11. Magnetic Locks
Chapter objectives
1. Learn the Basics in the Chapter Overview
2. Learn about Standard Magnetic Locks
3. Understand Magnetic Shear Locks and When to Use Them
4. Answer Questions about Magnetic Locks
CHAPTER OVERVIEW
Magnetic Locks are the workhorse of electrified locking mechanisms for most security contractors. They are easy to install, versatile, provide good security, and are generally dependable. Magnetic locks can be fitted to any door with a metal or wood surface near the top of the door, so they can be fitted to more than 90% of doors installed in commercial installations.
Magnetic locks work by using an electromagnet on the door frame and an armature mounted to the door. When the door closes, the armature comes into contact with the electromagnet. Magnetic locks can also function as a door position switch and lock engaged switch, when so configured. This is done by sensing the magnetic bond between the electromagnet and the armature. When the bond is made, the lock is locked, and thus the door is closed. All surface-mounted magnetic locks should be mounted on the secure side of the door.
Magnetic Locks are the workhorse of electrified locking mechanisms for most security contractors. They are easy to install, versatile, provide good security, and are generally dependable. Magnetic locks can be fitted to any door with a metal or wood surface near the top of the door, so they can be fitted to more than 90% of the doors installed in commercial installations.
Magnetic locks work by using an electromagnet on the door frame and an armature mounted to the door. When the door closes, the armature comes into contact with an electromagnet. Magnetic locks can also function as a door position switch and lock engaged switch, when so configured. This is done by sensing the magnetic bond between the electromagnet and the armature. When the bond is made, the lock is locked, and the door is closed. All surface-mounted magnetic locks should be mounted on the secure side of the door.
Keywords: Armature, Electrified Lock, Electromagnet, Install, Magnetic Locks, Switch
Author Information:
Thomas L. Norman, CPP, PSP, CSC, Executive Vice President, Protection Partners International
Standard Magnetic Locks
The Standard Magnetic Lock (Figure 11.1), also called a magnetic plate lock, is the primary version of the magnetic lock. The Magnetic Lock is a blissfully simple device. It comprises only an electromagnet and an armature. Connect the electromagnet to the door frame and the armature to the door, add power, and you have yourself a locked door.
The Standard Magnetic Lock places the electromagnet and armature both in vertical orientation so that the armature closes directly against the electromagnetic. This provides a large contact area and ensures freedom of the door when the magnetic lock is released. In an attack on a Standard Magnetic Lock, the attacker must overcome the tensile force of the lock to break the magnetic bond.
There is a seemingly endless combination of door configurations that quite honestly drives many sane people crazy. No, I mean it. If you ever want a three-month holiday from work, just read up on door lock configurations and you will finish the article in a state that can get you institutionalized by any psychiatrist. After more than thirty years in the security industry, I still find new door configurations. Magnetic locks solve most of those problems by using an assortment of adapters to work in a large variety of mounting and use configurations.
Magnetic Locks work with DC current only. Most Magnetic Locks can be ordered with an optional door/lock position/engagement sensor that alerts when the door is open. They are also useful for doors with misalignment problems or buildings with foundation problems resulting in frequent misaligned doors. Due to the nature of the design, Standard Magnetic Locks can suffer some misalignment and still function well. (This is not true for Magnetic Shear Locks.)
Because magnetic locks have no mechanical locking elements, they require power to work, meaning that they will unlock if power is lost. Accordingly, a back-up power source such as an uninterruptable power supply is required if the lock is used in a security-critical application.
Standard Magnetic Locks come in a variety of holding forces, the most common is 650 pounds (light security), then 1,200 pounds (medium security), and 1,500 pound and higher (high security). A magnetic lock with a holding force of only 650 pounds should be used for traffic control only. These locks can easily be defeated. Locks with holding force of 1,200 pounds are sufficient for aluminum framed glass doors, because when 1,200 pounds of force is exerted on such a door the glass will break. These are useful where the doors are not likely to engage aggressive attacks. For hollow metal doors and doors where aggressive use of force is possible, such as a door securing a psychiatric ward of a hospital, high security magnetic locks are called for.
Standard Magnetic Locks are not the perfect one-size-fits-all solution that some contractors think they are (see the section Cautions about Magnetic Locks).
