ELECTRIFIED LOCKING MECHANISMS

Electrified locking mechanisms allow doors to be locked and unlocked by a remote device. The device may be a simple electric push button or a motion sensor, or may be a sophisticated automated access control device such as a card reader or digital keypad. In addition, many access control systems allow the use of Boolean logic to augment the control of electrified devices. Boolean logic relates to the combination of conditions; for exam-ple, “if door A is locked and door B is locked then door C can be unlocked.”

This is useful in the design of mantraps and other high-security opera-tions. When considering failure and defeat mechanisms for locks, the addi-tion of remote-control devices requires that these other devices be included in the analysis.

Before describing the different types of electrified locking mechanisms, it is useful to clearly define two important terms: fail safe and fail secure.

These terms are usually applied in reference to fire/life safety codes and relate to doors in the path of egress from an occupied space that are required to be unlocked either at all times of occupancy or only during a detected fire emergency. Codes also state that the means of egress must be by a single action that requires no special knowledge, although there are some exceptions for banks, jewelry stores, and other high-security applica-tions. Turning a door handle or pushing an exit device (panic bar) are allowable single actions. Pressing a button, turning a key, using a card reader, or keying a number on a digital keypad before turning the handle do not constitute single actions. Local fire codes vary by municipality and the reader should refer to them when specifying any door locking mechanisms.

A fail safe locking mechanism is one that will unlock under any failure con-dition. The failure mode most commonly considered is loss of power, but failure of the mechanism itself and any connected control device need to be considered also. Most, but not all, locks related to code-required egress are fail safe to ensure that they provide free egress if a power failure occurs at the same time as a fire emergency. Note, however, that the lock for a door that normally provides free egress simply by turning a handle or depressing the exit bar from the secure side does not need to be fail safe. For example, an electrified exit device (panic bar) will mechanically unlock its door when pushed, regardless of whether or not the lock is electrically energized.

A fail secure lock is one that will remain locked when power is lost or other failure occurs. As noted above, a fail secure lock may be used on a door in the path of egress provided that free egress is available regardless of the state of the lock’s power or other control mechanisms.

Electric Deadbolt

The electric deadbolt is the oldest and simplest of all electrical locking devices. A solenoid (electro-magnet) moves a deadbolt, typically mounted on a door frame, either into or out of a strike plate on a door (see Figure 11). The mechanism can be either fail safe, automatically unlocking on the removal of power, or fail secure, remaining locked on the removal of power. The electric deadbolt is not normally recommended for application to doors required to be unlocked automatically in response to a fire alarm signal. This is because the bolt may bind in the strike plate if pressure is on the door when power is removed. This can occur in a panic situation when people are pressing on the door before the lock is de-energized. Some deadbolts are designed with tapered bolts to prevent binding but the reader should check the local building and fire codes before specifying this type of device for fire egress doors.

Electric Latch

Somewhat similar to an electric deadbolt is the electric latch, illustrated in Figure 11. It is also solenoid activated, mounts on the door frame, and

uses a strike plate in the door. Instead of a deadbolt, a beveled latch is used. It has an advantage over the deadbolt because the latch does not need to be withdrawn for the door to close since it is pushed into the lock mechanism against spring pressure as it rides up and over the strike plate.

Electric Strike

The electric strike operates as an adjunct to any standard mechanical lock. The operating principle is simple: electrical energy is delivered to a solenoid that either opens or closes a mechanical latch keeper or strike plate. (Note that the electric strike is not a lock but operates with a lock to hold the door closed or to permit it to be opened.) Such devices have been used for many years in apartment houses, business offices, commercial installations, and occupancies in general. Figure 12 illustrates a typical electric strike.

A typical application of the electric strike is to control passage in one or both directions. The lockset handle is fixed (i.e., will not turn) on the side(s) from which passage is to be controlled. The only means of access becomes remote, unlocking the electric strike by a button or switch within the secure space, or by an automated access control device, such as a card reader or digital keypad. If the knob or handle on the secure side of the door remains operational (i.e., it will turn), then egress can be free. If the knob or handle is fixed on both sides, egress can be achieved by the same types of devices described for access. Additionally, if the mechanical lock-set is equipped with a lock cylinder on one or both sides, the door can be unlocked with a key.

Figure 11. Electric deadbolt and electric latch.

A problem arises when electric strikes are used to control doors for re-entry from fire stairs to an occupied floor. Fire code may permit re-entry to be restricted, except at times when there is a fire emergency. At such times, code requires that doors be equipped with locks that are fail safe and unlock automatically for re-entry on a signal from the fire alarm system.

