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Fire Engineering: Class of Fire and Fire Extinguishers

Class of Fires:

As explained in another article, a fire can be extinguished by breaking the fire triangle (smothering, starving or cooling), and fire extinguishers are developed on these principles.

To understand fire extinguishment principles, we need first to understand the different classes of fire. These are based on the fuel type and defined as follows:

· Class A – Carbon-based combustible materials (e.g. wood, rubber, paper, fabric, plastics)

· Class B – Flammable and Combustible Liquids (e.g. petrol, oil, thinners)

· Class C – Flammable Gases (e.g. acetylene, propane, LPG, Butane)

· Class D – Flammable Metals (e.g. Sodium, potassium, magnesium)

· Class E –Energized Electrical Equipment (e.g. electrical cable, electric motor)

· Class F – Cooking Oils and Fats


Fire Extinguisher types:

Although every fire will be extinguished by breaking the fire triangle, the nature of the fire varies according to the fuel type, and the best way to extinguish the fire varies also.

Each fire extinguisher type acts in different ways and may not be ideal for some scenarios or classes of fire. In the list below we discuss the different types of extinguisher and how they work and also which fire types or scenarios they are suitable for, and in some cases, should not be used for.

Dry Chemical Powder (DCP)

  • Inhibition of chain reaction: Fine particles of DCP trap the free radicals, inhibiting the chain reaction.

  • Smothering: fine particles of powder form a cloud over the fire and thus displace the oxygen present in the vicinity of fire to effect smothering.

  • Cooling: This powder also absorbs heat while changing its state, and some cooling takes place.

  • For various types of metal fires, there are special powders available.

Carbon Dioxide (CO2)

  • Smothering: replaces air in the vicinity of the fire and forms an inert atmosphere.

  • Cooling: caused by sudden expansion of CO2, but this effect is negligible.

  • Active protection systems using CO2 are mainly used to protect electrical hazards such as generating & distributing plants, and high energy switch gears.

  • Suitable only for small fires.

  • Generally not suitable for outdoor fires.


  • Cooling: Excellent cooling ability as it absorbs 540 cal/gm of heat as ‘Latent Heat of vaporisation' from the fire.

  • Smothering: When water vapourises into steam, it expands 1600 times and the large amounts of steam formed displaces the oxygen in the vicinity of the fire, having a smothering effect.

  • Emulsification: emulsifies with various fuels, rendering them non-combustible

  • May be used on water-soluble fuels.

  • Many fire protection systems are based on water as the extinguishing media, such as sprinkler systems, and hydrant systems.

  • Dangerous if used on flammable liquids, live electrical equipment or cooking oils/fats.


  • Smothering: Foam spreads over the burning liquid surface, forming a blanket and prevents the access of air(oxygen) to the fuel.

  • Cooling: the water drained from the foam causes cooling,

  • Fixed Foam Protection systems: foam pourer/chambers/extinguishers are typically employed to store and handle flammable Liquids.

  • Dangerous if used on electrical fires.

Wet Chemical

  • Cooling: the mist cools the fire and lowers the temperature to stop the fire spreading, it also prevents splashing of hot oils/fat.

  • Smothering: the potassium salt reacts with the hot oil and causes saponification (creates a soapy foam). This non-combustible soapy foam coats the cooking oil’s or fat’s surface acting as a barrier between the fat/oil and the air (the oxygen source).

  • Dangerous if used on electrical fires.

Vaporising Liquid (a.k.a Halons - Halogenated hydrocarbon)

  • Interrupts the chain reaction: Due to the high temperatures of fire, Halons dissociate and thus ‘ions’ are formed, combining with active free radicals (chain carriers) of chain reaction to form complexes.

  • This is the fastest and cleanest of all extinguishing agents; however, CFC is being phased out of conventional usage.

  • Halon’s use is still common in Military & Space installations mainly for explosion suppression systems.


Fire Extinguisher Use

Around 80% of all fires in commercial properties get extinguished in the early stages by members of the public*.

The use of fire extinguishers is recommended to ensure the safety of building occupants and employees, especially for places where there is a higher risk of fire, such as:

  • cooking areas

  • certain manufacturing processes

  • welding

  • hot work.

