Access Control: How to Configure Complex Door Interlocking Systems

11 steps to help avoid pitfalls when designing and bidding multi-door mantraps

Door interlocks are being used more frequently for facilities requiring a higher level of security, such as bio-medical facilities built to serve the healthcare industry. While some projects use detailed specifications developed by the owner from prior sites, others may supply only vague requirements that the integrator must develop into a complete engineered system.

While simple two-door, one-room mantraps may be familiar to some integrators, the complexity increases dramatically as more related doors and connecting rooms are added. When automatic door operators, card access systems, etc., are interfaced with the door interlocks, care must be taken to ensure compatibility, and all of it must satisfy local building codes.

Security dealers and consultants should use the following detailed checklist — developed by Dortronics Systems — to design both simple and complex door interlock systems. With this checklist, many potential problems can be avoided; in addition, Dortronics engineers will work with you to suggest door hardware and accessory devices and recommend operating features.


Door Interlock Checklist

1. Define the primary purpose: Some mantraps are for security in applications such as banks, casinos or government sites. These typically incorporate card access and require that the doors be normally locked. Environmental control interlocks frequently use normally unlocked doors. Bio-medical and pharmaceutical projects, on the other hand, may use a combination of locked and unlocked doors to provide a higher degree of security and isolation but using some normally unlocked portals to facilitate faster traffic flow.

2. Determine door relationships: The next step in designing a door interlock system is to determine the traffic pattern and the door relationships. A door matrix chart can indicate which doors are inhibited when each of the controlled doors are accessed. A floor plan with each door numbered and each room labeled will easily illustrate the required logic. Shared doors between rooms and other doors in these rooms must all be controlled by a common interlock controller.

3. Verify normally locked or unlocked doors: The most common door interlocks are for two doors, both normally unlocked and used as an air lock. Opening either door causes the other door to lock, preventing it from being opened while the first door is open. Note that door switch contacts are usually not hardy enough to switch lock power — resulting in an early door switch failure. If two or more normally unlocked doors are interlocked, it may be possible to open both doors simultaneously, as the interlock controller will not see a door opening in time to prevent the other doors from being opened.

4. Monitor controlled doors: Door status switches are typically used to monitor mantrap doors; however, card access systems also require a door status input and they cannot share the same contact as the mantrap control. There are ways around this — either use two door position switches or a DPDT switch to isolate the two system circuits or “mirror” the door switch through a PLC relay output for the access control.

5. Select electric locking hardware: Maglocks and strikes are often used to secure swing doors. Clean rooms that require periodic wash-down need electrically-sealed locks and actuating controls — usually maglocks, although electrified locksets are sometimes used. Casinos frequently use a combination of both fail-safe maglocks and fail-secure strikes in the money counting areas. In the case of a power failure with maglocks de-activated, the doors remain secured by the strikes and mechanical locksets. Electric locks specified must be within tolerance of the power supply and control relay contacts. Detention locks and some electrified door hardware may require higher voltage power or a large current draw that could exceed the control relay rating.

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