Class D hazards vary widely in form and difficulty of extinguishment. Each combustible metal is different; each form that it is in presents different challenges (castings vs. dry turnings vs. wet turnings vs. powder vs. chips vs. shot vs. etc..) so that a repeatable, consistent way of rating these extinguishers according to fire fighting effectiveness has been impossible.
Class D hazards are completely different than any other Class of Fire. Agents that work on Class A, B, C and even K have no effect on Class D fires and may actually have an adverse effect. Conversely, agents that are effective on Class D fires have no effect on any other class of fire.
When dealing with Class D fires, it may take 8-15 lbs. of agent to extinguish 1 lb. of Class D burning material (Depending on the type of metal that is burning). A 15:1 ratio of extinguishing agent to burning material is unique to metal combustible fires and when coupled with the physical characteristics (powder, dust, pellets, and shavings) makes it extremely difficult for UL to run exacting tests. The result is no numerical rating. This in turn leads to Target 570 #5! (Refer to our label on the back of the Model 570 and 571 that lists approximate quantities and sizes of hazards by square feet and pounds of material.)
P.S. If you or your customers are interested in UL’s fire test procedures for Class D extinguishers and agents, they can be found in UL/ANSI Standard 711.