Door interlock systems for airlocks, mantraps and cleanrooms

What is a door interlock system?

An interlocking door system uses a vestibule between the secured area and the unsecured area, with a lockable door between each area and the vestibule. The doors are linked so that while one door is open or unlocked, the opposite door cannot be unlocked. Depending on what it protects, the arrangement is called an airlock, a mantrap or a sally port.

An authorised person must pass through one door and close it behind them before they can unlock the second. If someone forces their way into the vestibule while the first door is unlocked, they can go no further until an authorised person releases the second door.

How an interlocking door system works

1. 1

   Card at door 1

   The reader outside door 1 checks the card. Door 1 unlocks only if door 2 is shut: while door 2 is open, its door contact holds the door 1 reader disarmed.

2. 2

   Into the vestibule

   The person steps in and closes door 1. While door 1 is open, its door contact disarms both readers on door 2, so nothing on the far side will release.

3. 3

   Card at door 2

   With door 1 closed, the reader on the vestibule side of door 2 is armed again and a valid card releases door 2 into the secure area.

4. 4

   Out the same way

   The reader on the secure side of door 2 lets people back into the vestibule, and a push-to-exit button in the vestibule releases door 1. Norpass3 logs every card at every reader.

Because the interlock is wired in hardware, Norpass3 needs one setting: the door 2 controller set to single door operation. Access levels, time periods and every other feature are set up as for any other door. The wiring diagram is in application note AN-0507, listed under Documents.

Airlock and mantrap door systems

The wiring is the same whatever the vestibule is for. What changes is what the interlock is protecting, and so which extras the site needs.

A mantrap that must pass one person at a time needs a detection device in the vestibule, so the inner door stays locked unless only one person is inside. Optical, body-heat and weight sensors are all used for this.

Leaving the secure area: reader, button or handle

Exit through door 2 can be by card reader, push button or door handle, depending on how much the site needs to know about who has left and on the lock type.

Door interlock hardware

• Door controllers, one per door. The CRC221-1 reads in and out on a single door, holds up to 65,000 cards and connects to Norpass3 over TCP/IP or RS485. A CRC220 controller also works once set to single door operation. Both are designed and manufactured in South Wales.
• Readers: one outside door 1, and one each side of door 2 when exits are logged. The wall switch proximity reader fits a standard UK back box; the mullion mount proximity reader is rated IP65 for exposed doors.
• Door contacts: a changeover door contact on each door reports whether it is open and disarms the readers on the opposite door.
• Locks: the application note shows 12 V or 24 V DC maglocks. Each lock must be rated at no more than 30 V and 1 A, with suppression fitted at the lock; without it the relays and the controller's memory can be damaged.
• Exit devices: a push-to-exit button in the vestibule for door 1, or the touch-free proximity free exit device, which fits a standard UK back box and detects a hand at 3 to 20 cm.
• Software: Norpass3, licence-free, on any Windows PC.

Fire alarm release and power failure

Each CRC22x controller has a fire alarm input. Wired to the building's fire alarm, it releases the door that controller holds, so with both controllers connected, both doors of the airlock release and nobody is held in the vestibule. Maglocks release when their power is removed. Agree the release arrangement with the fire alarm installer and the site's fire risk assessment before commissioning.

Managing interlocked doors in Norpass3

Norpass3 treats each interlocked door as a normal door, so its security features carry over. The ones most relevant to airlocks and mantraps:

• Card & PIN on the inner door, set per door, for a second check before the secure area
• Buddy mode, which needs two valid cards before the door releases
• Anti-passback, so a card can't be handed back through the airlock to a second person
• Key-personnel counting and optional RollCall mustering, showing who is inside the controlled area
• Door alarms when either door is forced open, reported to the Norpass3 PC

The controllers hold the card list and store up to 1,700 events themselves, so the doors keep working if the network or the Norpass3 PC is down, and the events upload when the connection returns.

What affects the cost of a door interlock system

• The number of readers: logging exits adds a reader on the secure side of door 2
• The locks and door hardware, and whether the doors themselves need replacing
• Occupancy detection in the vestibule for a one-person mantrap
• Cabling between the doors, the controllers and the network, and interfacing to the fire alarm
• Whether the doors join an existing Norpass3 system or need a new one. Norpass3 is licence-free, so there is no software licence to buy either way

We supply through installers and distributors, so the price comes from the installer after a site survey. Our sales team can help put the equipment list together: sales@nortechcontrol.com.