To prevent fire, you first need to understand it. Every fire, from a smouldering bin to a burning warehouse, needs the same three things to start and to keep going.

Fires also behave differently depending on what is burning. A fire in a chip pan, a fire in a stack of paper and a fire fed by a gas leak each need a different response.

This module explains how fire starts, how it spreads, and how fires are grouped into classes, so that you can recognise hazards and choose the right action.

In this module you will learn about:

• The fire triangle: heat, fuel and oxygen.
• Common sources of ignition, fuel and oxygen at work.
• How fire spreads through a building, and why smoke is so dangerous.
• The classes of fire, and fires involving electricity.
• Dangerous substances, labels and safety data sheets.

A fire needs three things, known as the fire triangle:

• Heat, from a source of ignition such as a flame, a spark or a hot surface.
• Fuel, anything that will burn, whether it is a solid, a liquid or a gas.
• Oxygen, usually from the air around us.

Take away any one of the three and the fire cannot start, or it will go out. Fire prevention is about keeping the three apart. Fire fighting works by removing one of them: water cools away the heat, and a fire blanket or foam smothers the oxygen.

A fire needs three things to begin. The first is a source of heat. This might be direct flames, such as a gas burner or LPG heater, a hot surface such as an oven or hotplate, or sparks from electrical fittings, equipment or welding. The second is fuel. Many materials can burn, including paper and cardboard, which are common in workplaces. Some workplaces also hold paint and solvents in cans or bottles, gas cylinders, plastic packaging, hand sanitiser, or organic dust in places like bakeries and flour mills.

The third is oxygen. The air contains plenty of it, so it is available for most fires. But compressed oxygen in cylinders, if mishandled, can intensify a fire and become very dangerous. Together these make the fire triangle, and if any one is missing, a fire cannot start.

Common sources of ignition at work include:

• Faulty, damaged or overloaded electrical equipment, sockets and extension leads.
• Heaters, cooking appliances, lighting and other hot surfaces.
• Smoking materials, matches and lighters.
• Hot work, such as welding, cutting, grinding and soldering.
• Friction and sparks from machinery.
• Static electricity, and lightning.
• Deliberate fire setting, which is one of the most common causes of workplace fires.

Look out for the sources of ignition that most often start workplace fires. Open flames are the most obvious. Smoking is another, through cigarettes, matches and similar items. Hot work covers processes involving high temperatures, such as welding or cutting with abrasive wheels. Faulty electrical equipment is a frequent cause. Hot engines and machines produce heat while they run. Friction is created by loose machine bearings, parts of electric motors, or components that rub against each other.

Static electricity can build up on clothing and footwear and may be enough to ignite flammable vapours or dust. As a fire warden, being able to recognise these sources helps you spot problems in your area before they turn into fires, and report them to the person responsible.

Almost everything in a workplace can act as fuel, including:

• Paper, cardboard, packaging and waste.
• Furniture, furnishings, fabrics and foam.
• Wood, plastics and rubber.
• Flammable liquids such as paints, solvents, fuels and cleaning products.
• Flammable gases such as LPG, and flammable dusts.

Oxygen is always present in the air, but extra oxygen can make a fire much fiercer. Oxygen cylinders, some chemicals called oxidisers, and open windows or ventilation systems can all feed a fire.

Almost everything in a workplace can act as fuel. Paper, cardboard and plastics include packaging and office supplies. Foam products, such as the polyurethane foam used in packaging, will burn, as will wood and fabrics, for instance wooden furniture and textile furnishings, and items made of rubber. Flammable liquids include cleaning chemicals, cooking oils, hand sanitiser, paints and varnishes. Gas cylinders such as LPG and acetylene are fuel sources, as are solvents, including thinners, inks and products like cleaning agents.

Flammable and explosive dusts matter too: wood dust in workshops, flour in mills and metal dust in metalworking factories. Fuel canisters such as propane, and some aerosol cans that use LPG as a propellant, also burn, and so do chemicals and waste materials such as the gases produced at landfill sites or sewage treatment works.

