Which class of extinguishers is never assigned a rating of Class D?

In the context of fire extinguishers, Class D extinguishers are specifically designed for use on combustible metals such as magnesium, titanium, and sodium. They utilize dry powder agents to smother or chemically interrupt the combustion process, which is crucial when dealing with fires involving these materials.

Multi-purpose extinguishers are rated for Classes A, B, and C fires, meaning they can be used on ordinary combustibles (such as wood and paper), flammable liquids, and electrical fires. However, they do not have a Class D rating because their agents are not suitable for metals.

On the other hand, Class A extinguishers are rated for ordinary combustibles, Class B extinguishers for flammable liquids, and Class C extinguishers for electrical fires. Each of these classes addresses specific fire hazards but does not meet the requirements for Class D fires, leading to their classification.

Therefore, the distinguishing characteristic of multi-purpose extinguishers being not assigned a Class D rating aligns with the specific focus needed for handling combustible metal fires, for which dedicated Class D extinguishers are necessary.