EN ISO 14119
EN ISO 14119 Interlocking devices associated with guards
EN ISO 14119 describes the design requirements for minimising the override possibilities of safety switches.
The European Machinery Directive requires machinery to be fitted with safety components, such as safety switches and blocking devices.
The NEN/EN/ISO 14119:2013 also requires that machinery be designed to minimise the motivation for bypassing these safety mechanisms. This is in addition to the requirements of the ISO 1200s which state that the possibility of bypassing safety devices must be taken into account in the risk assessment.
ISO 14119 describes four steps to prevent safety switches and interlocking devices from being by-passed or over-ridden in a reasonably foreseeable manner.
The revised 2024 edition places additional focus on human behaviour in relation to safety systems, the mechanical robustness of interlocking devices, and the correct use of series-connected or sequential interlocks.
No motivation for manipulation or an override?
ISO 14119:2024 describes a structured four-step process to prevent reasonably foreseeable bypassing of safety switches and interlocking devices. The approach is intended to identify potential motivation for tampering at the design stage — and address it proactively.
Step 1 – Implementation of Basic Measures
The first step involves applying basic protective measures as outlined in section 4 of the standard. This includes correct mounting, selecting interlock types suitable for the risk level, and proper positioning. Type 3 safety switches (e.g. non-coded contactless types) may only be used where there is no reasonable expectation of bypass or tampering.
Step 2 – Evaluate Motivation for Tampering
After implementing basic measures, it must be assessed whether there is any motivation to bypass safety mechanisms. This could result from production pressure, long waiting times or limited access for maintenance.
No motivation detected: the process is complete.
Motivation exists: proceed to Step 3.
Step 3 – Eliminate or Reduce Motivation
Motivation for tampering can often be reduced or removed by better design choices or functional alternatives such as setup/maintenance modes, faster release mechanisms or improved workflow. If these measures are effective, no further action is required.
Step 4 – Apply Additional Measures
If motivation remains, the standard requires additional protective measures to reduce the risk of tampering or bypassing.
The following 4 steps are important and worth revisiting, this time as a flowchart.
Additional measures
If the motivation for overriding and/or tampering cannot be removed, ISO 14119:2024 requires that additional measures should be taken in order to minimise the risk of overriding safety switches.
These additional measures are categorised as followed:
- Install safety switches in locations that make tampering physically difficult, such as behind guards or out of reach
- Use coded actuators or permanently fixed actuators to prevent easy replacement
- Secure components with non-removable fixings to prevent unauthorised removal
- Monitor the status of interlocking devices through the control system to verify correct operation.
The emphasis lies on a combination of physical deterrence and behavioural consideration.
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Why would an operator want to bypass a safety switch?
Motivation to manipulate has been an important issue in reformulating ISO14119 with considerable emphasis on additional measures to prevent manipulation.
The current standard defines non-removable operating tongues and excludes the use of safety screws. Significant efforts have been made to improve the existing table of requirements, but the focus remains on additional measures and not on addressing the underlying problem – why would an operator want to bypass a safety switch?
When considering additional measures, it is essential to follow the four-step approach, taking into account the following key question: “Does the motivation to bypass or tamper exist?” If it does not exist or can be eliminated, no additional methods are needed.
However, the reality is that eliminating the motivation to manipulate during the risk assessment can not only provide a safer working environment, but can also lead to productivity gains. Focusing only on making it more difficult to ‘defeat’ safety switches does not eliminate the problem but only treats the symptoms.
New in the 2024 version
Testing procedures and verification
A new addition in the 2024 edition is the inclusion of test procedures for mechanical strength, such as impact resistance and retention force. These are detailed in Annex I and ensure that interlocking devices function reliably not only under ideal conditions but also in demanding environments where impacts or forceful pulling may occur. The standard also introduces plausibility checks: system-level diagnostics that identify abnormal switching behaviour or time delays. These checks reduce the need for manual inspections and enable early fault detection. For components with lower Performance Levels (PLc or PLd), annual inspections remain mandatory. For PLe-level systems, monthly verification is required.
Series connection and fault masking
Another key update in ISO 14119:2024 is the guidance on series connection of multiple interlocks on a single safety input, common in machinery with multiple access points. The standard warns of the risk of fault masking, where a fault in one device is hidden by a fault in another. Annex J explains how to assess diagnostic coverage (DC) and determine the achievable Performance Level (PL) for series-connected systems, helping designers to reduce this risk.
Trapped key interlocking
The integration of trapped key interlocking systems is another major update. These systems, formerly described in ISO/TS 19837 and now included in Annex K, rely on mechanical keys to enforce a fixed sequence of actions, such as ensuring power isolation before granting access. This type of interlocking is especially useful where software-free safety is required, such as in ATEX zones or in installations without PLC control. When correctly implemented, trapped key systems are now recognised as fully valid safety solutions.
