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How Fire Extinguishers Work

A fire extinguisher works to put out a fire because it removes one or more of the 3 essential elements of fire.

Foam and Water extinguisherTo maintain a fire you need HEAT, OXYGEN and FUEL.
Remove any one of these, and the fire will go out.

So for example a CO2 extingusher will put out a fire because it displaces the oxygen. An air-water extinguisher works because it cools the fuel, ie no heat. Dry chemical fire extinguishers and foam extinguishers work by separating the fuel from the oxygen. They form a film that covers the fuel and stops access to the oxygen. The extinguisher usually only provides a temporary solution, it is still important to ensure the rogue heat source is completely removed or cooled to prevent re-ignition.

Why are there different types of extinguishers?
Because there is more than one way to put out a fire, and because there are different fuels, and even different types of heat, that need different approaches to ensure the safety of the person trying to put out the fire. If you use the wrong type of extinguisher on a fire, you could make it worse, and/or put yourself in danger.

carbon dioxide sign Consider electrical fires for example. The most important thing to do to stop one, is to turn off the electricity, thereby removing the heat source. This is not always easy to do, as the power switch is often too close to the item on fire. If you try to put it out with a water, or foam extinguisher, you run the risk of getting electrocuted because water could conduct the current to the user. The ideal fire extinguisher to use is a CO2 extinguisher. Not only can it get in around difficult-to-reach objects, but it also leaves no residue. However once the CO2 clears, the fire will re-ignite unless the cause of the fire has been fixed.
AFFF id sign Air Water ID sign
Water or AFFF (Foam) extinguishers must not be used on deep seated cooking oil/fat fires, when the water hits the hot oil it will convert to steam, with a possible violent ejection of burning fuel from the container. The blast from a CO2 extinguisher on a fat, or a petroleum fire could cause hot burning fuel to be spread widely.
Wet chemical
A wet chemical fire extinguisher is ideal for oil and fat fires because it sprays a mist over the oil which chemically reacts to form "soap", thus providing a barrier between the hot fat and oxygen. As soon as the flames stop you must turn off the heat. Don't move the oil/fat until it has cooled- if the soap layer is disturbed it may re-ignite.
ABE sign
Dry chemical or ABE extinguishers work on the widest variety of fires by spraying a powder over the fuel. They go by various names; DCP- dry chemical powder, and ABE- referring to the fire types it can be used upon. The powder is derived from bird droppings (!) and works because the heat of the fire causes it to melt and form a film/layer over the fuel, cutting off its access to oxygen.

Fire extinguishers are rated for their ability to put out different type of fires. Read our article to learn more about fire extinguisher ratings.

What size of fire extinguisher do I need?

CO2 fire extinguisherThis is governed by the fire extinguisher Australian Standards AS2444. In general the number, size and type of extinguishers you need is determined by the area to be protected and the type of fire risk. Other considerations are barriers that prevent access to the extinguisher eg internal walls, machinery, pallets or storage shelves. The presence of a fire sprinkler system will also change the requirement. So the same building could require different extinguishers depending on how its used, and what is stored there. For this reason Councils require a Fire Safety compliance certificate whenever a new DA is requested.

dry chemical DCPThe size of a fire extinguisher is often referenced by its weight eg a 4.5kg extinguisher. However, the weight of a fire extinguisher does not necessarily describe its fire-fighting capacity. Different brands of fire extinguishers, vary in their ability to put out fires. When comparing prices and value for money look at the ABE rating, not just the weight. For example at the home fire extinguisher level, a 1kg fire extinguisher could vary wildly in its ability to put out a fire: a cheap one might have a 1A:10B:E rating, whereas our 1kg ABE extinguisher has a rating of 1A:20B:E. That's twice the ability to put out a B type fire.

Have a look at our article on choosing a fire extinguisher

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How to use a fire extinguisher

Signs above the extinguisher tell you what sort of fire it can be used on.

  • Always test the extinguisher before proceeding to the fire.
  • Ensure you have an escape route.
  • Most fire extinguishers only discharge for seconds, not minutes, so don't tackle a fire unless it is safe to do so and you are trained.
  • The rule of thumb is if you can not put a fire out with one extinguisher, then the fire is too big to fight - call emergency 000

To use a fire extinguisher, follow the PASS rule - acronym for Pull, Aim, Squeeze, Sweep

  • Pull the pin,
  • Aim the extinguisher at the base of the fire,
  • Squeeze the handle,
  • Sweep it back and forth.

For a more comprehensive article on how to use a fire extinguisher go to the NSW Fire Brigade

Pressure testing and recharge
If a fire extinguisher has been used
or discharged it should be replaced or refilled, even if it was not emptied. This is a requirement of the Australian Standard, but it also makes common sense. In particular if you use a dry chemical powder extinguisher, the powder residue left in the valve could lead to a slow leak of pressure, meaning it will be useless next time you try to use it.

Further reading


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