When we talk about normally open and normally closed, we may think we are referring to the door, but in electric locks we are referring to the state of the circuit.
In other words, NO (normally open) means that the lock in the absence of voltage (relay open) is closed and will only open when it receives voltage (relay closed); this philosophy typically applies to Fail Secure systems.
Fail Secure: If there is no power, the lock will remain closed
However, when we talk about NC (normally closed) means that the live lock remains closed (relay closed) and will only open when it is not energized (relay open).
This system is designed so that in the event of a power failure or building emergency, the door will remain open so that it can be opened; this philosophy is typically applied to Fail Safe systems.
Fail Safe: In the event of a power failure the lock will remain open.