Selecting the Right Electric StrikeBy: Brian Rhodes, Published on Jan 22, 2013
Despite being one of the most common components of access control, specifying the right electric strike can be deceptively complex. Understanding the particulars of each device can be overwhelming. In this note we describe which characteristics of strikes are most important and how to select the right one.
Strikes are basically moveable portions of the door frame, consisting of 3 main components, shown to the left:
- The Strike Box contains the internal components to the strike that sits inside the frame. Electric strikes are typically driven by one or more solenoids, either directly or via a simple geared carriage inside the box.
- The Strike Plate affixes the device mechanism to the frame and is responsible for the proper alignment the device in relation to the door locking hardware.
- The Keeper is the component of the strike that moves. When 'locked', the keeper is rigid and forms a positive stop - interfering with the latch to prevent opening of the door. When 'unlocked', the keeper swings out of the way of the latch and allows the door to open.
Unlike other types of hardware, strikes do not replace or improve the existing hardware mounted on a door. In fact, a strike totally relies on mechanical door locks for securing the door. The strike simply allows for the locking hardware to remain locked and still gain entry through the opening.
Fail Safe/Fail Secure: Unlike maglocks that 'fail safe' on power loss, strikes can be configured to 'fail secure' - meaning the keeper remains rigid regardless if they are powered or not. In order to preserve the life/safety compliance of the opening, the door hardware must accommodate free egress in an emergency. Whether through panic bars/exit devices, lever sets, or even latch sets, egress doors cannot be locked to prevent escape, and hardware must take no more than one, intuitive action to open the door.
AC or DC powered: Most modern strikes either allow selecting between AC and DC or available in AC or DC versions. All strikes are low-voltage, with either 12 or 24 volts standard. Final polarity and voltage selection depend on several design criteria:
- Noisy vs. Quiet: Due to differences in the way solenoids handle power types, AC electric strikes make a characteristic 'buzzing' noise when operating, while a DC model is quieter (or even 'silent'). In the past, solenoid reliability was tied to polarity types, but modern strikes can be purchased to exceed 'Grade 1' reliability, leaving polarity choices subject to matching existing power supplies or tolerated noise.
- Battery Backup vs. Low Current Draw: Since 12 VDC 'backup' batteries are common to many electronic systems, they are an inexpensive option for backup power compared to 24 volt cells. However, the current draw of 24 volt strikes is typically lower than 12 volt versions, so if multiple devices are to be powered from a single source or if overall energy consumption is a concern, 24 VDC devices are a popular choice.
Integrated Readers: A newer trend in strike design is integrating a proximity-style reader into the device and running both power and communication bus down a single cable. This integration allows a more streamlined installation and offers a less intrusive install by combining the reader and strike into a single unit. Variants exist that include 'read-in' and 'read-out' capability, with the unsecured side reader being connected through the frame or wall assembly via bluetooth.
Door/Latch Monitoring: A common option for many strikes is a latch monitor, or a simple contact switch that detects when a latch bolt is being contained inside the strike. This sensor also functions as a defacto 'door position' contact, since the door must be shut for the latch to be present. While not all strikes feature this switch, it usually does not add significant cost to the device but greatly enhances the reporting function of the device.
Selecting the right type of strike includes considering the 'form factor' of the device. Strike are available in two common types:
Mortise: The most common type of strike is the mortise variety, which typically requires a cutout in the frame for install. Mortise Strikes are used when the door locking hardware is a mortise or cylindrical lock - or any other lock whose latch is retracted into the door leaf during operation. Since the bolt protrudes into the frame, a mortise strike's keeper replaces a portion of the reveal. As we will cover in the installation section, installing a mortise strike requires the strike to be installed deep into the frame, often requiring frame material to be precisely cut away for a clean fit.
Surface: These types of strikes are used when the companion locking hardware is a 'rim style' device, meaning it is mounted to the inside surface of the door [link no longer available] rather than inside the door. Common 'rim' devices are 'exit devices [link no longer available]' or surface deadbolts. Even though a portion of the device protrudes on the 'door rabbet', the strike box may still require an additional cutout in the frame for proper mounting.
Strikes Vs. Other Hardware
The relative value of strikes compared against other electrified hardware types
- Inexpensive: Strikes are among the least costly electrified devices, with Grade 1 quality devices selling between $100 - $300. Compared to locks like maglocks that range between $400 - $ 1000 per unit. Even when considering the installation labor, the cost of a strike is between $175 and $375 per door.
- Reuses Existing Hardware: Another benefit of strikes are they are specified to work with door hardware already in use. Not only does this reduce the 'hard cost' of buying more locks, it saves on the 'soft costs' attributed to re-keying, installation labor, and redistribution of mechanical keys.
- Energy Efficient: Unlike maglocks that require a steady impulse of electricity to operate, a strike uses only intermittent impulses to operate. While the operational amperage is relatively small for either device, when multiplied over tens or even hundreds of doors, using strikes can cut hundreds of amps from a facility's electricity consumption.
- Wear: Unlike maglocks, strikes cannot be installed once and operate for years without attention.Since electric strikes have moving parts, they can wear or break over time. Components like load springs and solenoids require periodic maintenance attention.
- Adjustment is Vital: Since the strike is totally dependent on the door's locking hardware for security, the relationship between strike and latch is vital and the tolerances for movement are limited. Even during the course of normal operation, door hinges sag, door frames shift, and door latches fall out of throw range of the strike. Anywhere strikes are used, a companion door maintenance problem is vital to guarantee security.
- Extra Hardware: In some cases, additional hardware is required to protect the strike. Take the example below, where an exposed exterior strike is vulnerable to outside tampering unless additional 'latch protectors' are installed on the door:
Installer Skill Required
Properly mounting a strike takes considerable skill in precision measurement and often requires cutting metal. While the overall installation process is typically straightforward, the installer's craftsmanship and trade skills determines more than cosmetic quality - the operation of the device is affected as well.
For example, while frame cuts are simplified by the use of mounting templates included with the device, the position of the template is subject to accurate measurements and assumptions that the frames, doors, and existing hardware are square and properly mounted.
Successfully mounting a strike free of problems like "preloading [link no longer available]" (often caused by misalignment and warping of the door) requires the installer to apply a skillful eye to the door condition before beginning work, and be prepared to correct structural problems by shimming hinges or even replace a warped door leaf. Will will examine the problems, and their corrective actions in an upcoming report on "Maintaining Electrified Hardware".