Principles of heat transmission and fire spread

Fire spreads by heat transfer in four main ways: convection, conduction, radiation and direct burning.

Convection. The transfer of heat within a liquid or gas, caused by differences in density. A heated liquid or gas expands and becomes lighter, so it is displaced by heavier material and rises. As it rises, fresh air and oxygen are drawn in, which feeds the fire further, and the rising hot gases carry heat to the upper floors. In an enclosed space, the hot gases collect under the ceiling and spread along ceilings and walls, then bank down as they build up. Further superheating makes the fire rise again, carrying with it the products of incomplete combustion such as embers.

Conduction. The transfer of heat through materials. When enough heat is present, it can be enough to ignite fuel on the far side of another object. Combustible materials are the most susceptible to this.

Radiation. The transfer of heat by waves that travel until the heat is absorbed by other objects. A bar heater or open fireplace radiating onto a drying rack or curtain is an example.

Direct burning. The simplest way for fire to spread: direct application. A lit match can burn paper, for example. The more objects the fire touches, the more likely it is to spread quickly. Because fire is transient and can affect other objects, it spreads easily, and once it has spread it can move from place to place and become very hard to control.

Common causes of fire in the workplace

• Faulty equipment. Defective electrics such as loose wires, or faulty equipment that can overheat and spark, are common causes of workplace fires. Equipment must be inspected regularly and replaced at any sign of electrical damage.
• Clutter. If the workplace is not cleaned regularly, it becomes untidy and cluttered, and the build-up of dust and grease can be a fire risk, especially in poorly ventilated areas. In offices, clutter around desks and computers can cause problems, and so can untended items in the kitchen or break room.
• Combustibles on site. Combustible materials such as paper, wood and cardboard often accumulate and, if allowed to build up, can add a great deal of fuel to any fire that starts. To reduce the risk of a fire escalating because of incorrect storage, store these materials off-site in a locked container and dispose of them regularly.
• Human error. With many people using a workplace every day, human error is a common factor. Using equipment incorrectly, not reporting faulty machinery, smoking, and even leaving cooking food unattended can all cause accidents without proper training. All staff should have basic fire safety training so that the correct procedures are followed if hazards are found.
• Arson. Arson accounts for a large share of fires on business premises and can have devastating consequences. Deterrents such as CCTV and motion-sensor lighting should be installed, and access to the premises should be limited to authorised people. Fire safety equipment such as sprinklers throughout the building also limits the damage if the building does fall victim to arson.
• Chemical reaction. Reactions between chemicals can generate heat, for example with oxidisers.
• Mechanical heat. Dirt and dust can build up on machinery and equipment and block its ventilation, causing overheating and possibly a fire if left unattended. Regular inspection and cleaning limit the risk.
• Misusing equipment. Misuse is a major fire risk in offices and other workplaces, particularly in construction and manufacturing, where heavy plant and personal tools can generate significant heat and friction. Tools such as angle grinders throw sparks that can ignite surrounding debris, especially on building sites or in enclosed spaces. Heavy machinery also needs careful oversight and maintenance because it often contains complex electronics and has large power demands. Misuse adds strain that can cause motor shorting and other faults that easily lead to fire.
• Inadequate wiring. Older buildings can have inadequate wiring, which is both a fire and an electrical hazard. Warning signs:
  • you have to disconnect one appliance to plug in another
  • you rely heavily on extension cords or multi-way adaptors
  • fuses blow or circuit breakers trip often
  • lights dim when you use another appliance

If anyone is worried about their electrical installation, they should not try to fix it themselves but should seek the advice of a certified electrical contractor.

• Bonfires and waste burning. Avoid unnecessary open burning and use lawful waste arrangements. Heat, smoke and windborne embers can endanger people and property. A bucket of water or a large distance alone is not an adequate work-site burning plan.

Summary notes for Element 10 · Cheat sheet · Glossary. A study aid, not published or endorsed by NEBOSH.