Dry powder covers more fire classes than anything else in the range, which is why it is sold as the general purpose answer and why it turns up in so many vans and garages. It is also the extinguisher most fire safety guidance discourages inside an occupied building, for reasons that have nothing to do with whether it puts the fire out.
What it covers
General purpose ABC powder is rated for class A solids, class B flammable liquids and class C flammable gases, and is generally safe to use near live electrical equipment. That breadth is real and it is why powder is the standard choice for vehicles, forecourts, plant and external locations where one extinguisher has to answer several risks at once.
Why it is discouraged indoors
A discharged powder extinguisher fills the room with a dense cloud that destroys visibility instantly, which matters when people are trying to leave, and it is an inhalation irritant. The residue is corrosive to electronics and gets into everything, so the clean up after a small fire can cost more than the fire did. Guidance for occupied premises generally steers towards water, foam and carbon dioxide indoors and reserves powder for outdoor and vehicle use.
Class C and gas fires
Powder is the only common portable agent rated for flammable gases, but putting out a gas fire without stopping the gas supply creates an explosive atmosphere that will find an ignition source. The rule is to isolate the supply first, and only then deal with fire. An extinguisher rated class C is not permission to extinguish a gas fire and walk away.
Metal fires are different powder
Class D metal fires need a specialist powder designed for the specific metal, applied gently with an applicator rather than jetted. General purpose ABC powder is not class D rated and using it on burning metal can make matters considerably worse. If a site holds magnesium, sodium or lithium metal, the class D provision is a separate decision from the general extinguishers.
Questions people ask about dry powder fire extinguishers
What colour is a dry powder extinguisher?
Blue. A red body with a blue band or panel identifies dry powder contents.
Why is powder not recommended indoors?
A discharge destroys visibility in the room people are trying to leave, irritates the airway, and leaves a corrosive residue that is expensive to clean up. Water, foam and carbon dioxide are preferred inside occupied premises.
Is ABC powder suitable for metal fires?
No. Class D metal fires need a specialist powder and an applicator. General purpose ABC powder is not rated for them and can make a metal fire worse.