There are two main types of electric locking devices. Locking devices can be either "fail safe" or "fail secure". A fail-secure locking device remains locked when power is lost. Fail-safe locking devices are unlocked when de-energized. Direct pull electromagnetic locks are inherently fail-safe. Typically the electromagnet portion of the lock is attached to the door frame and a mating armature plate is attached to the door. The two components are in contact when the door is closed. When the electromagnet is energized, a current passing through the electromagnet creates a magnetic flux that causes the armature plate to attract to the electromagnet, creating a locking action. Because the mating area of the electromagnet and armature is relatively large, the force created by the magnetic flux is strong enough to keep the door locked even under stress.
Typical single door electromagnetic locks are offered in both 600 lbs.(272 kg) and 1200 lbs. (544 kg) dynamic holding force capacities. A "fail safe" magnetic lock requires power to remain locked and typically is not suitable for high security applications because it is possible to disable the lock by disrupting the power supply. Despite this, by adding a magnetic bond sensor to the lock and by using a power supply that includes a battery backup capability, some specialized higher security applications can be implemented. Electromagnetic locks are well suited for use on emergency exit doors that have fire safety applications because they have no moving parts and are therefore less likely to fail than other types of electric locks, such as electric strikes.
The strength of today's magnetic locks compares well with that of conventional door locks and they cost less than conventional light bulbs to operate. There are additional pieces of release hardware installed in a typical electromagnetic locking system. Since electromagnetic locks do not interact with levers or door knobs on a door, typically a separate release button that cuts the lock power supply is mounted near the door. This button usually has a timer that, once the button is pressed, keeps the lock unlocked for either 15 or 30 seconds in accordance with NFPA fire codes. Additionally a second release is required by fire code. Either a motion sensor or crash bar with internal switch is used to unlock to door on the egress side of the door automatically.
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Via via wikipedia.org