Electric Strikes: How, Where And Why?

March 4, 2011

As locksmiths, we control access. We install locks to keep people out and key those locks with different levels of security to permit only those authorized to gain entry. We install mechanical or electromechanical locks in order to accommodate the security and convenience requirements of our customers. For convenience, electromechanical lock hardware provides one feature that is not possible with mechanical lock hardware: the ability to remote release a locked door in order to provide access.

There are three basic methods of electromechanically locking and unlocking a door - the electrified mechanical lock and lock hardware, electromagnetic lock and electric strike. Depending upon the manufacturer or provider, electrified locking products can be available with different types of monitoring and/or release switches.

The electrified lock and lock hardware usually operates as an access control standalone as it can be unlocked and locked both using a key and electronically. Electrified locks incorporate either a solenoid or motor that controls the latch bolt mechanism.

Considerations when ordering an electrified lock are lock finish and function (usually Storeroom), voltage (usually 12 or 24 volts/AC or DC) and operation Fail Safe or Fail Secure. Door and lock monitoring/switches can include a REX (Request-to-Exit), Door Position Switch (DPS), Latch Bolt Monitoring (LBM) and Dead Bolt Monitoring (DBM).

Important: Before purchasing and installing any wired electromechanical hardware, contact your Local Authority Having Jurisdiction (AHJ).

Electromagnetic lock installations will operate either as standalone, having pull and push handles on the door or in conjunction with electrified mechanical lock hardware in order to electronically release both of the locking mechanisms to gain access. For standalone operation, there will usually be some type of bolt lock that provides an additional layer of security during non-open hours.

A dual locking example could be a door equipped with an electromagnetic lock and an exit device equipped with electrified trim. For simplification, the exit device trim is configured to Fail Safe operation. With the single action of power being removed, the magnetic lock will release the armature and rotating the outside lever will retract the exit device latch, permitting the door to be opened.

When ordering an electromagnetic lock, take into consideration the number of doors and if the doors swing (direct pull), are swing through or slide (shear). Once this has been determined, determine the door composition (hollow metal, wood, glass or aluminum stile), holding force, voltage (12 and/or 24 VDC), mounting method (surface or face), finish and accessories. Door and lock monitoring/switches can include a Door Position Switch, Magnetic Bond Sensor, anti-tamper switch and relock delay timer.

Delayed Egress is a special application for an electromagnetic lock. The Delayed Egress electromagnetic lock will not release for either 15 or 30 seconds (release time determined by application, codes and AHJ). The purpose of delayed egress is to prevent theft or prevent a handicapped individual from leaving an area. These locks can be installed onto back doors of warehouses, secure airport areas and especially in geriatric institutions. Delayed Egress locks must be wired into the fire panel in order to allow immediate release in case of an emergency.

Electric strikes permit opening of a door without retracting the latch or bolt. An electric strike is designed to take the place of the strike plate, securing the extended latch or bolt from a mechanical lock. Most electric strikes have a ramped surface (lip), like a strike plate for the latch to travel. At the final portion of the surface, a movable keeper, when closed, secures the latch or bolt as does a strike plate. When the keeper is released, pulling or pushing on the door causes the keeper to swing out, freeing the extended latch or bolt. The door can be swung open without using a key or turning the knob or lever.

Depending upon the electric strike, holding force varies from less than 500 to more than 2,000 pounds. The electric strike installed should be compatible with the door. For example, a 2,000-pound holding force electric strike serves no purpose when installed to control a narrow stile aluminum glass door.

Electric strikes are equipped with a solenoid or a magnetic coil that secures and releases the keeper. The difference between a magnetic coil and a solenoid is the method of operation. A solenoid wiring is like a relaxed coil spring and a magnetic coil is like a magnetic that pushes a release mechanism.

Electric strikes are available in different designs including varying body sizes, recessed and surface mount and fixed and removable faceplates. One important consideration when retrofitting an electric strike is its depth. This is the cavity made by the keeper. Depending upon manufacturer and model, the keeper varies.

ROFU Examples

For the purpose of this article, we will use ROFU Electric Strikes to discuss the choices and options available when installing an electric strike. ROFU uses six-digit part numbers plus the finish designation for their electric strikes. The first two digits are the Series. They include the 14, 15, 17, 18, 19, 24, 26, 34 and 39. ROFU electric strikes are manufactured in Germany.

The Series part numbers include the size of the electric strike body, the Keeper and the function. Function options are Fail Safe, Fail Secure and “field reversible” Fail Safe/Fail Secure having the ability to be reconfigured in the field. The first digit of ROFU’s part number indicates the function of the electric strike. The number 1 is Fail Secure. The number 3 is Fail Safe. The number 2 is field reversible.

