Electromagnetic Lock Basics

Important: Before installing electronic locks, it is important to check with the “Local Authority Having Jurisdiction” (LAHJ).

An electromagnetic lock is designed to provide security for a door or gate that closes against a solid jamb or stop. Electromagnetic locks are fail-safe locking mechanisms that require constant power to remain locked. They are designed to provide basic traffic control up to a medium level of security.

The basic electromagnetic lock is normally installed onto the jamb of an out swinging door. The ferrous metal armature plate is mounted onto the face of the door in alignment with the bonding surface of the electromagnet. When the door is closed and the electromagnetic lock is energized, the current flowing through the wire coils produces a magnetic field. This magnetic field attracts the armature plate mounted onto the door. The armature is drawn against the electromagnetic lock with sufficient force to prevent the armature (door) from being opened.

When power is removed from today's magnetic locks, the magnetic field immediately dissipates and the door is immediately unlocked. Earlier models of magnetic locks have suffered from residual magnetism that caused electromagnets to retain a portion of the magnetic field, building in strength over the years. This would eventually cause the door to remain bonded, resulting in difficulty separating the armature from the magnet (opening the door) in the unenergized state.

Electromagnetic locks have increased in popularity as the potential applications have increased and the costs have decreased. In this article, we will discuss the operation, types, advantages and disadvantages and generalized installation considerations.

An electromagnet is a piece of metal that is wrapped with wire and then electrified. The electromagnetic lock in its basic form is a locking device that has no moving parts, depending upon a produced magnetic field to lock the armature (strike) against the magnet. To electrify the magnet, loops (coils) of wire surround the metal, while electrical circuitry is used to control the direct current (DC). Current draw for most electromagnetic locks is minimal, costing much less than a conventional light bulb to operate.

The electromagnet component consists of a metal alloy formed in a specific shape. The goal is to produce as large a magnetic field as possible within a limited sized housing. The specific chemical composition of the alloy is different for each lock manufacturer and is a closely guarded secret. The reason is specific metals' molecular structure will align with each other to greatly enhance the magnetic flux density (armature holding power).

Securitron, a manufacturer of electromagnetic locks, uses special metal alloy formed in “E” shaped plates. The size and number of “E” plates and number of coils of wire used in a specific electromagnetic lock determine the current and the holding power of the magnet. The holding power of an electromagnet starts at approximately 200 pounds and goes up to to almost 2,000 pounds for the larger units. Electromechanical models can have a holding force of 4,000 pounds.

The shape of the electromagnet is usually rectangular with a flat face to maximize the electronic bonding capability of the armature. The early mounting choices were header mount and face mount. The header mount meant there were holes running vertically on the sides of the magnet housing. Face mount was horizontal holes. Over the years, manufacturers developed a variety of mounting variations for magnetic locks using specially designed brackets and spacers. As an example, top jamb mounting kits will accommodate in-swing door applications.

Electromagnetic locks have evolved in the way they operate and are installed. An early improvement eliminated magnetic residue. A later development was dual voltage (12/24VDC) electrical circuitry that could identify and accommodate voltage. Upgradeable electromagnetic locks were designed so components could be replaced, either to upgrade the capability of the electronic lock mechanism or to replace non-functioning components.

Mounting and installation modifications have included the addition of a splice chamber and trapezoid shaped mounting bracket and slot mounting in the lock's case. The splice chamber has been incorporated into the housing of some magnetic locks. The splice chamber normally contains the circuit board with sufficient area to connect the wires.

When installing a “block” style magnetic lock containing existing mounting holes either vertically or horizontally positioned, the installer would have to hold the magnet in position and start several of the mounting bolts in order to hold the magnet in place.

The development of the trapezoid mounting bracket enables this bracket to be first installed into the header. Then slide the electromagnetic lock into place and determine what final adjustments would be necessary. Several of the mounting holes in the trapezoid mounting bracket are elongated, permitting front to back adjustment in order to locate the magnet for maximum attraction. Once in position, existing set screws are tightened, securing the magnetic lock in place.

SDC (Security Door Controls) developed the interlocking EZ mount assembly on their EMLocks®. The EZ mount bracket is installed onto the header, using the magnet to set the bracket's position. A captive mounting screw locks the magnet in position. The EZ mount assembly simplifies the installation of the Emlock electromagnetic lock.

