sufficient fresh air (containing oxygen) in contact with the charred area to make it self-ignite.
  • Materials
    Some raw materials are prone to self-heating, which could lead to self-ignition. Examples include certain inks, hay, jute, flax, corn and other commodities of organic nature. If the conditions under which these commodities are kept are not correct, they could overheat and self-ignite, with catastrophic results.

Peter Wildman FCII. 1998. Commercial Property and Pecuniary Insurances – Assessment and Underwriting. The Chartered Insurance Institute.


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