What is Trapped Key Interlocking?

Trapped Key Interlocking


Put simply, Trapped Key Interlocking uses mechanical devices to keep operators safe by holding guards closed until it is safe to enter the area. The simplest way of achieving this is with a key switch and a door module with one key. The key is trapped in the switch while it switched on or trapped in the door module when the door is open. The key can only be in one of three positions:


  • Key trapped in switch in ON position – door module is locked closed
  • Between the key switch and door module –  key switch is off and door module is locked closed
  • Key trapped in the door module allowing door to be opened – key switch is off



The beauty of trapped key interlocking is the ability to force a process, removing human error, without wiring or programming. Using key exchange devices it is possible to ensure multiple different power sources (including electrical, mechanical, hydraulic and pneumatic) are isolated and discharged before access to the hazardous are is permitted.


Click here for an example of using trapped key to enhance a lockout tagout procedure.







Trapped Key Interlocking Definitions


Interlock / Interlocking Device

Mechanical, electrical or other type of device, the purpose of which is to prevent the operation of hazardous machine functions under specified conditions (generally as long as a guard is not closed).



Trapped Key Interlocking System

System fulfilling safety function(s) or part of safety function(s) and comprising of at least two trapped key interlocking devices which work together through the transfer of a key



Power Interlocking

The power supply is directly locked off by a trapped key interlock. This can be a bolt lock into a cam on a disconnect switch, or a key operated switch directly breaking contacts. The key can only be removed from the trapped key interlocking device once the power source has been isolated and locked in the off position – the power cannot be turned back on until the key is returned. This principle can be used not only for electrical switches but also for isolation of hydraulic, pneumatic, stored and mechanical energy. A key for the next operation (e.g. opening a gate or opening an alternate energy source) is only released once the power is isolated and locked in that position, forcing the process to be followed in the correct order.



Control Interlocking

Trapped key systems incorporated into the safety-related part of a control system (SRP/CS) by using switching elements linked into the control system to initiate a stop command. Unlike a “normal” interlock, the stop command is generated by a different component to the one that grants access. These components are functionally linked by the key so serve the same purpose. Turning the key switch generates the stop command, only then can the key be released in order to open the gate.



Safety Key or Personnel Key

Key which is released from a trapped key interlocking device (typically an access lock used in conjunction with whole body access) and retained by a person to prevent a hazardous situation, e.g. unexpected start-up. An example of a proactive inhibit function, giving personal control to an operator going inside a safeguarded space.




More information


ISO TS 19837 gives guidance on requirements of Trapped Key Interlocking Systems. Fortress’ mGard range meets and exceeds these requirements and is 3rd party certified to PLe and Category 4. Click here to read an article explaining how trapped key interlocks can meet performance level e.



Our animated video below explains trapped key interlocking, showcasing Fortress’ mGard range. The video looks at key switches, exchange modules, door modules and safety keys.