The armature or strike plate is a thick piece of ferrous metal that is attracted to the magnetic field produced by the electromagnetic lock. The armature is usually about the length and height dimensions of the electromagnet. It is normally is “float” mounted onto the door, allowing it to rock into position flush against the face of the magnet. To accommodate this needed movement, lock manufacturers usually include rubber washers to install between the armature and the face of the door. Secondary locating pins are installed into the armature, having holes drilled into the door face to permit movement without rotation. In addition, energy absorbing sex bolts are available with an internal spring designed to absorb and dissipate the force of a blow aimed at disengaging the magnetic bonding in order to gain unauthorized access.

Additionally, manufacturers have developed offset mounting hole and split armatures. The offset armature is designed for an installation where it is necessary to raise or lower its position to better accommodate the electromagnetic lock. The split armature (strike) can be used to enable one electromagnetic lock to secure a double door opening using traffic control holding force.

For applications that require delayed egress, some electromagnetic locks are designed to delay exiting through the door for 15 or 30 seconds while sounding an alarm, notifying personnel of an attempted exit. Once the delay period has expired, power is removed and the door can be opened. This type of lock mechanism complies with the NFPA Life Safety Code 101, Special Locking Arrangements. A few of the applications for this type of lock mechanism can be to deter shoplifting, employee theft, patient wandering or baby abduction.

A specialized application for the surface mount electromagnetic locks is for sliding doors. The electromagnet is recessed into the lock side of the jamb leg. The armature is recessed onto the lock edge of the sliding door. When the door is closed, the armature comes into close contact with the electromagnetic lock. When powered, the electromagnet secures the door.

Another specialized type of electromagnetic locks are the shear locks. These concealed magnetic locks were designed for providing traffic control for Herculite (glass) doors, while exhibiting no visible signs of a lock or locking mechanism. Unlike the standard electromagnetic lock, the shear lock provides its greatest power against the armature being pulled across the face of the electromagnetic lock (i.e. opening the door).

The face of the shear lock separate from the electromagnet has recesses into which raised shear locator posts built into the spring loaded armature engage when the door is closed and the electromagnet is powered. The purpose of the raised shear locator posts is to stop the armature (strike plate) from sliding off from the shear lock. When power is removed, the shear strike plate retracts, disengaging the shear locator posts from the magnet, releasing the door.

When the door is open and the shear electromagnetic lock is energized, the door can move to the closed position without correct alignment of the spring loaded strike plate to the magnet. When the electromagnetic lock is powered, the open door's shear locator posts keep this special strike plate far enough away that the electromagnet cannot gain sufficient holding power on the strike plate. Once the shear locator posts are able to enter the recesses in the electromagnetic lock, the armature is drawn into contacting the magnet face and locking the door. The shear locator post mechanism eliminates the need of timers or sensors, permitting the door to close before the electromagnetic lock is energized.

The shear type design permits locking of the door while maintaining the continuity of the door and frame. There are no exposed magnets or plates. In addition, the shear type locking mechanism can be used with both single and double swinging doors. Because the magnet and the strike assembly are both concealed, a door can swing in both directions. Shear locks are normally mounted into the header or threshold of a sliding or swinging door.

The Securitron MM15 E-Mag is a hybrid design electromagnetic lock that provides 4, 000 pounds of holding force, according to Securitron. The compact MM15 is equipped with two spring loaded, hardened steel clamps (mandibles) that extend from the lock housing. They are designed to place additional holding force onto the modified armature in the event an attempt is made to pull open the door. When this type of attempted forced entry occurs, the mandibles clamp down onto the edges of the armature adding a significant amount of holding force. Only when the force applied to pull open the door is eliminated do the mandibles release the armature. When the current is turned off, the MM15 releases and the door can be opened. The MM15 is designed for indoor applications only.

Another application for electromagnetic locks is to control access of manual and automatic gates. The lock is installed onto the fence post and the armature is mounted onto the swinging or sliding gate. The size of the electromagnetic lock is determined by the size and weight of the gate. For specific applications, contact the electromagnetic lock manufacturer.

Gate operators are designed to close gates. Many of these motorized units do not have speed adjustments. To keep the gate from slamming closed, shock absorbing strike mount kits are available. These kits are designed to absorb the excessive impact between the electromagnetic lock and the armature.

