Door Interlocking System: How Airlock Doors Work

A door interlocking system links two doors so that only one can open at a time. Walk into the space between them, the first door closes behind you, and only then does the second one open. It is how clean rooms keep contamination out, how laboratories hold their air, how secure lobbies stop tailgating and how windy entrances stop the street blowing straight into the building.

This guide explains how a door interlocking system works with automatic doors, where it is used in South Africa, what it costs to get right, and the safety rules that matter most. MC Automations supplies, installs, commissions and services interlocked automatic entrances. Our MC-125 sliding door operator supports two-door interlocking for airlocks and controlled lobbies.

What a door interlocking system is

A door interlocking system is a set of controls that watches the state of two or more doors and refuses to open one while another is open. The space between the doors is called an airlock, a vestibule, an interlock lobby or, in security work, a mantrap.

The principle is simple. The doors are never open at the same time, so there is never a straight path for air, dust, contamination or people from one side to the other. Everything that passes has to stop in the airlock first.

• Door A open: door B stays closed and locked or held.
• Both closed: either door may open on request.
• Door B open: door A stays closed.

Everything else in a door interlocking system, the timers, sensors, indicators, access control and emergency override, exists to make that rule reliable and safe.

Where a door interlocking system is used

The same principle serves very different purposes. Knowing which one you need decides almost everything else.

• Pharmaceutical and clean rooms. Keeping particles and pressure where they belong between rooms of different cleanliness.
• Laboratories. Containing what is inside and keeping the corridor out.
• Hospitals. Isolation rooms, theatres and sterile areas, often with hermetically sealed doors.
• Food and production areas. Separating clean and dirty zones and keeping pests and dust out.
• Security lobbies. Banks, data rooms, embassies and controlled sites where one person at a time is checked before the inner door opens.
• Windy and air-conditioned entrances. A vestibule with interlocked automatic doors stops the wind and holds the conditioned air on exposed lobbies.

A pharmaceutical door interlocking system and a security mantrap use the same logic but are specified, tested and signed off by very different people. In clean areas the site’s validation or quality team sets the requirements. In security work it is the security adviser. In every case, how the doors behave in an emergency is set by the fire strategy.

How the interlock knows a door is closed

A door interlocking system is only as good as its knowledge of whether each door is truly closed. There are three common ways to know.

• The operator’s own position. An automatic operator knows where its leaf is. The MC-125 supports two-door interlocking directly, so two sliding doors on MC-125 operators can be linked without a separate controller in many layouts.
• A door contact. A magnetic contact on the frame confirms the leaf is physically closed, independent of the operator.
• The lock. An electric lock with a status output confirms the door is closed and locked.

Where the doors are swing doors, a mix of sliding and swing doors, or more than two doors, the door interlocking system usually runs through a dedicated interlock controller or the access-control system, wired to each door’s lock and operator inputs. We design that per project rather than assume a standard layout. The MCW-100 swing door operator has an access-control input, an electric-lock connection and safety-sensor inputs for this kind of integration.

Sliding or swing doors in an airlock

Sliding doors are the usual choice for an automatic door interlocking system. They open without sweeping into the airlock, so a person, a trolley or a bed does not have to step back while the door moves. That keeps the airlock short.

Swing doors need room in the airlock for the leaf to sweep, which makes the airlock longer, and people inside have to stand clear of the swing. They are used where the existing doors must stay, where a sliding leaf has nowhere to park, or where a door has to carry a rating a sliding door cannot. Our sliding vs swing door guide explains the trade-offs.

In clinical and clean areas, the doors themselves are often hermetic sliding doors, which press onto seals when they close. A door interlocking system then controls which hermetic door may open. We do not publish laboratory certification or pressure-differential figures for our hermetic doors. If a project needs certified performance data, tell us at enquiry stage.

Sizing the airlock

The airlock has to hold the largest thing that passes through it with both doors closed. That is often not a person. It is a bed, a trolley, a pallet of materials or a person with a wheelchair and a companion.

• Measure the longest load, add room to walk beside it, and add clearance at each door so nothing stops in the closing path.
• With swing doors, add the depth of each leaf’s swing as well.
• Check that a wheelchair user can enter, wait and turn if needed.
• Check the clear opening of each door against the widest load, not the average person.

An airlock that is too short is the most common mistake in a door interlocking system. People end up holding one door open while the other is requested, which defeats the interlock and trips alarms. For MC-125 sliding doors, single slide clear openings run from 1,450 to 3,100 mm and bi-parting from 2,500 to 5,000 mm, so the opening is rarely the limit. The length of the airlock usually is.

Timers, indicators and the waiting person

A door interlocking system that feels slow will be fought by everyone who uses it. Good design hides the wait.

• Indicators. A light or sign at each door shows whether it is available or waiting, so people do not press buttons repeatedly or pull at a locked leaf.
• Hold-open times. Set long enough for the slowest regular user, then the door closes and releases the other side. The MC-125 hold-open time is adjustable from 0 to 8 seconds.
• Request priority. Decide which side wins if both are requested at once, and whether an occupied airlock blocks new entries.
• Alarms. A door held open too long should raise an alarm, not silently disable the interlock.

Activation also matters. In clean areas, doors are usually opened by a deliberate hands-free action, such as a no-touch switch, rather than by walking past. See our guide to the automatic door push button for the options.

Safety and emergencies

A door interlocking system must never trap people. That rule sits above every other requirement.

