Different designations are possible: EE function (emergency exit), double-sided lockability, BSZ
Standard locking cylinders cannot be opened if a key is inserted on the opposite side.
If the key stuck on one side is twisted, it cannot be pushed out with a key when trying to lock the other side to initiate the locking process.
The emergency and danger function is a protection system to unlock doors locked with double cylinders, even if a key is inserted inside.
Generally, the view has prevailed that the emergency and danger function generally ensures a double-sided lockability with double cylinders. However, this is not generally the case (see priority function).
The various manufacturers of lock cylinders distinguish in:
cylinders lockable on both sides (BSZ)
unilateral emergency and danger function
priority function (prio function)
The double-sided lockability is achieved by a special coupling inside the locking cylinder. Spring elements absorb the "excess length" of the key stuck on the inside of the door.
This makes it possible to insert the key outside "full" in the key channel and to be able to perform the locking process.
With the Priority function or one-sided emergency and danger function, has the key inserted on the outside always priority. This is not possible the other way.
The key on the inside rotates blind if a key is inserted on the outside. This means that if a key is inserted on the outside, it cannot be locked from the inside.
The use of the emergency and danger function prevents unintentional lockout.
The user saves, if he has an additional key, the key service for the emergency opening of the door.
Likewise, in the case of helpless persons, the emergency and danger function ensures access to the apartment.
However, there are also good reasons to manufacture locking cylinders not with a locking capability on both sides.
Therefore, the emergency and danger function is usually a selection option when ordering locking cylinders.