They are not the most effective of types for any class of fire. Pound-for-pound most foam units beat the powder on both class-A and B fires. Powder neither cools nor blankets a fire. A class-A fire put out with a powder extinguisher will stay hot, can smolder, and may reignite.
Plain water is often better.
Driving down the highway one day, I encountered a vehicle with a flaming tire. At the time, I had a dry powder extinguisher in my vehicle, so I stopped to help. Fortunately there was also a highway survey crew nearby, and they also responded, so we had about four or five dry powder units available to extinguish the fire.
We would empty an extinguisher, putting out the flames. After about fifteen seconds, the tire would flare up again. After repeating this cycle a few times, we finally realized that the wheel was extremely hot (due to a dragging brake shoe) and was the source of reignition. A bit of water applied to the wheel finally calmed things down and got the fire out.
The vehicle owner at that point opened his trunk, whereupon we saw a three gallon can of gas, uncapped. We were lucky that day.......
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