Dry powder fire extinguishers are great because they do not freeze outdoors in low temperatures which is especially important in the winter months that we experience in the UK.
When you compare the fire ratings to other extinguishers it is quickly apparent that powder extinguishers have a higher fire fighting performance than both water and foam extinguishers.
They can also be used to fight fires involving electrical appliances and flammable gasses which makes powder extinguishers extremely versatile.
Setting off a dry powder extinguisher indoors will create a lot of mess which can take considerable time to clean up. This is an important consideration when food items may be in close proximity such as in a butchers shop or a stall at a country market. Setting off a powder extinguisher in this scenario would likely lead to contamination of the food and require it to be disposed of. If used outside in a market scenario there is a real possibility of wind causing contamination of other stall holders food items downwind causing even more of a headache and likely claim on insurance.
Visibility is another issue. Discharging a dry powder extinguisher indoors can create a large powder cloud that can obscure escape routes and fire exits. This is a particular problem where occupants of the building may not be familiar with the layout such as in a shop or public building that they are visiting for the first time.
Breathing problems is the third disadvantage of using dry powder extinguishers indoors. Discharging a powder extinguisher indoors may cause breathing problems in people with chronic respiratory problems such as asthma and bronchitis. This may pose a safety hazard as there is the potential to cause a medical emergency which compromises the safe evacuation of the building in the event of a fire.
BS5306 Part 8 2012 acknowledges the problems associated with the use of dry powder extinguishers in buildings and states that they should not generally be specified for use indoors unless a health and safety risk assessment deems this appropriate.
In practice this means that for most buildings dry powder extinguishers are unlikely to be appropriate for use indoors. Exceptions may be workshops and garages where fire risks include running fuel fires or flammable gasses.