Best practices for selecting and installing door interlock technology

In its simplest form, a door interlock system is composed of two doors electronically connected so one cannot open until the other has closed.
Photos courtesy Dortronics Systems Inc.

Airlock control system
This is a low-security system used only for environmental control. Operation is simple: all doors are normally unlocked, and opening any door will lock the related doors so only one door in a designated area can be accessed at a time. This provides easy ingress or egress to an area without destabilizing the environment of the adjoining areas, whether these are interior rooms or the exterior of the building. Airlock control systems are the least secure type of door interlocking system since all of the doors are always open, and two or more doors can actually be opened simultaneously. Since these are not for security, airlocks typically use red and green lights as a visual indication to control traffic. If the airlock is in a biological-type lab setting where control of airborne pathogens is required, locks are utilized on the doors to ensure air quality is maintained.

Secure entry/free egress
A secure entry/free egress system provides a more secure entrance solution and will allow a faster method of egress through a combination of locked and unlocked doors. Exterior doors are normally secured and interior doors normally unlocked. An electronic access system controls entry from the exterior and a request-to-exit (REX) device is used on the interior of the outside door.

With this system, a valid access card will unlock the exterior door only if the interior door is closed. Unlocking the exterior door will lock the interior door. Once the exterior door is re-secured, the interior door is unlocked to allow access into the facility. In addition, opening the interior door prevents the exterior door from being unlocked.

From a practical perspective, envision an entryway into an apartment building. An access card or code entered into a keypad unlocks the exterior door allowing entry into a vestibule. Once the exterior door is closed and automatically locks, the interior door can be opened using the same credentials, or by having the individual buzzed by the resident. If the interior door remains locked, the individual in the vestibule can exit the exterior door enabling “free egress.” This type of system allows a faster egress since the interior door is already in an unlocked state and the exterior door only requires a push on the REX button to unlock.

Restricted entry and exit system
The highest level of security is provided with a restricted entry and exit system, whereby a door is unlocked by a request for access only if no other related doors are unsecured. Opening any one door keeps all other related doors locked. Restricted entry and exit systems will buffer simultaneous requests for access to prevent two or more doors from being unlocked.

A door interlock system.

Specific uses
Choosing a door interlock system will greatly depend on the facility’s specific environment. For example, banks and other financial institutions require a higher level of security than most commercial sites as a result of the greater risk associated with the sensitive nature of their business and the value of the assets on their premises. Traditionally, these organizations rely on card access and video surveillance systems to provide a heightened level of security, but it is becoming more common for them to also incorporate multiple door interlock systems.

Paired with a metal detector, an interlock system could provide the ability to automatically lock doors, preventing visitor access, if a possible concealed firearm is detected. In this type of application, an interlock door system provides unrestricted access to the interior vestibule, where customers then pass through a metal detector before entering the inner lobby through a locked door. Access to the interior is only allowed when the exterior door is closed, and no metal is detected. If both conditions are met, the exterior door is locked, and the visitor is allowed to enter through the interior door. Should the visitor be deemed suspicious, an alert sounds, and the individual is denied entry to the lobby and may only exit to the street.

Door interlock systems are also commonly used in cleanrooms. Unfortunately, many cleanrooms cannot be easily expanded or reconfigured, while remaining in compliance with the mandatory industry standards. Many modular cleanrooms are not supplied with door interlocks and must be retrofitted in the field. As a result, cleanroom manufacturers benefit from utilizing door interlock controls as an option for new and existing installations as a means of addressing these changing conditions.

The addition of door interlocks not only assures the cleanroom meets the necessary International Organization for Standardization (ISO) classifications by enforcing clean air control, but can also add a level of security within the controlled environment of the cleanrooms.

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