To accommodate these different applications, electromagnetic lock manufacturers have developed a number of different shape mounting brackets and fillers. These metal components ensure that the electromagnetic lock is solidly mounted. The fillers are used to compensate for the different styles of jamb stops.

Since there are no moving parts in an electromagnetic lock, there is no easy way to determine if the door is secured. For this reason, when electromagnetic locks are used for security applications rather than traffic control, order the locks with monitoring capabilities. The monitoring options include a bond sensor, signal sensor, door position switch, anti-tamper sensor and visual status indicators.

A bond sensor can monitor a portion or the entire surface of the armature. Should there be any variation beyond the established parameters, a signal will be initiated indicating the electromagnetic lock is not generating full holding potential.

A lock status sensor is designed to indicate when the door is secure, not just closed.

A door position switch (DPS) can provide a remote signal indicating if the door is opened or closed. This DPS is normally used for auto relock with an access control system.

An anti-tamper switch will indicate if the splice chamber cover has been tampered with or has been removed. This type of sensor is normally an alarm point on the panel or part of an access control system.

The LED visible from the rear of the electromagnetic lock gives a visual status of the lock. Check with the lock manufacturer to determine the operating colors for your specific magnet.

Surface/header mount electromagnetic locks are designed for swinging doors that open out from the secured side. Smaller electromagnetic locks can be used to secure office furniture doors/drawers and display cases. These surface mount locks can be used on doors where the armature is mounted onto the interior face of the door. Surface mount electromagnetic locks  can be header mounted or surface mounted. They are designed for the access control of hollow metal, wood, aluminum and glass door openings.

When installing an electromagnetic lock, the door and jamb material determines the type of fastener. When installing the unit onto a wood jamb, follow the manufacturer's fastener recommendations. When installed the lock onto a hollow metal or aluminum jambs, the use of blind nuts is recommended.

Choosing the power supply and access control mechanisms including keypad, key switch, momentary button, sensors, proximity cards or magstripe cards will usually vary from job to job. Contact the electromagnetic lock manufacturer for recommendations regarding the best way to apply and install their products. Most electromagnetic lock manufacturers will provide wiring diagrams, riser diagrams, and other information that can provide your customer with the best access control system for their needs.

Several manufacturers offer a complete package including the electromagnetic lock, keypad or card reader, and power supply. Alarm Lock offers a complete package electromagnetic lock, power supply and remote control. This electromagnetic lock is wired to a receiver designed to be operated by up to 100 battery powered remote controls.

Some of the electromagnetic lock manufacturers include:

Alarm Lock Systems, Inc., 345 Bayview Avenue , Amityville , NY 11701 . Telephone: 631-789-4871. Fax: 631-789-3383. Web Site: www.alarmlock.com.

Dortronics Systems, Inc., 1668 Sag Harbor Turnpike, Sag Harbor , NY 11963 . Telephone: 800-906-0137. Web Site: www.dortronics.com.

Dynalock Corp., 705 Emmett St. , Bristol , CT 06010 . Telephone: 860-582-4761. Fax: 860-585-0338. Web Site: www.dynalock.com.

RCI, 2697 International Parkway , Parkway Five, Virginia Beach , VA 23452 . Telephone: 800-265-6630. Fax: 800-482-9795. In Canada , 210 Shearson Crescent , Cambridge , ON N1T 1J6 . Telephone: 800-265-6630 or 519-621-7651. Fax: 519-621-7939. Web Site: www.rutherfordcontrols.com.

ROFU International Corp, 2004-B 48th Ave. CT. E, Tacoma , WA 98424 . Telephone: 253-922-1828. Fax: 253-922-1728. Website www.ROFU.com.

Schlage, 111 Congressional Blvd., Carmel , IN 46032 . Telephone: 800-847-1864. Web Site: www.Schlage.com.

Securitron Magnalock Corp., 550 Vista Blvd. Sparks , NV 89434 . Telephone: 775-355-5625 or 800-MAGLOCK. Fax: 775-355-5633. Website: www.Securitron.com.

SDC Security Door Controls, P. O. Box 6219 , Westlake Village , CA 91359 . Telephone: 805-494-0622. Fax: 805-494-8861. Website: www.sdcsecurity.com .