DO NOT USE water extinguishers on energized electrical equipment. Because of this, carbon dioxide or halon extinguishers are preferred in these instances because they leave very little residue. ABC dry powder residue is mildly corrosive to many metals. For example, residue left over from the use of an ABC dry powder extinguisher in the same room with a piano can seriously corrode piano wires. Carbon dioxide or halon extinguishers are provided for most labs and computer areas on campus.
All ratings are shows on the extinguisher faceplate.
These extinguishers are capable of putting out more than one class of fire. Class A and B extinguishers carry a numerical rating that indicates how large a fire an experienced person can safely put out with that extinguisher. Class C extinguishers have only a letter rating to indicate that the extinguishing agent will not conduct electrical current. Class C extinguishers must also carry a Class A or B rating. Class D extinguishers carry only a letter rating indicating their effectiveness on certain amounts of specific metals.