A fire door retainer is a legal means of holding a fire door open under normal circumstances. In the event of a fire the fire door retainer will release the door, allowing the door to close.
Fire Extinguisher Rating
Fire extinguishers are thoroughly tested on the classes of fire that the extinguishant inside is suitable for. The ratings given to the extinguisher indicate the maximum size of each type of fire that the extinguisher successfully tackled under test conditions.