Access Control: How to Configure Complex Door Interlocking Systems

Aug. 5, 2014
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.

6. Determine how users will access controlled doors: When card access is used with a door interlock, the card reader lock relay should be used as a request-for-access input to the interlock. A valid card will activate the lock relay for a preset time. If the related doors are secure, the interlock will unlock the door following the state of the request-for-access input. Multiple requests-for-access are held pending until the door can be unlocked or the access request times out.

7. Keep in mind automatic doors & gate openers: Automatic pedestrian doors require special considerations. Automatic swing doors are typically powered open and spring closed with a built-in time delay and various safety sensors. Automatic sliding doors are powered open and closed, plus they incorporate mechanical hardware to facilitate an emergency egress. Sliding doors are best specified with factory-installed electric locking hardware. When locks are used on automated doors, care must be taken to ensure that the door lock is disengaged before activating the door opener.

Automatic gates for vehicles use vehicle detectors and incorporate some logic to keep the gate open until the vehicle has cleared the gate area. Vehicle gates and overhead doors are power-open and power-close using a 3-button control for open, stop and close. The best designs use the interlock controller to disable the open signal when the door should be inhibited but allow operation of the close and stop functions at any time.

8. Provide user feedback: Traffic lights for normally unlocked doors can use lock relay contacts to operate LED indicators, as the lights follow the lock status. Normally locked doors should have lights that indicate when doors may be accessed requiring separate control relays for the door lock and the traffic signals. Some systems have used three colored traffic lights (red, amber and green) requiring two traffic light relays per door in addition to the lock control relay.

Traffic lights can be super bright LEDs on wall switch plates or may be illuminated caps of the push-button switches. Most bio-labs use waterproof push button switches with an LED indicator built into the mechanism for periodic wash-downs. The LED operation is independent of the switch action to indicate door lock status or door availability. Some touch-sensitive Piezo switches also incorporate an independent LED indicator and an adjustable time delayed output.

9. Determine the type and model of the interlock controller: The number interlocked doors will determine the model of interlock controller. Additional devices to be monitored and/or controlled, plus special timed functions, may require additional inputs and outputs. Be sure to discuss these requirements with a factory engineer to ensure that all operations can be satisfied with the specified controller.

10. Don’t forget emergency override egress: In an emergency, the fire alarm relay on the lock power supply can be used to unlock maglocks and failsafe strikes. Other override controls include a push-button with a time-delayed relock to override the door interlocks, or a custom PLC-controlled sequence of operations as specified by the owner or consultant.

11. Settle on standard or customized door interlock controls: Discuss your required operation with your vendor for locking hardware suggestions and PLC programming recommendations. Dortronics, for example, offers a number of standard “canned” operations in addition to an extensive library of custom programs that can be modified to a customer’s requirements.

Bryan Sanderford is the National Sales Manager for Dortronics Systems. To learn more about the ins and outs of PLC-based interlocks, call or email Dortronics a request for the Mantrap Solutions — Selection Guide for Interlocking Door Systems. To request more information about the company, please visit