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Guide to Fire Extinguishers Back

Guide to Fire Extinguishers

Below we attempt to demystify the distinction between various types of fire extinguisher. This knowledge is essential for those seeking to gain fire awareness training and fire marshal training, as well as those looking to buy fire extinguishers online.

In this guide we refer to ‘fire classes’. There exists five different classes of fire and each is distinguished by the type of fuel keeping the fire alight. In the United Kingdom this standard is defined in the European Standard ‘Classification of Fires’. This standard is applied across the European Union. This classification system allows firefighters and lay fire marshals to select the appropriate fire extinguisher based on the class of fire he or she is faced with.

Classes of fire referred to throughout this post include:

  • Class A: Ordinary combustibles e.g. wood, fabric, paper
  • Class B/C: Flammable liquid and gas
  • Class C or Class E: Electrical
  • Class D: Metal
  • Class K/F: Cooking oils and fats (kitchen pans etc.)

We now provide an overview of each type of fire extinguisher and note which class of fire each extinguisher is designed to tackle.

Foam fire extinguishers

Foam fire extinguishers are designed for use on Class A and Class B fires. Foam fire extinguishers may also be used to tackle an electric fire provided the user is able to stand one metre away from the fire. A foam fire extinguisher should never be used to tackle a Class F fire (chip fat fire etc.).

A foam fire extinguisher is red with a distinctive cream label as illustrated below:

A foam fire extinguisher works by killing the fire’s flame. AFFF foam is the active agent contained within the extinguisher’s cylinder (pronounced A-triple-F). The foam’s outer layer seals the fire’s harmful vapours. The foam creates a barrier between the fire and oxygen. Therefore, the fire is unlikely to re-ignite once extinguished. This outer layer is often referred to as a ‘foam carpet’.

Carbon dioxide (CO2) fire extinguishers

A carbon dioxide fire extinguisher suffocates the fire of oxygen. This type of fire extinguisher is ideal for use on Class B (liquid) fires and also fires involving electric equipment (Class C). This is because unlike water, carbon dioxide is non-conductive.

A carbon dioxide fire extinguisher is distinguished by a horn-shaped nozzle. The extinguisher’s cylinder is red with a black label, as illustrated below:

A carbon dioxide fire extinguisher poses several inherent risks. Firstly its use poses a risk of asphyxiation to the user. This is particularly the case when this extinguisher is used in a confined space should as a computer server room. This is because the carbon dioxide replaces the room’s supply of air.

Secondly a carbon dioxide fire extinguisher does not cool the fire. The fire is only extinguished for want of oxygen. The remaining heat is capable of re-igniting the fire once enough carbon dioxide has escaped from the room.

Water fire extinguishers

The classic water fire extinguisher is designed for use on Class A fires (e.g. those involving paper, wood or fabric). Water cools the flames. A water fire extinguisher is not suitable for use on a Class B (liquid) or Class (electrical) fire. Water is an electrolyte and therefore conducts electricity. If used to tackle an electrical fire, the current may travel across the water and electrocute the extinguisher’s user.

Wet chemical fire extinguishers

A ‘wet chemical’ fire extinguisher is suitable for use on Class F fires (e.g. chip pan fires). This variety of fire extinguisher is red with a distinctive yellow label as illustrated in the picture below:

Water is useless when tackling an oil based fire. This is because water does not cool oil. To combat water’s inherent weakness, wet chemical fire extinguishers employ potassium acetate. This chemical cools hot oil and thus extinguishes the associated fire. Potassium thickens the oil. The oil is thus cooled preventing re-ignition of the fire. A foam coat is produced over the surface of the oil thus starving the flame of oxygen.

Water additive fire extinguishers

A water additive fire extinguisher is essentially an ‘improved’ version of the classic water fire extinguisher outline above. A water additive improves the fire rating of water and reduces the size of the extinguisher unit without compromising on power. A three little water additive fire extinguisher is the equivalent of a thirteen-litre water fire extinguisher.

The water additive fire extinguisher is suitable on Class A organic fires. An additive removes water’s surface tension thus making the water more ‘wet’. This improves the water’s ‘stickiness’ properties. Water with this additive is better soaked up by the burning matter. In turn this improves the waters ability to put out fire.

Powder fire extinguishers

Powder fire extinguishers are suitable for tackling Class A and B fires as defined above.

Powder fire extinguishers are distinguished by a red cylinder and dark blue label as illustrated below:

This variety of extinguisher utilises a gas (usually nitrogen) to propel powder into the fire. The powder then forms a ‘blanket’ over the flames, starving the fire of oxygen. The powdered agent is typically made up of either sodium bicarbonate or ammonium phosphate.

Similar to carbon dioxide, the powder does not cool the burning material. Thus, a risk of re-ignition exists. Also, corrosive phosphoric acid is formed when powder mixes with water. Therefore powder will damage otherwise repairable machinery such as computer equipment.

Metal powder fire extinguishers

Metal powder fire extinguishers are suitable for use on Class D fires (metallic). Flammable metals include magnesium, potassium, lithium and sodium. These metals are common in industry. Metal powder fire extinguishers are classed as a form of foam fire extinguisher. The active agent contained within this style of extinguisher varies according to the flammable metal it seeks to cool. The active agent is commonly sodium chloride. The powder works by forming a foam carpet over flames thus cutting off the fire’s source of oxygen.

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