What is a door interlock system and how does it work?
A door interlock system controls the relationship between two or more doors.
In a typical arrangement, one door must be closed before another door can be released. This helps manage movement through a controlled area and prevents selected doors from being used at the same time.
This guide explains how door interlocks work, where they are used and what to check when planning a system.
What is a door interlock?
A door interlock uses control equipment, door position information and compatible locking devices to manage a sequence of door operations.
A common example is a two-door lobby.
A user enters through the first door, allows it to close, and then requests release of the second door.
The controller uses the configured interlock conditions to decide when each door may be released.
How does a two-door interlock work?
A typical sequence is:
- The user requests access through the first door
- The controller checks whether the second door is closed and the release conditions are met
- The first door is released
- The user enters the lobby and closes the first door
- The controller receives confirmation that the first door is closed
- The second door can be released when the relevant conditions are met
The exact sequence depends on the controller, wiring and system configuration.
Some installations also require confirmation of the locking condition.
What equipment does a door interlock system need?
The main hardware may include:
- Interlock controller or module
- Compatible electric locking devices
- Door status contacts or outputs
- Access control readers or keypads
- Exit buttons
- Power supplies
- Status indicators
- Emergency release equipment
The required combination depends on the number of doors and the intended operating sequence.
Cables, fixings and any additional interfaces also need to be considered.
Why is door status monitoring important?
The controller needs reliable information about whether the relevant doors are open or closed.
This can be provided by suitable separate door contacts or compatible door status outputs.
A command to lock a door does not confirm that the door has physically closed.
Door position feedback allows the controller to make release decisions using the actual door condition.
Where the design also requires lock status feedback, this must be specified separately.
Is an interlock the same as access control?
Access control normally decides who is authorised to enter.
An interlock controls the relationship between door operations.
The two functions can work together.
For example, a valid card may authorise entry, but the interlock can delay the release until another door is closed.
A valid credential does not necessarily override the interlock conditions.
Where are door interlocks used?
Typical applications include:
- Controlled entrance lobbies
- Secure access areas
- Laboratories
- Cleanrooms
- Healthcare premises
- Industrial facilities
- Data centres
- Restricted internal areas
The reason for interlocking varies by application.
Some systems manage security, while others support controlled movement between areas.
A basic door interlock does not itself control air pressure or establish a particular cleanroom performance.
What is the difference between a two-door and multi-door interlock?
A two-door interlock manages the relationship between two doors.
A multi-door system can manage more doors, subject to the controller’s capabilities.
The required logic may involve:
- A shared interlock group
- Selected relationships between doors
- Particular entry and exit sequences
- Manual control functions
- Different operating conditions for different areas
Do not assume every multi-door controller supports every possible sequence.
Write down the required operation before selecting the equipment.
What are the ILM200 and ILM600?
Our interlock range includes the ILM200 for two-door interlock applications and the ILM600 for systems controlling up to six doors.
The ILM200 is a compact control module, while the ILM600 provides a larger multi-door control arrangement.
The appropriate choice depends on the door count, operating sequence, power requirements and connected equipment.
Check the individual product information before specifying either model.
Can interlocks use magnetic locks?
Compatible magnetic locks can be used in suitable interlock installations.
Other electric locking devices may also be suitable, depending on the controller and door requirements.
Check:
- Lock operating voltage
- Current draw
- Fail-safe or fail-secure operation
- Door status feedback
- Lock status feedback where required
- Controller output ratings
The locking equipment and controller must be selected as part of one system.
What happens during an emergency?
The normal interlock sequence must be considered alongside the required emergency release arrangements.
Emergency operation may need to override normal access restrictions, according to the specified system design.
Do not assume that the controller’s normal logic provides all the emergency functions required for the installation.
The door use, locking equipment and emergency release strategy should be assessed together.
What should you check before ordering?
Check:
- Number of doors
- Required operating sequence
- Which doors must be interlocked
- Door position monitoring
- Lock monitoring where required
- Reader and exit button inputs
- Lock voltage and current
- Power supply capacity
- Battery backup requirements
- Status indication
- Manual control requirements
- Emergency release arrangements
Summary
A door interlock system manages when selected doors can be released in relation to one another.
Successful operation depends on suitable control equipment, reliable door feedback and a clearly defined sequence.
Choose the controller and associated hardware around the complete application, including normal access and emergency operation.