(Some codes require re-entry to be gained only on every fourth floor or

“nth” floor of high-rise office buildings.) However, the doors must remain latched to prevent from being blown open by expanding fire gases and thus permit smoke to enter the stairway. A fail safe electric strike will not keep the door latched when power is withdrawn. A solution is discussed below.

Stairtower Lock. To meet the specific fire codes related to stair doors, both for new construction and retrofit on older multi-story buildings, a special hybrid electric locking device, known as a stairtower lock, was developed. The arrangement is illustrated in Figure 13. The mechanism consists of a frame-mounted electric solenoid device that does not pro-trude beyond the frame but controls the dead-locking mortise latch bolt of a regular mortise-type door lock. Removal of power to the device releases the dead-locking latchbolt mechanism to allow the door handles to oper-ate. One of its advantages is that drilling the door for electrical wiring, which could negate the door’s fire certification, is not required.

Figure 12. Typical electric or breaking strike.

Electric Lockset

The electric lockset is simply a regular mortise lockset that has been electrified to control the ability to turn the handle. As the lock is contained within the door, the door must be drilled to allow power wiring to be fed to the hinge side. The cabling must then be fed either through or around the door hinge. Figure 14 shows an electric lockset and different types of elec-tric hinges and an armored cable door loop.

This type of electric lock is becoming increasingly popular for auto-mated access control applications. The normally fixed, unsecure side handle of a storeroom function electric lockset is controlled by an access control device (e.g., a card reader), while the secure side handle remains opera-tional at all times for unimpeded egress. Some models offer an option of a sensor switch in the lock that is activated when the inside handle is turned.

This provides a request-for-exit signal to the access control system to auto-matically shunt any magnetic switch or local horn associated with the door so that alarms are not sounded for a valid egress. It should be noted that

Figure 13. Stairtower lock.

Figure 14.Electrified lockset and power transfer devices.

this option will require the specification of a four-wire (instead of two-wire) transfer hinge to accommodate both the lock power and the sensor switch cabling.

Exit Device

Also known as a panic bar or crash bar, the exit device is commonly used on doors in the path of egress from structures with high occupancy.

Figure 15 illustrates some of the types. The rim-mounted device requires little modification to the door, as it is surface mounted. The mortice-mounted device requires the locking mechanism to be mortised into the edge of the door in the same manner as a regular mortise lock. Vertical rod devices are used on doors with double leaves where there is no fixed frame or mullion to accept a latchbolt. The rods, which move into holes or strike plates in the frame header and floor to restrain the door, can be surface mounted or concealed within a hollow door.

Exit devices can be electrified to permit remotely controlled re-entry via a push button or card reader/keypad. One special application of this type of hardware is the delayed egress locking system. Developed as a compro-mise between safety and security, this system is usually applied to doors intended to be used only for emergency fire egress. Instead of allowing immediate egress when pushed, activation of the bar starts a 15- or 30-second delay, after which the door will unlock. Special signage is required to inform users of the delay, the system must be connected to the fire alarm system, and the delay must not occur in the event of a fire or other defined life safety emergency. Although it will not usually provide enough of a delay to permit interception of an escaping thief, the system will sound a local alarm and report that alarm condition to a central moni-toring location. A CCTV camera can be used to identify the perpetrator and any articles being carried, and to record the incident.

Much controversy has surrounded the delayed egress system. Readers intending to use them should carefully check with their local building offi-cials and fire marshals as some communities require application for a special variance before they can be installed, and some communities have banned them completely.

Electromagnetic Lock

The electromagnetic lock, also known simply as a magnetic lock, utilizes an electromagnet and a metal armature or strike plate. When energized, the magnet exerts an attractive force upon the armature and thus holds the door closed. Figure 16 illustrates this type of locking mechanism. Although the lock is usually mounted at the head of the door, it can be mounted on the side. This location, while reducing the door passage width, provides a considerably more secure door.

Figure 15.Exit devices.

Magnetic locks are rated by the pounds of force required to separate the armature or strike plate from the electromagnet, and are available from 500 to 2000 pounds. Although most applications will need only a single magnetic lock installed on a door, multiple locks can be used for high-secu-rity requirements.

Shear Lock. An adaptation of the electromagnetic lock is called the shear lock. The electromagnet is concealed in the header of the door frame and the armature is mounted in a channel in the top of the door. The plane of both pieces is horizontal, in contrast to the regular type where it is ver-tical. The armature “floats” up and down on vertical pins so that it can be

“sucked” up to the magnet when energized. The face of the armature rests in an indentation in the magnet to prevent the two faces from sliding or shearing when the door is pushed. Although the shear lock is considerably more attractive than the regular magnetic lock because it is totally con-cealed, it is not very forgiving to installation variances between the door and the frame. As little as a 1/8-inch sag after installation or door distortion due to wedges being used to keep the door open will render the lock inop-erative.

