The electric strike provides remote electrical control to unlock a door. The electric strike was originally patented in the late 1880s so New York City high rises could have a locked common entry door, while residents could grant access and physically unlock the door from inside their apartments.
Today, there are a number of different styles of electric strikes. The basic style is the recessed electric strike mounted into the doorjamb at the location corresponding to the centerline of the lock's latch.
The standard recessed mounted electric strike appears as a modified strike plate with a strike lip and an attached case. Instead of a fixed opening for the latch to enter and become trapped, a movable jaw or keeper is in the front of the strike faceplate. As the door closes, the latch slides along the electric strike's lip. The latch retracts as it slides over the movable jaw that is in the locked condition. As the latch passes over the jaw it extends within the opening in the electric strike, becoming trapped. The strike opening will usually be either one-half of an inch or deeper to accommodate the different length latch/bolts.
Once the latch is within the opening, the straight edge of the extended latch cannot slide back over the jaw. The door can be opened in one of two ways -- retract the latch by operating the knob/lever or electrically release the movable jaw. When electric strike is remotely released, the jaw tilts out of the way as the door is opened by having the latch slide out of the electric strike in the extended condition.
Most recessed electric strikes are designed for a 1-3/4" thick door in a standard jamb. When installed, the edge of the electric strike lip is flush with the edge of the jamb. This enables the lock's latch to use the lip as a ramp to slide into the electric strike without contacting or damaging the jamb material. To accommodate thicker doors and larger jamb sizes, some electric strikes have optional lip extensions, available in different sizes. Lip extensions for fire-rated electric strikes must be factory installed.
The internal mechanism for controlling the operation of an electric strike is different for most manufacturers. Most are controlled by a solenoid, operating a mechanism that controls the jaw or keeper. For the purpose of this article, I will use Adams Rite electric strikes as the examples and illustrations.
The Adams Rite 7100 Series is an ANSI/BHMA Grade 1 electric strike designed for operation with cylindrical latches that have a 1/2" to 5/8" projection. The 7100 Series electric strike has a 1" x 3-3/8" x 1-5/8" deep zinc-aluminum alloy case. The strike lip is designed for operation with a 1-3/4" thick door, when installed into a standard jamb. The faceplate is 1-1/4" x 4-7/8" to fit into an opening designed for an ANSI strike plate. The faceplate has radiused corners to accommodate aluminum and wood installation. The radiused corners can help prevent splitting of the wood or cracking of an aluminum jamb.
The AR 7100 Series electric strikes are available with 12, 16, or 24 volt AC and DC solenoids. The voltage and duty of the solenoid determine the operation of the electric strike. A solenoid is the housing that surrounds a coil of wire around a metal rod. When the solenoid is powered, the rod moves in one direction. When power is removed, the rod moves back to the original position. The difference between an AC or DC solenoid operating on 12, 16 or 24 volts is the number of windings. In addition, there is a difference between an intermittent duty and a continuous duty solenoid. An intermittent duty solenoid is designed to be energized for no more than 30 seconds. The intermittent duty solenoid is used in a fail-secure electric strike application. A continuous duty solenoid is designed to be energized without break or interruption. A continuously duty solenoid can be used in a fail-safe or fail-secure electric strike application. A continuous duty electric strike requires less power than an intermittent duty electric strike.
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