Fire
Fire needs three things, spreads in four ways and can end a business in minutes. This element explains how fires start and grow, how to prevent them and stop them spreading, how to detect and fight them, and how to get everyone out safely.
Pages in this chapter (1 of 14)
10.1 Fire Principles
The fire triangle
The fire triangle is a simple way to understand what a fire needs. Each side of the triangle stands for one of three components that must be present for a fire to burn.
Fire is a chemical reaction. Take away any one of the three components and the fire cannot start or keep burning.
Fuel. A fire cannot start unless there is something to burn, and businesses and homes are full of fuel. Sources include wood, paper, plastic, rubber, foam, loose packaging, waste and furniture. Any of these can feed a fire.
Heat. A sufficient heat source can ignite a suitable fuel/air mixture. For liquids, flash point is the test temperature at which enough vapour is produced for momentary ignition by an external source; it is different from auto-ignition temperature and is not a universal ignition temperature for every material. Combustion itself also gives off heat as it burns, which raises the temperature of the fuel still further. Sources of heat include heaters, lighting, naked flames, electrical equipment, smokers' materials such as cigarettes and matches, and anything else that can get very hot or make sparks.
Oxygen. Oxygen keeps the combustion reaction going, because it reacts with the burning fuel to release carbon dioxide and heat. The atmosphere is 21 per cent oxygen, so there is enough to start a fire as long as the other two components are present. The main source of oxygen for a fire is the air around us. In an enclosed building, the ventilation system provides it. Extra oxygen can sometimes come from materials that are used or stored on the premises, such as:
- oxygen supplies from cylinder storage and piped systems, for example medical oxygen, or oxygen used by contractors in welding
- some chemicals, called oxidising materials, which can give a fire extra oxygen and help it burn. The manufacturer or supplier should identify these chemicals on the container and can advise on their safe use and storage. Oxidising materials are chemicals that can easily break down to yield oxygen or an oxidising substance. Common examples include halogens, potassium nitrate and nitric acid
Classification of fires
In the UK/European system used here, fuel classes include A, B, C, D and F. Live electrical equipment adds a hazard, not a separate fuel class. Other national systems use different letters, so identify the system and the extinguisher's marked rating.
| Class | Fuel involved | Example |
|---|---|---|
| A | Organic solids | Paper, wood |
| B | Flammable liquids | Petrol, solvents |
| C | Flammable gases | LPG, methane |
| D | Burning metals | Aluminium swarf |
| F | Cooking oils and fats | Deep fat fryers |
| Electrical hazard | Live equipment can make firefighting dangerous | Check suitability and isolate only if safe; not a separate fuel class in this system |
Electrical fires do not have a class of their own, because they can fall into any of the classes. It is not the electricity that burns, but the surrounding material that the electric current has set alight.
Summary notes for Element 10 · Cheat sheet · Glossary. A study aid, not published or endorsed by NEBOSH.