I went to see an architect on Friday, for what I thought would be a 2-hour meeting to discuss the security requirements for a new project.  3 1/2 hours later (time flies when you’re talking about hardware!) I emerged to the sunlight (and the parking ticket), after literally resorting to cheerleading to get the architect through one more floor of the building (Her: “Lori, my brain hurts.”  Me: “Come on!  You can do it!!”).

I’ve worked on courthouses, mental health facilities, museums, public safety complexes, and this building probably has the most security on it that I’ve ever seen.  Along with two types of card readers there are fingerprint readers, vascular scanners, and security interlocks with all doors controlled by guards. Whenever you have this level of security, you have code issues.  Luckily, the security consultant was aware of the code requirements so we were able to work things out without any bloodshed.  While it’s on my mind, I thought this would be a good time to talk about interlocks, also known as “mantraps.”

An interlock is a series of doors meant to control access and egress either for security or as an airlock.  In a clean room or lab, for example, occupants would enter a vestibule  through one door, wait until that door closes, and then continue through the next door.  This prevents air-flow directly into the clean room.

In an interlock used for security purposes, the doors work the same way but instead of preventing air-flow the interlock is controlling the movement of people.  Interlocks are common when there’s a cash room, so someone can’t grab the cash and easily make their escape. The video below illustrates how an interlock works:

Electromagnetic locks are typically used as the locking devices in an interlock, along with a special controller that sequences the locks.  When one door is open, the other door is locked and can’t be opened until the first door closes.  This application gets even trickier when you add card readers or other access control devices, and sometimes automatic operators.

Because the position of one door controls the locking/unlocking of the other, interlocks can be an egress problem.  If you’re in a lab and the only means of egress is through the interlock, you would not be able to open the door to the interlock if the outer door is propped open.  The interlock can be made safer by using emergency over-ride buttons which unlock both doors (and sometimes sound an alarm), and fire alarm contacts which will unlock the doors automatically during a fire alarm.

I have not seen anything pertaining to interlocks in the International Building Code or the Life Safety Code, although I did read something in one of the Canadian codes and I’ve heard rumors that there might be something in the code used in New York City.  But because interlocks are usually located in areas that are not occupied by the general public in buildings that have special security/air-flow requirements, the facilities are sometimes granted a variance by the code officials.

If you need help specifying hardware for an interlock, drop me an email and I’ll connect you with a consultant in your area.

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