Besides the oxygen in the air, other sources of oxygen can feed a fire if they are not properly managed. Oxygen cylinders and canisters are widely used in medical facilities, workshops, repair shops, factories and warehouses. Oxidising agents are another source. Ammonium nitrate, a common agricultural fertiliser, and the peroxides found in many cleaning products can act as oxidisers, and some other chemicals do the same. Pure oxygen can make a fire stronger and faster.

The storage, handling and use of oxygen cylinders and oxidising materials therefore needs careful planning and supervision by trained people. Oxygen canisters must be stored apart from flammable materials, especially LPG and acetylene. Organic fertilisers also have to be stored safely, and there are strict rules for bulk storage.

Once a fire has started, it spreads in four ways:

• Conduction: heat travels through solid materials, such as steel beams and pipes, and can set fire to things in another room.
• Convection: hot smoke and gases rise, spread across ceilings and travel up stairwells and shafts.
• Radiation: heat radiates out in all directions, igniting materials nearby without any direct contact.
• Direct burning: flames reach and ignite materials next to them.

This is why fire doors, fire-resisting walls and good housekeeping are so important. They break the paths that fire uses to spread.

Smoke is the biggest killer in fires. It contains toxic gases, including carbon monoxide, which can make a person unconscious within minutes.

• Smoke spreads far faster than flames, and can fill a corridor or stairway in seconds.
• It makes it impossible to see, so people become lost even in buildings they know well.
• Modern furnishings and plastics produce thick, highly toxic smoke.

Never go into smoke to look for a fire, and never re-enter a building that is on fire. If you have to move through smoke to escape, keep low, where the air is cleaner.

All fires need the same basic elements, but the type of fire depends on the fuel involved, which is why there are six classes. Knowing the class matters for three reasons. First, it lets your employer judge the most serious fire risks in the workplace, introduce suitable safety measures and provide the right fire-fighting equipment. Second, in an emergency it tells the fire and rescue service what kind of fire they are dealing with.

Third, it guides your own actions. For a small fire that can safely be handled with an extinguisher, knowing the class helps you pick the right type. Remember that you should only try to put out a fire if you have been properly trained and it is safe to do so. Your first priority is always to evacuate safely.

Fires are grouped into classes according to the fuel that is burning:

• Class A: ordinary solid materials, such as paper, wood, textiles and many plastics.
• Class B: flammable liquids, such as petrol, oil, paints and solvents.
• Class C: flammable gases, such as propane, butane, natural gas and acetylene.
• Class D: burning metals, such as magnesium, aluminium swarf and lithium.
• Class F: cooking oils and fats, such as in deep fat fryers.

Electricity is not a class of fire, because it is a source of ignition rather than a fuel. But fires involving live electrical equipment need special care.

Class A fires involve solid materials. They are the most common type of fire in offices, shops and warehouses. Typical fuels are paper, cardboard, wood, furniture, fabrics and waste.

Class B fires involve flammable liquids. Many of these liquids give off vapour that can catch fire well below room temperature, and the vapour can travel a long way to a source of ignition.

Water must never be used on a burning liquid. It can spread the fire or cause it to flare up violently.

Class A fires involve solid flammable materials and dusts. They include wood, including shavings and sawdust, plastics that burn and give off hazardous smoke such as polyurethane, paper, cardboard and packaging such as straw, and fabrics such as curtains, clothing and carpets. Dusts such as coal, grain and flour are common in food and processing sites, and sewage and other waste also burn. Class A fires are put out with water or foam.

Class B fires involve flammable liquids, many of which give off vapour that can ignite well below room temperature. Examples are oil-based paints, varnishes, solvents and thinners; petrol kept in containers for mowers, chainsaws and disc cutters, which is subject to strict safety rules; cleaning chemicals showing a flammable symbol; and diesel-soaked rags and clothing. Class B fires are fought with foam or powder.

