Carbon dioxide is the agent chosen where a discharge must leave nothing behind. Its band is black. It is stored as a liquid under high pressure and leaves the horn as a gas and a cloud of dry ice, displacing oxygen at the seat of the fire. Because it evaporates completely it does no damage to electronics, which is the single reason it is standard provision in server rooms, switch rooms and near laboratory equipment.
The rating and what it excludes
It is rated for class B flammable liquids and carries an electrical marking. It is not rated for class A. A carbon dioxide unit discharged at a burning waste bin will knock the flame down and the paper will re-ignite, because nothing has been cooled and the gas disperses in seconds. A room whose only extinguisher is carbon dioxide is under provided for the fuel it actually contains.
The horn and the cold
The gas leaves at very low temperature. Older units have metal horns that will cause a cold burn on contact, which is why modern units use a frost free horn and why training covers holding the handle rather than the horn. This is the most common minor injury associated with any portable extinguisher.
Confined spaces
The mechanism is oxygen displacement, so a full discharge in a small unventilated room reduces the oxygen a person is breathing as well as the oxygen feeding the fire. The instruction is to discharge and leave, and to ventilate before re-entering. This is the one agent where the room itself is part of the hazard assessment.
Noise
A carbon dioxide discharge is extremely loud. People who have never used one frequently drop it. That is a training point rather than a defect, and it is worth covering in any building where the provision includes them.
Questions people ask about co2 fire extinguisher details
What is the co2 fire extinguisher class rating?
Class B, with an electrical marking. It has no class A rating in the current standard.
Why does a fire extinguisher with black label sit next to a server rack?
Because it leaves no residue. Water, foam and powder all destroy the equipment they are used to protect, and the cost of a wrongly chosen discharge in a server room is usually larger than the fire.
Does it need a different service regime?
It is a high pressure vessel, so it is weighed to confirm the charge rather than gauged, and it is subject to a periodic pressure test at a longer interval than the basic service. The service record should show both.