A dry chemical tri-class extinguisher should NOT be used on which type of fires?

A dry chemical tri-class extinguisher is typically effective on Class A, B, and C fires. Class A fires involve ordinary combustibles such as wood, paper, and textiles. Class B fires involve flammable liquids, while Class C fires pertain to energized electrical equipment.

However, dry chemical extinguishers should not be used on Class D fires, which involve combustible metals like magnesium, titanium, and sodium. The reason for this is that dry chemical agents can react with metals and potentially cause explosions or worsen the fire. Firefighting substances designed for Class D fires, such as specific dry powders, are essential for safely controlling these types of flames. Hence, the tri-class extinguisher's designation is incomplete concerning Class D fires, which makes it ineffective and dangerous to use in those situations.