Magnetic locks have several basic forms:
• Basic surface-mounted magnetic lock
• Magnetic lock with escutcheon filling the top of the door frame for a more aesthetic look
• Magnetic gate locks
• Magnetic locks built into the door frame
Standard Magnetic Lock Applications
The following illustrations show the three most common applications for magnetic locks. These include:
-Figure 11.2 Outswinging door configuration.
-Figure 11.3 Inswinging door configuration.
-Figure 11.4 Double door configuration.
These illustrations indicate how surface mounted magnetic locks are applied to the most common door swing applications.
Magnetic Shear Locks
The second major variation of magnetic locks is the Magnetic Shear Lock (Figure 11.5). Unlike the vertically oriented Standard Magnetic Locks in which the armature pulls directly away from the electromagnet, the Magnetic Shear Lock is vertically oriented so that if someone is attacking the door, they must overcome the shear force of the magnetic lock, which is much greater than the tensile force of a Standard Magnetic Lock. Magnetic Shear Locks are available in holding forces reaching up to 3,000 pounds. These are very strong indeed.
In Hollow Metal Doors, Magnetic shear locks can be mounted inside the door frame with the armature mounted inside a pocket in the top of the door, making for a completely invisible lock when the door is closed.
Magnetic Shear Locks can also be used with aluminum frame glass doors, frameless glass doors, and solid core wood doors. In most of these cases, the armature will be mounted to the door and will be visible.
Most Magnetic Shear Locks are concealed; easy concealment is one of their strong points. However, to ensure complete safety and to provide the higher security only available with the shear lock version, surface-mounted versions are available. These can be mounted to accommodate either inswinging or outswinging doors (see the section Cautions about Magnetic Locks).
Magnetic Gate Locks
Indoor and outdoor gates can also be locked using magnetic locks. Magnetic Gate Locks (Figure 11.8) provide high security and, if mounted correctly, are generally very reliable. Since gates do not align as precisely as doors, their natural ability to accommodate some misalignment makes them a good choice.
When mounting Magnetic Gate Locks outdoors, it is important to ensure that wiring is secure from weather and is unaffected by animals and unavailable to passersby. Some Magnetic Gate Locks are available already fitted with threaded conduit connectors to make a seal-tight connection to the rigid conduit, making for a secure installation.
Cautions about Magnetic Locks
Problems with magnetic locks fall into several areas:
• Egress Assurance
• Operational Warnings
• Maintenance Warnings
Egress Assurance
Magnetic locks are not free mechanical egress locks; that is, power to the lock must be dropped for the lock to unlock. And this is where the problem begins. To exit through a door that is locked with a magnetic lock, the door must be equipped with some kind of Request-to-Exit sensor, and Request-to-Exit sensors are not all created equal (Figure 11.9).
There are two broad types of Request-to-Exit sensors: those that require no special knowledge to operate and those that require the users to know how to operate the Request-to-Exit sensor. Examples of those that require no special knowledge include infrared or infrared/microwave motion detectors over the door and a Request-to-Exit switch inside a Panic Bar. Examples of Request-to-Exit sensors that require special knowledge include Exit Push Buttons and Emergency Pull Station Exit Switches. There are several kinds of Request-to-Exit sensors, and some are more reliable than others.
Another way to categorize Request-to-Exit sensors is by reliability. A switch that directly controls power to the magnetic lock has one degree of separation, whereas a motion detector that notifies an exit sense input to an Access Control Panel has seven degrees of separation. The motion sensor must first sense motion with an infrared circuit (1), and it must process that detection (2) and activate a notification relay (3). This will be communicated to an Access Control Panel across wiring (4), where it will be sensed on a Request-to-Exit input (5). The Access Control Panel must process the request (6) and activate the Lock Relay (7), which will drop power to the magnetic lock. (The final cable is not included because if it fails, there would be no power to drop.)
I just described eight chances for failure. The more chances, the more likelihood of failure. Things can go wrong with electronics. And they do. I have been locked behind a door where an Access Control Panel Request-to-Exit input failed unexpectedly; and with the building manager of course. Luckily, we were on a construction inspection tour and the contractor was there to open the lock and release power.