• Class C fires involve flammable gases. The only safe way to deal with a gas fire is to shut off the supply, if this can be done safely. Never try to put out a burning gas leak while the gas is still flowing, because the escaping gas could explode.
• Class D fires involve burning metals. They burn at very high temperatures and need specialist extinguishers. Ordinary extinguishers can make them worse.
• Class F fires involve cooking oils and fats at very high temperatures. They need a wet chemical extinguisher or a fire blanket. Water on burning oil causes a violent fireball.

Class C fires involve flammable gases. These include LPG cylinders, often propane or butane, and acetylene cylinders. Oxygen cylinders belong here too, because oxygen makes fires worse, so they must be stored away from flammable materials and gases. Aerosols are a serious hazard in a fire, and should be kept in a designated area away from sources of ignition and heat. Class C fires are highly dangerous because they can cause explosions, so the safe storage and handling of flammable gases needs a thorough risk assessment.

Class D fires involve combustible metals, notably metal swarf, which is common in engineering factories when metals such as aluminium are ground or polished. Other metals used in processes such as chemical manufacturing should also be regarded as potentially combustible. Class D fires are fought with specialist powder.

Flammable and explosive dusts are not a separate class of fire, but they deserve special attention as a source of fuel. Many dusts will burn. Examples are wood dust in woodworking shops, flour and grain dust in mills, organic dust from food manufacturing, and dust from sewage sludge drying plants. Dust from working with materials such as leather can settle on surfaces over time, and if a heat source such as welding equipment is nearby, the accumulated dust can ignite.

Dust of these kinds can cause large fires and even explosions in workplaces, which is why proper control measures are essential. If there is an incident involving dust, call the emergency services immediately, exactly as you would for any other class of fire.

Electrical faults are one of the most common causes of fire at work. Typical causes include:

• Overloaded sockets and extension leads, and extension leads plugged into each other.
• Damaged cables, plugs and equipment.
• Equipment left running, or covered so that it cannot cool.
• Poor or overdue maintenance of electrical installations.

If equipment is on fire, switch off the power if it is safe to do so. Never use water or foam on live electrical equipment. A carbon dioxide extinguisher is normally used.

Electrical fires start when faulty or overheating equipment ignites flammable material inside or around it. Typical examples are short circuits in control panels, especially at higher voltages, overheated or misused equipment, damaged cables and wires, and poorly maintained equipment. Electricity is not a class of fire, because it is a source of heat and not a fuel. A CO2 extinguisher is used to put them out. Class F fires involve cooking oils and fats that burn at very high temperatures.

These include vegetable oils such as sunflower, olive and maize oil, and animal fats such as lard and butter, often used in deep-fat fryers. They are most often found in commercial kitchens, food factories and edible oil processing. A wet chemical extinguisher or a fire blanket is used.

Many workplaces store or use dangerous substances that can catch fire or explode, such as solvents, fuels, gas cylinders, aerosols and some cleaning products.

• Containers carry hazard pictograms: a red diamond with a black symbol, such as a flame or an exploding bomb.
• Each product has a safety data sheet, which explains how to store and use it safely, and what to do in a fire.
• Dangerous substances must be kept in the smallest quantities needed, in suitable containers and stores, away from sources of ignition.

As a fire warden, report any substances that are stored badly or left in escape routes.

Employers should use safety data sheets, known as SDSs, to help with their risk assessment, but a safety data sheet is not a substitute for a full risk assessment. The employer, or a competent person, will study the key information in the sheet to assess and reduce the fire hazards linked with the chemical. Employers must also pass the contents of the safety data sheet on to their employees and to anyone else who might be affected, and they may need to present it in a more accessible form.

It is important to understand that a safety data sheet does not release the employer from their other duties, including the duty to provide information, instruction and training on fire safety and on hazardous substances.