What type of fire extinguisher must not be used on electrical fires?

The reasoning behind why a water fire extinguisher must not be used on electrical fires is rooted in the properties of water and the nature of electrical fires. Water is a conductor of electricity, meaning that if it is used on an electrical fire, it can create a significant risk of electrocution for the person using the extinguisher. Additionally, applying water can also spread the fire, especially if it results in electrical equipment becoming further energized or if conductive pathways are created.

In contrast, other types of extinguishers, such as foam, powder, or CO2, are specifically designed to tackle fires without posing such risks. For example, CO2 extinguishers and dry powder extinguishers can effectively suffocate or interrupt the combustion reaction of electrical fires while being non-conductive and safe for use around energized equipment. Thus, using a water fire extinguisher in this context is highly dangerous and should always be avoided.