The latch is the part that makes a fire door work, because a door resting against its frame is not closed. The lock is a security question that has to be answered without compromising escape, and getting those two the wrong way round produces either a door that does not hold back fire or one that traps people.
The fire door latch
A fire door needs a latch that engages positively when the closer brings the door to. Without one, air pressure, warping and the seals themselves will hold the leaf a few millimetres off the frame, which is enough for smoke and eventually for fire. Latches, like every other component, are part of the tested assembly, so a latch swapped for a lighter one is an alteration to the certification.
Locks and escape
A door on an escape route must be openable from the escape side without a key, without special knowledge and in one action. That rules out a thumbturn nobody has explained, a keypad on the escape side and, absolutely, a padlock. Where security genuinely requires locking, the solutions are panic hardware, an emergency exit device, or an electrically held lock released by the alarm and by a local break glass.
The fire door keep locked sign, and what it means
Fire doors to cupboards, risers and plant rooms that are not escape routes are often required to be kept locked, and carry a keep locked shut sign. The sign is not decoration: it is telling whoever finds the door that the correct state is locked, so an unlocked riser cupboard is a finding rather than a convenience. Doors on escape routes carry fire door keep shut instead, which is a different sign and a different meaning.
Electric locking
Access controlled doors on escape routes need the lock to fail safe: released by the fire alarm, released by a local emergency break glass, and released on loss of power. All three paths need testing, and the break glass needs to be where somebody escaping would find it. This is an interface on the fire alarm cause and effect matrix and it is one of the items most often found untested after installation.
Questions people ask about fire door lock
Can a fire door be locked?
A fire door that is not on an escape route, such as a riser or store cupboard, can be locked and is often required to be. A door on an escape route must be openable from the escape side without a key in one action.
Why does a fire door need a latch?
Because the closer alone leaves the door resting against the frame, and the seals and air pressure will hold it fractionally open. A latch pulls the leaf into the frame where the seals can work.
What does fire door keep locked mean?
That the correct state of that door is locked, normally a cupboard, riser or plant room that is not an escape route. It is different from fire door keep shut, which is about escape route doors.
Are fire door latches different from an ordinary latch?
They do a different job. An ordinary latch holds a door shut; a fire door latch has to hold the leaf tight into the frame hard enough that the closer cannot leave it resting on the stop, because a door standing a few millimetres proud is a gap for smoke. It also has to have been tested as part of a door assembly, which is the part nobody checks. A door with a closer and no latch, or with a latch that has been eased so it no longer engages, is not doing what it was certified to do.