Which type of extinguisher is typically not effective on Class B fires?

Water fire extinguishers are typically not effective on Class B fires, which involve flammable liquids such as gasoline, oil, paint, and solvents. When water is used on Class B fires, it can actually spread the burning liquid, making the situation worse rather than extinguishing the fire. Water is effective on Class A fires (ordinary combustibles like wood and paper) due to its cooling properties, but it can lack the necessary characteristics to address the unique challenges posed by flammable liquids.

In contrast, foam, dry chemical, and carbon dioxide extinguishers are specifically designed to smother and suppress Class B fires. Foam extinguishers create a barrier between the fuel and the oxygen, making them effective for certain flammable liquids. Dry chemical extinguishers interrupt the chemical reaction of the fire, while carbon dioxide extinguishers displace oxygen, effectively extinguishing flammable liquid fires. Thus, the use of water on Class B fires poses a significant safety risk, underscoring why it is not a suitable option for tackling these types of fires.