Class A fires are combustible solids, for example wood, textiles, paper, rubber or plastic. The finer the class A material, and therefore the greater surface area it has, the easier it is to ignite and the more rapidly it will burn. For example a solid lump of timber will take a significant effort to light on fire, but wood shavings can ignited readily. Taking it a step further, fine wood dust suspended in the air can be explosive.
When most people imagine a generic “fire” they are imagining a class A fire, whether it is a house fire, a bonfire, or a bushfire. The most common way to extinguish a class A fire in these contexts is by cooling the fuel until it is no longer able to combust, via applying water. Perhaps its lucky that class A fires are so common, as it is the only class of fire that can be safely extinguished with water.
Class A fires can also be extinguished by separating the fuel and allowing the ignited material to burn out in a safe location. An example of this would be a hay fire or a fire in loose waste, where machinery or hand tools could be used to separate the burning material from the rest of the fuel. This isn’t always practical and should only be considered where circumstances allow.
If you can remove the air, or oxygen source, from the class A fire it will go out. The most common way of doing this is by smothering it with either foam or a dry powder agent. In general, class A fires are the most straightforward to extinguish.
Solids are, generally speaking, a denser source of fuel than their liquid or gas counterparts. This generally results in what is known as incomplete combustion. Incomplete combustion produces the most dangerous fire by-products, such as carbon monoxide and dioxins.
This extra density often allows solids to retain heat much better than their liquid or gas counterparts, increasing the risk of re-ignition if the fire isn’t completely extinguished or oxygen is reintroduced.
Class B fires are flammable liquids, for example petrol, paints or oils. Flammable liquids are generally easier to ignite than their Class A counterparts. A major consideration with class B fires is where it is burning, a fire burning confined inside a drum is a vastly different situation to burning fuel rolling down a sloping driveway.
Liquid fuel fires are practically impossible to extinguish by cooling (many liquid fuels burn readily at room temperature) or by separating the burning fuel from the unburnt fuel. This leaves smothering as the most viable option. This is typically done with a foaming agent, although any mechanism that prevents the fuel accessing oxygen will work suitably well, for example by putting a cover on a drum of burning fuel.
If the fuel fire is uncontained applying a foaming agent may be insufficient as the fuel will continue to spread in area until it reaches an equilibrium, so factors such as the topography of the area need to be taken into account, it may be necessary to contain the fuel spill before extinguishing actions can be completely effective.
Liquids are, generally speaking, much easier to ignite than their solid counterparts, often not even requiring pre-heating. This means open flames, sparks or other ignition sources pose a greater risk of reignition and precautions need to be taken to ensure they do not reignite the fuel.
Liquid fuels are also able to run and spread, unlike solid fuels which can be assumed to remain relatively stationary. This means any change to the fuels environment or containment could rapidly spread the fire. Surface area is a key component of fire growth and heat release rate, and liquids are capable of rapidly expanding their surface area if they are allowed to break containment.
Class C fires are flammable gases, for example LPG, natural gas or propane. Flammable gases are extremely easy to ignite. Some gases, such as natural gas, are lighter than air and tend to rise and disperse, while others such as LPG are heavier than air and tend to pool on the ground.
By far the safest way to extinguish a gas fire is to starve it by shutting down the gas supply. Simply extinguishing the flame while allowing gas to continue to leak will result in gas filling the area and pose a much greater risk of explosion than had nothing at all been done. Sometimes circumstances might dictate that the flame needs to be extinguished in order to shut down the gas supply, but that is a decision and operation best left to firefighting professionals.
The greatest risk a gas fire poses is the chance of an explosion if the gases are allowed to accumulate. As such, shutting down the gas supply and isolating, removing or shutting down ignition sources is the number one priority in a gas fire.
The nature of flammable gases makes it hard to identify the location and concentration of the fuel, so everyone should be evacuated to a safe distance and not return until the fire department and utility providers have had time to control and assess the situation.
Class D fires are flammable metals, for example magnesium, titanium and lithium. Different flammable metals have different characteristics so there is no universal approach or guide to extinguishing them.
Class D fires generally require specific extinguishing agents or techniques to effectively extinguish them, for example magnesium, sodium and potassium fires can be extinguished with sodium chloride based dry powder, while lithium fires can be extinguished with copper based agents. If you have a significant quantity of flammable metals that pose a risk in your environment you should seek the advice of a competent fire safety practitioner to find out what the most effective agent for your needs will be.
If fuel-specific extinguishing options are not available and the burning material lends itself to it the safest method might be to separate the burning metal from other fuels and allow it to burn itself out, for example a lithium battery might be able to be shifted away and allowed to burn in an open air environment.
While flammable metals often require significant heat to ignite, once on fire they can be extremely energetic and have unique properties that make extinguishing them a challenge. For example, magnesium fires will split water into its component hydrogen and oxygen, providing more fuel and oxidiser for the fire. To learn what properties your specific flammable metals have you should consult the materials SDS.
Class E fires are fires that have an electrically energised component to them, in this sense they aren’t truly a fuel class but rather a modifier to another class of fire. If you are able to remove the electrical factor from consideration, by unplugging the appliance or shutting down the sites power, it becomes whatever class of fire it would otherwise be.
Water and water based agents, such as foam or wet chemical, are not safe to use on fires with an electrical component. Agents such as dry powder or CO2 are safe for use with electricity and should be the first choice for handling a class E fire. If you are able to turn the electricity off you can deal with the fire according to the fire class that remains.
Shock risk is the prime concern with class E fires, and can be avoided by not using an extinguishing agent that is conductive. A lesser talked about risk however is that of reignition. If the source of electricity is still powered it could be generating heat or spark that could reignite the fuel. This is of particular concern if you extinguished the fire with a CO2 extinguisher, as the CO2 will dissipate and allow reignition.
Class F fires are fires that involve cooking oils or fats, and while technically a flammable liquid they are distinct from class B fires due to the higher flash point and auto ignition temperature of cooking fats. This distinction has a significant effect on the fires properties and is why cooking oils and fats have their own fire class.
Much like their class B relatives, class F fires are generally impractical to extinguish via cooling or separation of fuels and smothering is once again the mechanism of choice. Specialised fire extinguishers, called wet chemical fire extinguishers, exist for dealing with the unique properties of class F fires.
These extinguishers work by forming a soapy blanket over the surface of the burning oil and preventing the fire accessing the oxygen it needs to continue burning. If the fire is small enough, like in the case of a cooking pot, the same effect can be attained by covering it with a pot lid or fire blanket. Remember to turn off the source of heat to prevent reignition.
Unlike class B fires foam is ineffective on class F fires, this is due to the higher temperatures and the fat itself breaking down the foam, leading to possible reignition. Using water on a class F fire can cause an explosion that will spray burning oil, spreading the fire and burning anyone unlucky enough to be nearby.
Generally class F fires will occur inside a confined environment like a pot or deep fryer so burning liquid flow is less likely to be an issue than with a typical class B fire. This changes if someone decides to relocate the pot of burning oil. In the heat of the moment it can often seem to be a good idea to move the fire outside of the kitchen, however this increases the risk of spillage. Instead you should try to either use the lid of the pot, a fire blanket, or a wet chemical fire extinguisher to put out the flames. Once it is cool it can be safely relocated.