What are the benefits of using a mechanical trapped key interlock over electronic or electromechanical door interlock systems?


Namely the vast reduction in installation and commissioning costs (no wiring to the gate and no programming of a safety plc required) and the ability to operate in harsh conditions where electronic components can become unreliable.

Generally, for Performance Level d and always for performance level e you will require a dual channel system (e.g. Category 3 or 4) with monitoring between the channels. However, despite not meeting the typical architecture of a high PL electromechanical or electronic safety related part of a control system, it is very possible and often easier to reach a Performance Level d or e using purely mechanical trapped key systems. This is done by meeting Category 3 or 4 behaviour rather than the circuit diagrams designed for electrical components.



A representation of Figure 5 in ISO 13849-1 2015 showing the relationship between Category Architectures, Diagnostic Coverage, Mean time to Dangerous Failure and the Performance Level.


(Cat B-4) – Category Architectures

(DC none – High) – Diagnostic Coverage

(MTTFd Low – High) – Mean time to Dangerous Failure

(PL) – Performance Level

Category 3 & 4 Behaviour

Category 3 & 4 Behaviour

In either Category 3 or 4, if all the faults that can lead to a dangerous failure have been excluded, either single faults of an accumulation depending on the Category, there is no longer a need for Diagnostic Coverage (DC). This allows the fully mechanical system to meet either PLd or PLe without monitoring and therefore without any wiring or programming. Careful consideration of all possible faults is needed and for peace of mind Fortress has obtained 3rd party certification on our mGard range of Mechanical Trapped Key products as suitable to be used in a PLe system for reassurance that all possible failure modes have been well considered.


Products Mentioned in this Article



Control of hazardous energy in sequence

Trapped Key

Mechanical Trapped Key Range