An electronic lock (or electric lock) is a locking device which operates by means of electric current. Electric locks are sometimes stand-alone with an electronic control assembly mounted directly to the lock. Electric locks may be connected to an access control system, the advantages of which include: key control, where keys can be added and removed without re-keying the lock cylinder; fine access control, where time and place are factors; and transaction logging, where activity is recorded. Electronic locks can also be remotely monitored and controlled, both to lock and to unlock. Operation Electric locks use magnets, solenoids, or motors to actuate the lock by either supplying or removing power. Operating the lock can be as simple as using a switch, for example an apartment intercom door release, or as complex as a biometric based access control system. There are two basic types of locks: "preventing mechanism" or operation mechanism. Types Electromagnetic lock The most basic type of electronic lock is a magnetic lock (informally called a "mag lock"). A large electro-magnet is mounted on the door frame and a corresponding armature is mounted on the door. When the magnet is powered and the door is closed, the armature is held fast to the magnet. Mag locks are simple to install and are very attack-resistant. One drawback is that improperly installed or maintained mag locks can fall on people, and also that one must unlock the mag lock to both enter and to leave. This has caused fire marshals to impose strict rules on the use of mag locks and access control practice in general. Additionally, NFPA 101 (Standard for Life Safety and Security), as well as the ADA (Americans with Disability Act) require "no prior knowledge" and "one simple movement" to allow "free egress". This means that in an emergency, a person must be able to move to a door and immediately exit with one motion (requiring no push buttons, having another person unlock the door, reading a sign, or "special knowledge"). Other problems include a lag time (delay), because the collapsing magnetic field holding the door shut does not release instantaneously. This lag time can cause a user to collide with the still-locked door. Finally, mag locks fail unlocked, in other words, if electrical power is removed they unlock. This could be a problem where security is a primary concern. Additionally, power outages could affect mag locks installed