RX for REX:

REX is an abbreviation used in access control for devices and circuits involved in providing request to exit functionality.

REX devices customarily are pushbuttons, exit bars or motion sensors. REX functions can also be implemented using upgrade kits designed for these applications, or by performing site specific applications engineering.

Frequently, locksmiths installing access control systems will be supplied with terminal points for the connection of REX or remote door release devices. How the internal circuitry is designed to operate may vary. It is the system designer/locksmith installer's responsibility to understand the features and operation sequence of the equipment, and to ensure that the configuration of the REX and the locking device provides the required life safety.

Typically a REX device will produce contact activations as a consequence of someone attempting to egress through a door. These contacts are then used to signal and switch various system operations.

REX devices are used for three primary reasons. First, access control systems may control doors which are also being monitored for security management, to indicate exception conditions on remote monitoring equipment.

“Forced Door” conditions occur when the controlled door is opened without a valid card to gain entry or without a REX to egress. “Propped Door” or “Door Ajar” conditions occur when the controlled door remains open, unlocked or ajar for longer than the system is programmed to allow.

Second, access control systems may utilize electrically actuated bolts, electromagnetic lock, shear locks, or a combination of locking devices which do not provide an integral means of manually unlocking the door to allow free egress.

For the majority of applications, building and Fire Codes will mandate that any door along the path of egress (and most internal doors) allow free egress at all times.

There are exceptions, but these exceptions generally apply to institutional applications (such as detention facilities or psychiatric wards), government high security and special locking arrangements (such as delayed egress).

Suitability of an opening for anything other than Free Egress will always be ultimately determined by the AHJ (Authority Having Jurisdiction). There may be times when more than one AHJ may be involved, in which case conflicts between them have been known to occur, as each may interpret the code differently.

The bottom line is that for those situations where the electric locking device does not have any integral means of unlocking it for free egress, the REX components are critical elements in the system design.

An example would be where an access control panel is used to control a door equipped with an electromagnetic lock. Although connecting a normally open momentary switch to the REX terminals on the access controller could be expected to trigger the door output on the access controller to unlock the electromagnetic lock for the pre-programmed unlock time, this is not considered a safe installation.

Most approved REX devices are available with two sets of output contacts, and when configuring access controls for doors using electromagnetic locks, the second set of contacts should be wired in series between the door output and the locking device. In this way, as long as the button is depressed, the electromagnet is unlocked, even if the access controller output fails to release the lock. A failure in the access controller, a failure in the REX switch, or a broken wire could all prevent the REX button from triggering the access controller.

All possible failure modes should be carefully considered, and countermeasures designed into each system you install. As a security professional, you should always strive to achieve the safest system possible, rather than one that passes the AHJ's inspection, or meets the client's budget.