Carbon dioxide is the extinguisher for live electrical equipment and for flammable liquids where residue would be a disaster. It is black banded, it leaves nothing behind, and it carries two specific hazards that ordinary extinguishers do not: the discharge horn gets extremely cold, and the gas displaces oxygen in an enclosed space.
How it works
Carbon dioxide is stored as a liquid under pressure and discharges as a gas and a cloud of dry ice particles. It smothers the fire by displacing oxygen at the seat of it, with some cooling from the expansion. Because it is a gas it leaves no residue at all, which is why it is the standard choice around electrical equipment, server racks, laboratory apparatus and anything a powder discharge would destroy.
What it is for
Live electrical equipment, and class B flammable liquid fires. It is not rated for class A: it will knock the flame off a paper fire and the fire will re establish from the embers, because there is no cooling to speak of and nothing left in place afterwards. That is why a building with a carbon dioxide extinguisher beside the electrical panel also needs water or foam for ordinary combustibles.
The two hazards
The horn reaches temperatures far below freezing during discharge, so a non frost free horn must be held only by its handle; cold burns from grabbing the horn are a common injury. And in a small enclosed room the gas displaces breathable air. Ventilate after use, do not use one in a confined space if you can avoid it, and leave the room once the fire is out.
Sizes and where they go
Carbon dioxide extinguishers are heavier than their capacity suggests because the cylinder holds a liquefied gas under high pressure. Siting them by the electrical intake, the server room and the plant room rather than on the general escape route is normal, because they answer a specific hazard rather than a general one and somebody grabbing the nearest extinguisher for a bin fire should not find this one.
Questions people ask about co2 fire extinguisher used for
What colour is a CO2 fire extinguisher?
Black. The body is red like all extinguishers, with a black band or panel identifying carbon dioxide.
Can CO2 be used on a paper fire?
It will knock the flames down but it is not rated class A and the fire will usually re establish, because carbon dioxide leaves nothing behind and does little cooling.
Why does the horn get so cold?
The liquefied gas expands rapidly as it discharges, which drops the temperature sharply. Hold the extinguisher by its handle and use the frost free horn grip if it has one.