Annex K: Trapped keys

Last edit: 10/07/2023

Trapped keys are devices used to ensure safe access to potentially live or dangerous plant or equipment in an industrial setting.

A trapped key interlocking system relies upon the transer of key(s) between a key-operated switch and an access lock.

Typically, in trapped key interlocking systems the access lock and the output system are physically separated and functionally linked by the transfer of the key.

These systems shall ensure a sequence of operation for equipment to ensure the intended risk reduction.



1 movable guard

3    actuator

5    actuating system

6    output system

7    intermediate key transfer

8    Type 5 interlocking device (access lock)

9    Type 5 interlocking device (key operated switch)

10  trapped key interlocking system





Other interesting requirements regarding Trapped Keys in the Annex K are:

6.4.2 Key coding. Key coding shall prevent two or more devices unintentionally sharing the same key, which can result in a hazard. For example, a key that shuts down and allows safe access into a particular machine shall  not allow access or control of another machine.

6.4.3 Key retention. In order to prevent the key from being easily removed from the Type 5 device in the trapped position it shall not be possible to remove the key out of the Type 5 device with a force less than 250 N. All of the parts that are intended for blocking the key, including the key itself, shall  be able to withstand a torque of at least 5 Nm applied to the key.
Furthermore, at torque values greater than 5 Nm, it shall be ensured that the key does not shear out ofthe device resulting in a loss of the safety function (e.g. through design features such as predetermined breaking points).

6.4.4 Reproduction of keys. Keys for type 5 interlocking devices shall be designed in a way that they are not easily reproduced (for example with hand tools or by a locksmith), other than by the original device manufacturer.