Knowing that fire extinguishers are different is something we’re often told at health and safety appraisals or other workplace seminars, but very often we aren’t told exactly what are the differences between the varieties of an extinguisher.
Depending on the workplace or home environment it’s important to know what fire extinguisher will work best on any particular fire. Doing so can often be the difference between life and death.
1. Water Extinguisher
It shouldn’t be a surprise that water remains the best substance for putting out fires. The majority of fire extinguishers contain water and these are the best for extinguishing Class A fires (organic materials such as paper, wood, and fabrics). The one area where they should not be used is anywhere near electricity as water acts as a conductor and you can run the great risk of electrocution.
There are three types of water extinguisher:
Pump extinguishers are best used in places with low temperatures, such as unheated warehouses or barns where there is a possibility of the water icing up. As the name suggests the water is manually pumped out like a bicycle pump.
Air-pressurised water (APW) extinguishers use air-pressure to push the water out and are probably the most common fire extinguishers.
Water mist (WM) extinguishers use a nozzle to turn the water into a fine spray, which is useful in places where magnets and electricity are found and are most commonly found in hospitals where they don’t affect things like MRI scanners.
Additionally, most water extinguishers contain additives that can help break the surface tension of water and antifreeze chemicals and alkali metal salt solutions that reduce the freezing point of water. The latter is also very corrosive so it’s not recommended for all fires.
All water extinguishers are red, with red panel above the operating instructions.
2. Dry Powder
A Class D fire takes place in combustible metals and dry powders might be preferable as water will be insufficient to cool the extreme heat produced by burning metal. There are a number of dry powders that can be used to fight fires, all of which cool the metal and smother the fire by cutting off the main fuel of a fire – oxygen.
Dry powders included sodium chloride (salt) which is used on most metal-based fires; copper powder, which is used on lithium fires; dry graphite which, like copper powder, is best used on very hot metal fires such as lithium as well as other metals; and sodium carbonate is best used on stainless steel that could otherwise be destroyed by the more corrosive sodium chloride.
Dry powder extinguishers are red with a blue panel above the operating instructions.
Foam is best for fires that involve Class B or Class F fires, which involve petrol, oil or other combustible liquids, where water is not very effective. Like dry powder, foam starves a fire of oxygen.
The most common foam extinguisher is aqueous film-forming foam (AFFF), which is often used in kitchens, airports and at motorsports, where fuel ignition is likely to be a factor. Other foam extinguishers include alcohol-resistant aqueous film-forming foams (which gives us the risible acronym AR-AFFF). As the name suggests these are used on fires that involve alcohol-based liquids. Also, compressed air foam system (CAFS) is most commonly used in remote places that don’t have easy access to water.
Foam extinguishers are red with a cream panel with red text above the operating instructions.
4. Carbon Dioxide
More commonly referred to as clean agents because of their environmental friendliness, carbon dioxide stops fires by displacing any oxygen and, because it’s so cold, can reduce any immediate heat. It’s not good on standard household fires involving furniture materials, metal or fuels but is most commonly used if a person is on fire although it can cause frostbite and suffocation.
Carbon dioxide extinguishers are red with a black panel above the operating instructions.
5. Wet Chemical
Another extinguisher that is mostly used to tackle burning oil and petrol, wet chemical extinguishers usually contain either potassium acetate, potassium carbonate or potassium citrate. They extinguish fires by forming a soapy foam blanket that keeps out oxygen.
Wet chemical extinguishers are red with a canary yellow panel and black text above the operating instructions.
For home use, a standard water extinguisher is most likely to be used, except in the case of deep fat fires, but in a workplace environment where there could be hazardous materials, it’s important to know which extinguisher is which.