Where extinguishers are installed in a building, there should be some occupants trained in their use (e.g. fire wardens or members of staff).

*Ghosh, B (2008) Assessment of the benefits of Fire Extinguishers as fire safety precautions in New Zealand buildings. University of Canterbury

How to Select the Right Fire Extinguisher?

In New Zealand and Australia, fire extinguishers have a coloured band at the top of the cylinder for easy recognition. Because there are no universal extinguishing agents available, it’s essential to know the different fire extinguisher types and to select the correct type for the class of fire which may exist in the area.

The table below lists the different types of fire extinguisher and their uses however the following should also be taken into consideration:

· Depending on the local requirements, the same fire extinguisher types will have different recommendations. It is always good to follow the manufacturer/supplier’s instructions.

· Class D fires use only special-purpose fire extinguishers - seek expert advice.

When to Use a Fire Extinguisher?

Remember only to use a fire extinguisher when:

  • It is safe to do so, considering the location and size of the fire

- Note: Once started, your extinguisher will only last 10-15 seconds.

  • You are confident that you understand how to use the extinguisher correctly.

  • Everyone has been evacuated at your safe meeting place.

  • Emergency services have been called.

  • You can safely access and if necessary retreat from the fire.

  • Remember, life is more important than property.

- Don’t put yourself or others at risk.

How to Use a Fire Extinguisher?

Use the ‘PTASS’ technique when operating a fire extinguisher:

  • Pull the safety pin (or remove the clip).

  • Test the extinguisher with a quick squirt to make sure it is working.

  • Aim at the base of the fire with the nozzle. Remember to maintain a safe distance.

  • - Most extinguishers are designed to be operated from 2-3 metres away.

  • Squeeze the handles.

  • Sweep the extinguisher from side to side while aiming at the base of the fire.

When to Use a Fire Blanket?

Fire blankets are usually used for class F fires.

Kitchens are the most common place where there is a high chance of a fire breaking out with simple accidents involving burning fat oils. Also, offices, garages and vehicles can benefit from a readily available fire blanket.

Fire blankets can also be used on clothing. If the clothing somebody is wearing catches fire, you can wrap the blanket around the person extinguishing the fire by smothering the flames. It can even be used around yourself if your clothing catches fire.

Note that fire blankets have limitations, and the effectiveness of a blanket against a cooking oil or fat fire depends on obtaining a good physical seal between the blanket and the rim of the container. Suppose any physical constraints of the user or the container’s shape or size make this unlikely or impossible. In that case, the effectiveness of the blanket will be significantly reduced. Also, smaller sizes of blankets may not be satisfactory for clothing fires on adults.

Correct use of fire blankets can save not only valuables but also someone’s life.

Note that fire blankets single use only, so make sure you dispose of and replace your fire blanket once it has been used.

How to use a Fire Blanket on a Burning Oil Pan?

  • It’s important to remember that a fire blanket is only useful in cases of minor fire.

  • Turn off all electricity/gas supply if safe to do so (e.g. the gas on the stove). NEVER move a container with burning liquid.

  • Pull the tapes to remove the fire blanket from its container

  • Hold the blanket in a shield position, and if possible, wrap the top edges of the blanket around your hands. Be careful, and do not let your clothing catch fire.

  • Gently place the blanket down over the pan/container to smother the fire. Do NOT try throwing it over the fire as a sudden rush of air could spread the flames. Ensure that no oxygen is reaching the fire completely cover the fire with the fire blanket.

  • Leave the pan to cool completely (around half an hour), and don’t touch the fire blanket until it is completely cool.

  • If the oil pan fire is too big or you’re unable to extinguish the fire with the blanket, immediately evacuate the area and call emergency services.

How to use a Fire Blanket on a Burning Person?

  • Pull the tapes to release the blanket from its container

  • Hold the blanket in a shield position, and if possible, wrap the top edges of the blanket around your hands. Be careful, and do not let your clothing catch fire.

  • Wrap the fire blanket around the person who is on fire to smother the flames


At Nelligan, we have the expertise to help with the specification and detailing of Fire safety systems on your Fire Safety Design, including Fire Engineering solutions and Passive Fire Protection systems.


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