The second digit indicates the keeper type. The keeper is the movable forward part of the strike face opening that secures the latch or bolt. ROFU offers four different keeper types. They include the 33mm fixed (5) and 44mm fixed (4), the adjustable (7 and 8), and the rim mount (9) keepers.

The 33mm keeper is for residential or light commercial applications. The keeper is nickel plated brass that accommodates up to ¾” latch. The 15 series electric strike bodies are compatible with existing products on the market.

For commercial applications, the fixed 44mm nickel plated brass keepers can accommodate up to ¾” latch. The 17 and 18 Series electric strikes have a 44mm cadmium plated brass keeper that has ¼” horizontal adjustability, and will accommodate a ½” latch.

ROFU electric strikes are also identified by whether they are equipped with a solenoid or a magnetic coil. The first two digits, for example the ROFU 17, 18, 24 and 26 series electric strikes are equipped with solenoids. The 14, 15, 19, 34 and 39 series electric strikes have magnetic coils.

Note: The advantage of a ROFU solenoid equipped electric strike is the solenoid models “cannot be surreptitiously unlocked using an ‘earth’ (permanent) magnet,” according to the manufacturer.

The third and fourth digits are faceplate sizes and shapes to accommodate different door materials and lock applications. Hollow metal and aluminum door faceplates are generally smaller than wooden door faceplates. Some faceplates are positioned at the centerline and can be reversible for a left hand (LH) or right hand (RH) doors. These ROFU electric strikes are equipped with removable faceplates. For example the “02” faceplate is used in conjunction with a cylindrical lock and the standard 1-1/4” by 4-7/8” ANSI Strike. This faceplate can be removed and a different faceplate installed. For wooden doors, ROFU offers centerline, symmetrical (non-handed) faceplates up to nine inches long.

To provide options for one electric strike that fits popular prep, the ROFU non-handed 1800 kit comes with three faceplates, the 02, 03 and 04 faceplates. They are 1-¼” by 4-7/8”, 1-¼” by 6-7/8” and 1-7/16” by 7-15/16”. This adjustable keeper, Fail Secure solenoid equipped electric strikes comes with built-in MOV (Metal Oxide Varistor) that is used to protect against excessive transient voltage.

Most mortise locks required handed faceplates as the latch is normally not along the centerline. Most handed ROFU electric strikes are equipped with a faceplate that is part of the strike body. The keeper must be positioned to not only accommodate the latch, but also not to interfere with the deadlatch retracting when the door is closed. Some electric strikes, for example the 1440/3440, have the keeper opening starting at the centerline to accommodate a variety of mortise locks.

The fifth and sixth digits of the electric strike part number indicate the strikes’ available voltages. ROFU electric strike solenoids and magnetic coils are available in AC or DC voltages and as intermittent or continuous duty. The fixed voltages are 12, 24 and 48. They are also available as 8-16 volts AC and 3-6 volts DC to accommodate non-standard wiring applications.

The current draw of an electric strike is determined by the solenoid or magnetic coil and its operation. Most ROFU electric strikes draw less than .250Amp. For example, the 18 Series Fail Secure electric strikes in 12 VAC have a current draw of .160Amp. The 24 VDC electric strikes have a current draw of .130Amps. The higher the voltage, the lower the current (Amp) draw.

Remember: An AC electric strike creates an audible buzz when powered. The buzz occurs because the alternating current is a wave pattern. The volume of the buzz varies depending upon the voltage and the solenoid or coil. A DC electric strike does not buzz when powered. A buzzer can be wired in for the purpose of notification.

An intermittent duty solenoid or magnetic coil is designed for momentary on operation only. Normally an intermittent duty magnetic coil or solenoid is used in a Fail Secure electric strike. Power is provided only long enough (usually 10 seconds or less) for a person to open the door. Continuous duty electric strikes are powered all of the time, with power removed only long enough for a person to open the door. Continuous duty electric strikes are always DC.

ROFU electric strikes are available with a radiused face plate for aluminum door jambs that have been radiused. These installations require a special lip bracket that will be supplied automatically when a radiused face plate is ordered. Radiused faceplates are indicated as RAD.

One of the features available with an electric strike is the ability to monitor the opening. Monitoring capabilities available with ROFU electric strikes include the latchbolt. The Latchbolt Monitor Switch notifies when the latchbolt is projected into the electric strike.

For more information, contact ROFU International Corporation, 10029 South Tacoma Way, Suite E-11, Lakewood, WA 98499-5199. Telephone 800-255-ROFU (7638). Web Site: www.rofu.com.