Which types of extinguishers do not have a numerical rating?

The correct answer focuses on the types of fire extinguishers that do not feature a numerical rating, specifically Class C and Class D.

Class C extinguishers are designed for fires that involve electrical equipment, and their effectiveness is not quantified by a numerical rating, as it is assumed that they are effective for such hazards due to their nature of being non-conductive rather than based on a specific capacity to combat a certain volume or area of fire.

Class D extinguishers are used for fires involving combustible metals, such as magnesium or sodium. These extinguishers are often rated based on their effectiveness against specific classes of materials rather than a numerical designation, as the behavior of combustible metals requires different extinguishing agents and techniques.

In comparison, Class A extinguishers, which are intended for ordinary combustibles like wood and paper, and Class B extinguishers, which are for flammable liquids, do have numerical ratings that indicate the amount of water needed to extinguish a specific type of fire. With this context, it becomes clear why Class C and Class D are categorized without numerical ratings.