Query the Access Control Expert: Electronic Latch Activation

Oct. 27, 2008
Little Trigger Happy

Q: From the electronic hardware perspective, I've run across a situation lately that concerns me. It involves the ‘rx' switch and whether the lock relay gets activated when an RX signal comes in to an access control panel. I've been to two jobs recently where the electronic latch retraction panic was getting activated as people were exiting the door. Does every access control system on the market have an option to determine what the RX signal does? Thanks, Rick.

A: Thanks for your question. The short answer is that not every access control panel allows that the REX be configured. If you close the circuit across the REX terminals, you‘ll get the output relay to trigger for the programmed door unlock time. Many systems ask you if you want the output relay to actuate when the REX is activated. On some systems, there are additional configurable input points (sometimes referred to as alarm zone inputs) you can manipulate to meet your requirements.

Bear in mind that the REX has two purposes. One is to allow remote door unlocking (like a receptionist or from an interior telephone). The other is to permit egress.

On an exit device that lets you out by depressing the bar, a RX is not required to actuate the latch retraction. However, if your system is also monitoring door position for security, and/or to reset the unlock time, a REX trigger to the ACP would be required to suppress a forced door alarm. If you are using an electromagnetic lock, you should provide redundant means of ensuring safe egress (another important topic).

Keeping the time period the door is unlocked to a minimum after an authorized entry is obviously good security practice. Having a door reset circuit allows you to program in a long enough door unlock time to allow a full range of users to have adequate time to open the door (typical, disabled, or deliveries); but then relock the door.

In the case of latching electrical hardware, you can program the system to relock as soon as the door is opened. For doors with deadbolt type locks, you need to tell the system to wait to relock until the door recloses. Timing issues may also occur with pairs of magnetically controlled doors which support two-way traffic (another important topic).

If you're monitoring for forced and/or propped door, then the door open time and the door prop time may need to be independently controlled time constants. You can implement external timers and door monitoring devices if this functionality is required (another important topic).

As you pointed out, with a mechanical exit device, you certainly do not require or want the device to actuate on egress since you are mechanically releasing the lock anyway. If your controller does not allow it to configure the REX other than to actuate the lock relay and the system is monitoring door position, you could connect the REX in your exit device to a separate timer relay and the contacts from that relay across the DPS terminals in the access control panel.

Set the time for enough time for a typical egress. You can get more sophisticated and integrate the timer with the door position sensor so that the relay resets as soon as the door recloses, thereby preventing piggy-backing. If the system is not monitoring door position, simply disconnect the REX in the exit device from the ACP.