Ask the average person in the street what a fire extinguisher contains and the odds are water extinguishers will come top of the list. The problem is that although water has been used to extinguish fires for millennia the types of fire a water extinguisher can tackle are very limited particularly in a home or office environment. These limitations are not widely appreciated by the general public who mistakenly see water as a catch all solution for fire fighting.
In practice water extinguishers should only be used for tackling fires involving common combustible materials like wood, paper and similar solids collectively termed Class A fuels by fire safety professionals. However there are some fairly common solids specifically metals like magnesium, aluminium and sodium that water will not extinguish effectively although fortunately such fires are rarely encountered outside specialist manufacturing facilities.
More common are fires involving flammable liquids like petrol and oil. Using a water fire extinguisher on such fires is a useless endeavor and potentially dangerous. Burning liquids will simply float on the water, as the water runs off it carries the fuel with it spreading the flames further. Similarly water fire extinguishers have no effect on burning flammable gases like butane propane and natural gas as the water molecules are too large to displace the air borne oxygen supporting the combustion process. It is for this reason that fires on oil rigs are so difficult to extinguish even though they are literally surrounded by water. At best applying water will merely prevent fire spread in the solid structure.
Also common in the home and at work are fires in electrical equipment from televisions and cookers to computers and photocopiers. Once again using a water fire extinguisher has some major drawbacks. Firstly if it is not possible to turn off the power, or more likely, if in the heat of the moment (pardon the pun) you forget to do so, then selecting the red labeled water extinguisher from the fire point is to put your life at risk. The water will quite probable suppress or even extinguish the fire but it is also a great conductor of electricity. The continuous jet of water between the extinguisher in your hands and the target fire connects you to the electricity coursing through the equipment. End result electrocution!
Even if you have the presence of mind to switch off the power applying water to electrical equipment you will further damage circuitry and electronics and may contaminate other equipment like your digital box or a printer not directly involved in the fire making the chances of repair minimal and the asset losses greater.
You should also be wary of using water extinguishers in the kitchen environment. Spraying a burning cooking pan with a high pressure jet of water is a recipe for disaster. For one the water jet will likely splatter and spray the burning fats around the kitchen spreading the fire. Secondly however hard you try the water will not extinguish the fire as like petrol the fat will just float on top, the pan will overflow further and the fire will spread.
When used for common Class A type fires water outshines any other type of extinguisher for economy and efficiency but knowing its limitations will ensure the safety of you, your family and work colleagues.