For this reason, I strongly recommend that you interface every motion detector Request-to-Exit sensor through an interposing relay having two contacts, such that one contact signals the Access Control Panel and the other contact directly controls power to the lock. An even better option is to use a Request-to-Exit switch that has two contacts, thus eliminating the relay that could be another point of failure.
Because Request-to-Exit sensors can cause magnetic locks to fail locked, I also recommend having two means to release the lock from the inside. Typically, this includes one method that requires no special knowledge and a second one that does. Common combinations include:
• Exit Motion Detector and Exit Push Button
• Panic Bar Switch and Exit Push Button
• Exit Motion Detector and Panic Bar Switch
You might be asking why you would have a panic bar on a door with a magnetic lock. This is a common practice on doors that do not have latching locks. In such cases, the panic bar is not mechanical; there is just a panic bar with an integral Request-to-Exit switch.
Operational and Maintenance Warnings
Safety Concerns:
• The placement of the Request-to-Exit Motion Detector is also critical. There is only one place to put these units and that is immediately centered above the door. I have seen motion exit detectors in a variety of unusual places, but they are most often placed on the ceiling 6 to 10 feet (2 to 3 m) in front of the door. Usually this is because someone in authority in the building has requested it so that the lock will be unlocked before anyone reaches the door. This might work fine for someone walking straight to the door, but that is not always the case. Sometimes people stop for a moment and close out a conversation before leaving through the door. In such cases, as neither person is in the area sensed by the motion sensor, they will find the lock engaged and not unlocked as they would imagine.
• When using Exit Push Buttons it is important to remember when wired as described earlier, if the Access Control Panel should fail to unlock the door, it will be up to the Exit Push Button to directly release power to the magnetic lock to unlock the door. In such cases, the Exit Push Button must be able to hold power from the magnetic lock long enough for the user to reach for the door handle and open the door. The best type of Exit Push Button for this function is a Pneumatically Controlled Exit Push Button. This type of Exit Push Button has a small, air-filled cylinder activated by the push of the button. The cylinder holds the button in by means of a vacuum generated by pushing the button long enough to allow the door to be opened. Although Pneumatic Exit Buttons are pricy, they are the best solution possible when using Exit Push Buttons.
Security Concerns:
• Request-to-Exit Motion Sensors on double frameless glass doors have another concern. Double frameless glass doors do not come together perfectly. There is always a small gap between them when closed. This gap is large enough to allow a person outside to pass a yellow note tablet through the gap where it can be sensed by the motion detector, triggering the Access Control System to unlock the door. This obviously presents a serious security concern. I once had a client whose Security Command Center manager gleefully demonstrated his “superior knowledge” by telling many employees about this security exploit. The building in question had many elevator lobby doors equipped with double frameless glass doors. While it might have made him momentarily popular with other employees, it was not the case with upper management who may have set his career path on a different course.
• It is important to mount all surface-mounted magnetic locks on the secure side of the door. When the lock is mounted on the unsecure side of the door, access to the lock by unauthorized users is possible, resulting in a compromise of the lock, allowing illegal entry. It is startling to see the number of magnetic locks that are mounted on the unsecure side of the door.
Maintenance Concerns:
• Magnetic locks may use a reverse-mounted diode to dissipate the electromagnetic force (back EMF) that the magnet generates back onto the power supply when it disengages power. I have seen this cause the lock to delay opening by up to half a second when commanded to open. Most modern magnetic locks use a Metal Oxide Varistor (MOV) instead of a reverse-mounted diode to dissipate the back EMF. The MOV does not cause the same delay as the diode.
• Magnetic Locks cannot have anything on the surface of the electromagnet or its armature (i.e., cellophane tape, paper, lacquer, etc.) as this can cause the magnet to fail to lock securely. Magnetic locks should be cleaned annually with a moist, soapy cloth and then wiped with a clean moist cloth.
Chapter Summary
1. The Standard Magnetic Lock, also called a magnetic plate lock, is the primary version of the magnetic lock.
2. The Standard Magnetic Lock places the electromagnet and armature both in vertical orientation so that the armature closes directly against the electromagnet.