Figure 16. Electromagnetic lock.

Manufacturers are continually trying to make design improvements to counteract the poor installation and usage of doors, but shear locks have had to be replaced with the regular type in a number of situations.

A very valuable feature of regular electromagnetic locks is that they involve no moving parts and are much less maintenance sensitive than mechanical or electromechanical devices. As long as the surfaces of the magnets and the armature are kept clean and in alignment, and provided there is assured electrical power, the devices will operate as intended. Bet-ter electromagnetic locks have built-in switches to monitor the bonding of the magnet and armature and to monitor the door position. These sensors are important to void the simple defeat mechanism of placing a nonmetallic sheet between the magnet and the strike plate to reduce the bonding power.

Electromagnetic locks are intrinsically fail safe because removal of power will release the strike plate. In high-security applications, backup power should be used to ensure that the lock fulfills its function in the event of a power failure. It is possible to use electromagnetic and electric bolt or strike plate locks in combination. For example, consider an area under ingress and egress access control that must remain secure against unauthorized entry but permit free egress during a defined emergency.

These types of locks are also used in delayed egress systems as described above for egress devices. A switch in the magnet senses that the door has been pushed and starts the required countdown. The same cave-ats as described above should be observed.

Access Control during an Emergency. An electromagnetic lock, used in conjunction with in and out card readers and an auxiliary electromag-netic bolt or latch, can maintain security, even during life safety emergen-cies. Normally, all ingress and egress is via card reader when the door is locked against both entry and exit, except via card readers that control the electromagnetic lock. In an emergency, the electromagnetic lock fails safe (permitting free movement in both directions). However, an electric latch or deadbolt, installed on the same door and normally unlocked, fails secure but can be retracted by turning an inside (secure side) handle or lever. This lock permits free egress but prevents re-entry from the unsecure side, even to those with approved cards, because the electric latch or bolt is not con-nected to the card readers. Upon termination of the emergency, the latch is restored to the unlocked state, the magnetic lock is restored to the secure state and normal traffic resumes. If access to the secure space from the out-side might be required during an emergency, a key-operated cylinder can be incorporated in the deadbolt or latch to operate from the outside.

Accountability for egress would be lost during the emergency mode but could be addressed by installation of one or more CCTV cameras and recorders to monitor movement through the door.

Selected Bibliography

Books

Allen, Sam; Locks & Alarms; 1984; Tab Books.

Ellis, Raymond C. Jr.; Security and Loss Prevention Management; 1986; American Hotel and Motel Association — Security Committee.

Healy, Richard J.; Design For Security, 2nd ed; 1983, John Wiley & Sons, New York (see especial-ly Chapter 9).

James, J.D.; Locks & Lock Picking: How Locks Work and How to Pick Them: A Basic Guide for Law Enforcement, Security, Military; 1987; R & R Publishing.

Mayers, Keith A.; Dictionary of Locksmithing; 1979; Mayers.

Mele, Joe A., Edgar, James M. and McInerney, William D.; The Use of Locks in Physical Crime Pre-vention; 1987, National Crime Prevention Institute, Butterworths Publishing Co., Stoneham, MA.

Roper, Carl A.; The Complete Handbook of Locks & Locksmithing; 1976; Tab Books.

Schum, John L; Electronic Locking Devices; 1988; Butterworths Publishing Co., Stoneham, MA.

Steed, F.A.; Locksmithing; 1982; Tab Books.

Periodicals, Monographs, and Standards

Burglary Resistant Electric Locking Mechanisms; Standard 1034; Underwriters Laboratories, Inc., Northbrook, IL.

Key Locks, Standard 437; Underwriters Laboratories.

Key-Locked Safes (Class KL), Standard 786; Underwriters Laboratories.

Locksmith Ledger [The]; Nickerson & Collins, Co., Des Plaines, IL.

Security Management; American Society for Industrial Security, Arlington, VA.

— Channell, Warren T.; Electronic Locks: Finding the Right Fit; January 1996.

— Cytryn, Jay.; A Primer of Locks for All Occasions; May 1985.

— Dunckel, Kenneth D.; Electronic Safe Locks: A New Current; November 1991.

— Dunckel, Kenneth D.; Safekeeping for Safe Combinations; November 1990.

— Hall, George P.; The Key to Electronic Locks; January 1996.

— Miehl, Frederick; The Ins and Outs of Door Locks; February 1993.

— Roland, Fred W.; Inns and Outs; November 1986.

— Spargo, Robert C.; Smart Locks Balance Safety and Security; November 1986.

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