CO2 (Carbon Dioxide) fire extinguishers are the only fire extinguishers recommended for fires involving electrical equipment. CO2 is safe to use on and around electrical equipment, as the gas itself is non-conductive, and once used, there is no sticky foam or messy powder left behind. They are also effective on Class B fires (flammable liquids). Carbon dioxide fire extinguishers are painted bright red with a black panel above the operating instructions. They have a distinctive horn-shaped nozzle at the side on the smaller models with 5 kgs and above having a hose and horn. CO2 fire extinguishers should not be used on Class D fires, involving flammable metals. The CO2 gas can react with some volatile metals, and as with other non-D rated extinguishers, can also blow burning metal over a wide area, making the situation worse. CO2 extinguishers are the most popular with film units, due to the low secondary damage caused when used. If paired with a foam extinguisher, 4 out of 5 fire categories are covered.