A fire door has one job with two halves: to hold a fire inside the compartment it started in for long enough that people can leave, and to keep the escape route clear of smoke while they do. Everything about how fire doors are specified, fitted and checked follows from those two sentences.
Holding the fire in
Buildings are divided into compartments by walls and floors that resist fire for a stated period. Every opening in a compartment boundary is a weakness, and the fire door is what restores the boundary at a doorway. A door rated below the wall it sits in, or held open, reduces the whole compartment to the performance of that opening, which is why a wedged door is not a small thing.
Keeping the route usable
Smoke, not flame, is what stops people escaping and what kills most of those who die in fires. A smoke rated door keeps cold smoke out of a corridor or stairway during the minutes when people are using it. That is why the S in a rating matters so much on escape routes, and why a brush seal on an unrated door is not a substitute for a tested smoke performance.
What is not a fire door requirement
A fire door does not have to be locked. It does not have to be self locking. It does not have to be a particular colour or material. It does not have to have a vision panel, and where one exists it does not have to be wired glass. It does not have to be heavy. And a fire door keep shut sign, while normally required, is not what makes the door a fire door. The requirements are the tested assembly, the correct gaps and seals, positive latching and self closing.
Why they fail in practice
Almost never because the door burned through sooner than tested. Overwhelmingly because the door was open, because it did not latch, because the seals had been painted or removed, because ironmongery had been changed, or because the gaps had grown. Every one of those is a maintenance and management failure rather than a product failure, which is the argument for the checking regimes.
Questions people ask about what is the purpose of a fire door
What is the purpose of a fire door?
To hold fire inside the compartment where it started for the tested period, and to keep smoke out of the escape route so people can leave. Both depend on the door being shut.
Which of these is not a fire door requirement?
Being locked, being self locking, being a particular colour or material, having a vision panel, or being heavy. The requirements are a tested assembly, correct gaps and seals, positive latching and self closing.
Why do fire doors fail?
Usually because they were open, did not latch, had painted or missing seals, had ironmongery changed, or had gaps outside tolerance. Product failure is rare; management failure is the norm.