A lock or latch on a fire door is a hole cut into the leaf at the point where the door is held closed. It is therefore part of the tested assembly and not an accessory, and hardware substitution is one of the routes by which a compliant doorset quietly stops being one.
Why the lock is part of the test
The mortice for a lock case removes material from the core at mid height, exactly where the door needs to stay intact and where the latch has to keep holding. A doorset is tested with particular hardware, and the certification then covers a defined range of alternatives. Fitting a lock outside that range is a change to a tested assembly with no evidence behind it.
Intumescent protection
Lock cases, latch cases and hinge blades are usually fitted with intumescent liners or pads that expand and fill the void as the timber around them chars. These are supplied with certified hardware and are routinely left in the packet by installers who do not know what they are. A lock fitted without its intumescent protection is a defect that cannot be seen once the door is hung.
Latching is the function that matters
The purpose is not security, it is keeping the door shut against the pressure a fire generates. A door with a closer and no latch will be pushed open. A latch that does not engage reliably because the keep is misaligned is the same failure. This is why the routine check asks about latching separately from closing.
Electric locks and access control
Electric strikes, magnetic locks and access control devices on fire doors have to be both fire tested with the doorset and correct in their fail behaviour: fail safe where the door is on an escape route, and always releasing on alarm. Retrofitting access control to fire doors is common in commercial buildings and is where the two requirements are most often traded against each other by somebody who only knows about one of them.
Questions people ask about fire rated door locks
Can any lock be fitted to a fire door?
No. It has to be within the doorset's certified hardware range and fitted with its intumescent protection, to the manufacturer's instructions.
What does a fire rated door latch actually do?
It holds the leaf against the frame so the door stays shut under the pressure differential a fire creates. Without it, the closer alone is not enough.
Is a bathroom style privacy latch acceptable?
Only if a certified version within the doorset's range is used. Standard privacy sets are generally not tested with fire doorsets.