Final answer:
Dry chemical extinguishers are used on Class C fires because they do not conduct electricity, which is safe for electrical fires. Historically harmful substances like carbon tetrachloride have been replaced by safer, non-conductive chemicals in extinguishers.
Step-by-step explanation:
Dry chemical extinguishers can be used on Class C fires because dry chemicals do not conduct electricity. This trait is critical because Class C fires involve electrical equipment, and using a conducting agent like water can increase the risk of electric shock. Instead, dry chemical extinguishers often contain compounds such as monoammonium phosphate or sodium bicarbonate which smother the fire and interrupt the chemical reaction of the fire, without completing an electrical circuit.
Historically, substances like carbon tetrachloride were used in fire extinguishers for electrical fires, but they are no longer recommended due to the production of toxic gases such as phosgene when they decompose under heat. Today, more environmentally friendly and safer alternatives are sought, including various foaming agents that can spread across surfaces to extinguish fires effectively. For instance, fluorinated solutions are commonly used in modern fire-fighting foams for large-scale fires of organic liquids.