Choosing an extinguisher

To choose an extinguisher, you need to decide what kinds of fire pose the biggest risk in your household. Clothing set alight by a heater? Workshop fires? Deep-fryer oil fires in the kitchen?

How they work

A fire is a chemical reaction between oxygen and a fuel. To sustain a fire, three things are necessary - the fuel, oxygen, and sufficient heat. So, to put out a fire, you need to remove (or substantially reduce) any one of those three. Fire extinguishers work by removing oxygen and, depending on the type of extinguisher, by cooling.

New Zealand Standard Code

Extinguishers that comply with the voluntary New Zealand Standard are marked with a code according to the kind of fire they are suitable for. Look for the following letters:

• A Wood, cloth or paper.
• B Flammable liquid (eg, petrol, diesel).
• C Flammable gas.
• D Flammable metals (not usually relevant for domestic use).
• E Electrical.
• F Burning cooking oil or fat.

Extinguishers may also display a number which relates to the size of fire that can be put out.

Types of extinguisher

• Dry-powder extinguishers often contain sodium bicarbonate (baking soda) or similar chemicals. When the powder is sprayed, it tries to smother the fire by excluding oxygen. As well the chemical breaks down at around 70°C and then releases carbon dioxide, which also excludes oxygen. Dry-powder extinguishers don't cool the fire.
  
  
• Foam extinguishers are water-based. The water is expelled with a foaming agent that increases its volume many times, and the foam then smothers the fire and removes oxygen. The heat of the fire also turns some of the water in the foam to steam - and this action cools the fire. But because foam extinguishers are water-based they are not recommended for use on electrical fires. They could give you an electric shock. (See below for more.)
  
  Foam extinguishers use expanded water droplets to help smother a fire. This raises the extremely remote possibility of a person using a foam extinguisher receiving an electric shock if the foam found its way into exposed electrical connections and at the same time the user was touching something metal or was standing in a puddle of water. As far as we know, the possibility is theoretical and no one has ever received an electric shock from using a foam extinguisher on a cooktop fire.
  
  In our view the efficiency of foam extinguishers far outweighs the theoretical risk of shock. Of course, you should never use a foam extinguisher on a switchboard or other electrical fire where the risk of electric shock becomes much more real.
  
  
• Carbon dioxide extinguishers stop the supply of oxygen to a fire. This type is multi-purpose and especially good for electrical equipment fires as there is no mess.
  
  But the carbon dioxide comes out with some force, which means on liquids or on fires in loose material the flames can spread.
  
  
• Fire blankets work by excluding oxygen from the fire. To do this, they must form a tight seal around the burning object and stop air from leaking in. That's difficult to achieve in many cooking-oil fires, because the handle of the pan prevents the blanket from sealing. The British consumer magazine Which? published the results of a test which showed that one or two wet tea-towels had an almost 80-percent success rate in putting out cooking-oil fires, compared with a 10-percent success rate for fire blankets.

Other things to consider

In addition to the type of extinguisher, you should also consider these features.

Size
The bigger the fire, the bigger the extinguisher must be. Household models in the 0.9 and 1.1kg range are generally rated 1A, which means they must be able to put out a fire of 50 pieces of 45mm x 45mm x 500mm dry pine, stacked in a crib arrangement, after at least eight minutes of burning. That's not a very big fire!

A canister this size should be fine if you are on hand to deal with a small fire early. But the small, spray can-style extinguishers are unlikely to be big enough.

If you want protection for a boat, or a rural property far from the local firefighters, we suggest you get professional advice on suitable-sized extinguishers.

Date
Don't buy an extinguisher with a manufacture date more than a year old, unless it is serviced before you buy.

Refilling
Most household-sized dry powder models are refillable. Service agents are listed under Fire Protection in the Yellow Pages.

All dry powder extinguishers should have an annual check on the pressure and be shaken to ensure the powder hasn't settled or gone lumpy.

Foam models cost around $20 to refill, and should also be checked annually.

Discharge pattern
Ideally, most extinguishers should discharge into a compact area 2 to 5 metres away, so you can target the seat of the fire without getting too close.