Which extinguisher classes have no numerical value assigned?

Class C and D fire extinguishers are indeed the correct answer as these classes do not have numerical values assigned to them. Class C extinguishers are specifically designed for electrical fires, and these types of fires do not have a numerical rating because they can vary greatly in their intensity and behavior based on the electrical equipment involved.

Class D extinguishers are designed to combat fires involving combustible metals, such as magnesium or sodium, and their ratings also do not use numerical values due to the specialized nature of the fires they are designed to extinguish. These materials behave very differently from standard combustible materials, which makes a numerical system less applicable or useful.

In contrast, Classes A and B have numerical ratings based on how many square feet of fire they can effectively extinguish. This measurement provides a clear indicator of their effectiveness in combating common types of fires, like those involving ordinary combustibles (Class A) or flammable liquids (Class B).

Therefore, the absence of numerical values in Class C and D indicates the specialized nature of their applications and the diverse characteristics of the fires they are intended to suppress. This distinction is important for understanding the capabilities and appropriate usage of fire extinguishers in different fire scenarios.