3. Magnetic locks have several basic forms including:
• Basic surface mounted magnetic lock
• Magnetic lock with escutcheon filling the top of the door frame for a more aesthetic look
• Magnetic gate locks
• Magnetic locks built into the door frame
4. The second major variation of magnetic locks is the Magnetic Shear Lock.
5. Unlike the vertically oriented Standard Magnetic Locks in which the armature pulls directly away from the electromagnet, the Magnetic Shear Lock is vertically oriented so that if someone is attacking the door, they must overcome the shear force of the magnetic lock, which is much greater than the tensile force of a Standard Magnetic Lock.
6. Indoor and outdoor gates can also be locked using magnetic locks.
7. Problems with magnetic locks fall into several areas:
• Egress Assurance
• Operational Warnings
• Maintenance Warnings
8. Magnetic locks are not free mechanical egress locks; that is, power to the lock must be dropped in order for the lock to unlock.
9. There are two broad types of Request-to-Exit sensors; those that require no special knowledge to operate and those that require the users to know how to operate the Request-to-Exit sensor.
10. Because Request-to-Exit sensors can cause magnetic locks to fail locked, I also recommend having two means to release the lock from the inside. Typically this will include one method that requires no special knowledge and a second that does. Common combinations include:
• Exit Motion Detector and Exit Push Button
• Panic Bar Switch and Exit Push Button
• Exit Motion Detector and Panic Bar Switch
11. Safety Concerns:
• The placement of the Request-to-Exit Motion Detector is also critical.
• When using Exit Push Buttons it is important to remember when wired, as described previously, if the Access Control Panel should fail to unlock the door, it will be up to the Exit Push Button to directly release power to the magnetic lock in order to unlock the door.
12. Security Concerns:
• Request-to-Exit Motion Sensors on double frameless glass doors can be easily compromised.
• It is important to mount all surface-mounted magnetic locks on the secure side of the door.
13. Maintenance Concerns:
• Magnetic locks may use a reverse-mounted diode to dissipate the electromagnetic force that the magnet generates back onto the power supply when it disengages power.
• Magnetic Locks cannot have anything on the surface of the electromagnet or its armature.
1) The Magnetic Lock comprises:
a. An armature and a plate
b. A plate and the door frame
c. An electromagnet and a plate
d. An electromagnet and an armature
2) The _________ mounts to the door frame
a. Armature
b. Electromagnet
c. Door
d. Hinges
3) The Standard Magnetic Lock is the primary version of the magnetic lock
a. True
b. False
4) The Standard Magnetic Lock places the electromagnet and armature both in _______ orientation
a. Triangular to the door frame
b. Vertical
c. Horizontal
d. Away from the door
5) Standard Magnetic Locks come in a variety of holding forces including:
a. 650 pounds
b. 1,200 pounds
c. 1,500 pounds
d. All of the above
6) Magnetic _____ Locks are the second major variation of magnetic locks
a. Hollerith
b. Wiegand
c. Proximity
d. Shear
7) Magnetic Shear locks are oriented _______ to the door
a. Horizontally
b. Vertically
c. Triangular to the door frame
d. Away from the door
8) Most Magnetic Shear locks are concealed
a. True
b. False
9) _________ and __________ gates can also be locked using magnetic locks
a. Open and closed
b. Swinging and open
c. Sliding and open
d. Indoor and outdoor
10) Problems with magnetic locks fall into several areas:
a. Egress assurance
b. Operational warnings
c. Maintenance warnings
d. All of the above
11) Magnetic locks are free mechanical egress locks
a. True
b. False
12) There are two broad types of Request-to-Exit sensors:
a. Those that require no special knowledge to operate and those that require the user to use a code
b. Those that require special knowledge to operate and biometric
c. Those that require no special knowledge to operate and those that require the users to know how the operate the Request-to-Exit sensor
d. All Request-to-Exit sensors require special knowledge
13) Request-to-Exit sensors can include:
a. Infrared
b. Switch inside a Panic Bar
c. Push-to-Exit Button
d. All of the above
Answers: 1) d, 2) b, 3) a, 4) b, 5) d, 6) d, 7) a, 8) a, 9) d, 10) d, 11) b, 12) c, 13) d
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