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, including confined space 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.
Carbon dioxide extinguishers, and the two things they must not be used on
A carbon dioxide unit smothers a fire by displacing oxygen and leaves no residue, which is why it sits next to electrical equipment and in server rooms and kitchens with electrical risk. It has no cooling effect to speak of, so it must not be used on a deep seated class A fire in paper, wood or textiles, where it will appear to work and then let the fire re establish. And it must not be used on cooking oil or fat, because the blast of gas will throw burning oil out of the pan, which is a wet chemical extinguisher's job. Two more cautions belong on the label rather than in a manual: in a small unventilated room the gas that puts the fire out is also displacing your oxygen, so leave the room, and the horn reaches an extremely low temperature in use, which is why frost free horns exist and why the old ones are held by the handle only.
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.
Where do CO2 fire extinguishers belong in a building?
Beside the electrical risk rather than on a general corridor: server and switch rooms, plant, kitchens with electrical equipment. They are usually provided in addition to a class A unit, not instead of one.
Is a fire extinguisher carbon dioxide unit safe in a small room?
It puts the fire out, and in a small unventilated space it also lowers the oxygen you are breathing, so the rule is discharge and get out rather than stay and watch. That is also why a carbon dioxide unit is not the right first choice for a windowless cupboard or a small plant room where somebody might be working alone, and why fixed gas suppression systems in server rooms carry alarms, delays and interlocks rather than just a nozzle. In an ordinary office or shop with a door and normal ventilation, a portable unit used properly and then vacated is not the hazard people imagine.
What is CO2 extinguisher use limited to?
Electrical equipment and flammable liquids, which is classes E and B in everyday terms, plus the mixed electrical risk that is the real reason it hangs where it hangs. It is not for wood, paper or textiles beyond knocking down a surface flame, not for cooking oils, not for metal fires, and not for lithium battery fires, where nothing portable will stop a cell once it is in thermal runaway and the answer is evacuation and the fire service. Where a room needs both, the standard arrangement is a water or foam unit for class A fires with a carbon dioxide unit beside it for the electrics.