Electric Strike Basics
To understand the electric strike, one first must understand the term, “strike.” In door hardware context “strike” means a metal plate or assembly that is installed into or onto a door frame on the lock side to receive a latch or bolt and hold it secure. When locking a pair of doors with a single point lock, the strike is installed on the inactive leaf – that is, the door that is locked in place with flush bolts. When locking a pair of doors with vertical rod exit devices, the strikes are installed into or onto the header and threshold, or sometimes just the header.
At right is a picture of a standard ANSI (non-electric) strike in a hollow metal frame. When the latch contacts the lip of the strike it is forced back into the door, allowing the door to close. When the door is fully closed the latch drops into the "D"-shaped hole, securing the door. One would say that the door is then positively latched.
The term, "positive latching" relates to compliance with fire code in the sense that fire rated doors are required to be equipped with positive latching devices, that is, locks that latch positively. Since the electric strikes you may install must comply with fire code, this is an important concept.
How Electric Strikes Work
Electric strikes are not called "electric door openers" because they do not open the door. They only allow a person to open the door. The force that opens the door comes entirely from the person.
An electric strike holds the latch, just like a non-electric strike, but can also electrically release it. I have included a couple videos showing how a couple of different electric strikes release a latch. The first example is a cylindrical lock on a hollow metal door and frame released by a classic style electric strike in which the keeper swings out like a door; the second application is a mortise exit device on an aluminum storefront door and frame. The electric strike in this application has twin keepers that swing out together like a pair of doors.
Electric Strike Anatomy
An electric strike is a simple mechanism. Above is an illustration of a classic, single-keeper electric strike showing its visible external parts when not installed.
- The Faceplate - has mounting holes in it through which screws are inserted to fasten the electric strike to the door frame.
- The Keeper - is a movable cavity into which the latch projects when the door is closed. The keeper pivots outward when the strike is activated, releasing the latch.
- The Lip - bridges the gap between the faceplate and the edge of the door frame and provides a path for the latch to enter or exit the electric strike.
- The Body - contains the internal electrical and mechanical parts of the electric strike.
- Height and width dimensions
- Round or square corners
For installation in wood door frames a larger faceplate with square corners is recommended. For hollow metal frames, like the electric strike in my illustration above, the faceplate should be 4-7/8 inches tall by 1-1/4 inches wide with square corners. For aluminum frames a round corner faceplate is generally called for, and there are a variety of sizes to accommodate various applications.
For typical installation of an electric strike to release a cylindrical lock in a hollow metal door and frame will need to be at least 1/2 inch deep in order to accommodate a standard 5/8 inch deadlatch, given a standard 1/8 inch gap between the frame and the door.
For the cylindrical lock application described, the keeper will be centered in the strike (as pictured above). For installation with a mortise lock, the strike will need to have an offset keeper, because with a standard mortise lock prep the hole for the latch in its non-electric strike will be offset (as shown in the illustration below).
Electric strike lips come in different lengths for different applications. The lip must extend far enough to reach slightly past the surface of the door frame. A standard electric strike has a lip that protrudes slightly from a hollow metal frame. For other applications, such as a center hung glass and aluminum storefront door, an electric strike would need an "extended lip." Lip extensions are available for many strikes to accommodate these variable conditions.
For installation in a pair of doors, a shorter lip is not required, but it does make for a much more aesthetically pleasing installation.
Electric strike bodies are designed for specific applications. Most electric strikes are designed for hollow metal or aluminum doors and frames. For wood doors and frames, strikes are available with different body designs that leave more room in the prep for the mounting holes. If the screws are too close to the body cutout in a wood frame, the wood may split, weakening the installation.
In a pair of wood doors in which the strike will be installed in the inactive leaf, there is little room for error. It is best to get an electric strike that is designed for this specific application.
Electric Strike Applications
Although I write about this in more depth elsewhere, the simple rule is:
The electric strike you choose will depend upon the lock you use.
Therefore when you go shopping for your electric strike, the first thing you need to know is what lock you will be releasing. The second thing you need to know is the characteristics of the opening: it's composition and dimensions. An electric strike application is made up of the type of lock and the type of opening.
Here are some typical electric strike applications:
- Cylindrical lock, hollow metal door and frame
- Mortise lock, aluminum storefront door and frame
- Rim exit device, hollow metal door and frame
- Rim exit device, aluminum storefront door and frame
- Mortise lock, hollow metal door and frame
- Mortise exit device, hollow metal door and frame
For more information, please see my articles on cylindrical locks, aluminum storefront doors, exit devices and mortise locks.
Electric Strike Pitfalls
The most common (and expensive) mistake made when specifying electric strikes is to specify them for use on interior stairwell doors because in most jurisdictions stairwell doors must be unlocked and positively latched when the fire alarm has been activated and/or when the building experiences a power outage. If the strike is fail secure, the door will not be unlocked. If the strike is fail safe, the door will not be positively latched. Worse, fire rated frames are only supposed to be modified in a fire rated shop, so if you have cut electric strikes into the door frame the field, the AHJ (authority having jurisdiction) would be fully within their rights should they order you to replace all the frames, in addition to removing all your electric strikes. You can see how this very rapidly might add up to a big loss.
Another electric strike no-no is vertical rod exit devices. There is no law against this, but the application works so poorly it is hard to justify. First, the bottom rod must be removed from each exit device to be released by an electric strike. With the bottom rod removed, when you pull on the door you will probably be able to pull the door out a couple of inches. Add to that the electric strike which will also add a certain amount of mobility, and you've got a door that feels very insecure. And if later the door sags a little over time the door will not be secure at all.
© 2014 Tom Rubenoff
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