• Emergency override. There must be a clear way to release the interlock in an emergency, usually a break-glass or override switch at each door and a link to the fire panel where the fire strategy calls for it.
• Escape routes. Where an interlocked entrance is on an escape route, how the doors behave in a fire is set by the building’s fire strategy and confirmed by the fire consultant or local authority, not by the door supplier.
• Power failure. The fail state of each door, open, closed, or on battery backup, is decided with the fire strategy. Battery backup is optional on the MC-125.
• Safety sensing. Every door still needs presence sensing that stops it closing on someone, regardless of the interlock.

We commission the override, test it with the people who will use it, and write down how it works. See our guides to automatic door safety requirements and automatic doors and load shedding. Guidance on South African standards is available from the South African Bureau of Standards.

Security lobbies and access control

In security work, a door interlocking system adds identity to the logic. The outer door opens for anyone, or for a card. The person is checked in the lobby, by a reader, a guard or a camera. Only then does the inner door release. Tailgating is harder because the outer door has closed before the inner one opens.

The MC-125 supports electric locks with multiple access-control inputs, including card readers, keypads, remotes and intercoms. We wire the operators, locks and access control as one system and agree the behaviour with the site’s security team. See automatic door access control.

Door interlocking system price

People often ask for a door interlocking system price as if it were a single product. It is not. The cost depends on what the interlock is protecting and how many parts it connects.

• The doors. Two automatic doors, or more, each with an operator, leaves, glass and sensors. Our MC-125 sliding operator is supplied from R27 590 excluding VAT and the MCW-100 swing operator from R15 900 excluding VAT, as published on our sister company Coetz.
• The interlock control. Built into the operators in simple two-door sliding layouts, or a separate controller for mixed or multi-door layouts.
• Locks and status contacts. Needed where the doors must be held, not just closed.
• Indicators, override switches and access control. Usually the part that grows most on a security project.
• Hermetic doors. Where the airlock is in a clean or clinical area.
• Commissioning and testing. Including testing with the validation or security team.

Send us the layout, the purpose and the largest load, and we will quote the whole door interlocking system rather than a box. Our automatic door prices guide explains the other cost drivers.

Commissioning and testing

An interlock is a sequence, so it is commissioned as a sequence. We test every combination: each door alone, both requested together, a door held open, an occupied airlock, power failure, battery operation where fitted, and every override. The settings are written down at handover, and the override is demonstrated to the people who will use it.

In clean and clinical areas, the site’s own validation testing usually follows ours. We provide what we tested and how, so the validation team can build on it.

Servicing a door interlocking system

A door interlocking system has more parts that can drift than a single door. Contacts loosen, locks wear, sensors cloud and people find ways to defeat the interlock when it annoys them.

As standard we service automatic doors every six months, more often on high-traffic or critical entrances. On an interlock, every visit re-tests the sequence and every override as well as each door’s running gear and safety sensing, and ends with a written record. Our technicians work on our own operators and on other manufacturers’ doors. See automatic door servicing and our service level agreement.

Interlocks in South African conditions

Three local conditions shape how an interlocked entrance is designed here. The first is load shedding and power cuts. Every door in the airlock needs a defined state when the power goes, and the override has to work without mains power. Where people must keep passing through during an outage, battery backup on each operator is worth the cost.

The second is wind. On exposed coastal and Highveld entrances, a vestibule with interlocked doors is one of the most effective ways to stop the wind and hold the conditioned air, and it protects the inner door from wind load. The third is dust. Fine dust clouds sensors and status contacts and works into tracks, so the interlock’s sensing is cleaned and checked on every visit. See our guide to outdoor automatic doors and the South African climate.

What to send us

You can get a long way before a site visit with a few pieces of information.

• A sketch or drawing of the two rooms, the space between them and the doors.
• What the airlock is for: cleanliness, containment, security or wind and air.
• The largest thing that will pass through, with its length and width.
• Who uses it, how often, and whether hands are usually full or gloved.
• Any access control, fire panel or building management system it must connect to.
• Who signs it off: the validation team, the security adviser or the fire consultant.

Mistakes we see

• An airlock too short for the bed, trolley or pallet that actually passes through it.
• No indicator, so people pull at a locked door and press every button.
• An interlock that can be switched off at the door and never switched back on.
• No tested emergency override, or one nobody knows about.
• Swing doors in an airlock with no room for the swing.
• Hold-open times set for the fastest user, so slower people get caught between doors.

Door interlocking system: quick answers

• A door interlocking system lets only one of two doors open at a time.
• A door interlocking system is used in clean rooms, laboratories, hospitals, security lobbies and windy entrances.
• A door interlocking system on two MC-125 sliding doors can often run from the operators themselves.
• A door interlocking system with swing doors or more than two doors usually needs a separate controller.
• A door interlocking system must have a tested emergency override and must never trap people.
• A door interlocking system is sized to the largest load that passes, not the average person.
• A door interlocking system price depends on the doors, locks, controls and access control, not one box.
• A door interlocking system is serviced every six months as standard, with the sequence re-tested each visit.

Door interlocking system questions

Planning an airlock, clean-room entrance or security lobby? Request a quote with a sketch of the layout, the purpose and the largest load, and we will design the door interlocking system around it. We are on 010 412 0062, Monday to Thursday 08:00 to 16:30 and Friday 08:00 to 14:00.