Preventing the Defeat of Interlocking Devices
22 September 2026 
Most people working with industrial machinery will have seen an interlocking device defeated at some point. An actuator removed from a guard, a spare actuator used to keep a switch made, or a more creative method to allow a machine to continue running. Defeating interlocking devices exposes people to hazards and can result in serious accidents. But, how far should machine builders and integrators go to prevent defeat?
BS EN ISO 14119:2025+A11:2026 Section 8 sets out a methodology to establish whether defeat is reasonably foreseeable, why somebody might want to defeat the interlocking device, and how motivation to defeat can be reduced and managed.
The first step is to apply the measures required elsewhere in the standard. The machine builder or integrator must then assess whether motivation to defeat exists. Experience from similar machines is particularly useful when making this judgement.
For many machines, motivation to defeat an interlocking device is not an unlikely or unusual event. Interlocks typically interrupt a process when opened; increasing downtime. Production pressure to minimise downtime creates a strong incentive to bypass safeguarding especially quick activities like fixing a jam, making a small adjustment, or cleaning a small mess.
This is why Section 8 requires the designer to consider whether the motivation itself can first be eliminated or reduced through design or suitable operating modes. The standard gives precedence to appropriate operating modes and identifies activities such as setting, tool changing, fault finding, maintenance as examples where they may be beneficial. Process observation is also a key point in machinery safety and guarding should not prevent it, consequently creating a motivation to defeat.
This should not be interpreted as meaning that providing an operating mode will always eliminate the problem. On most machines, some foreseeable motivation remains simply because bypassing an interlock allows some interventions be completed quicker. Where that motivation continues to exist, the methodology then leads into Table 5. This only applies where foreseeable motivation to defeat remains after completing the previous stages.
The additional measures address the practical ways in which an interlock could be defeated. One obvious pattern in the table is that low and medium coded devices require more additional measures than high level coded ones. However, coding alone does not prevent someone from removing the genuine actuator from the guard and using it to operate the switch independently.
This is why table 5 includes the use of non-detachable fixing. However, it is often assumed that both the actuator and the interlocking device must be fixed in a non-detachable method. For typical interlocking devices such as type 2 and type 4, non-detachable fixing is required for the actuator and only recommended for the interlocking device dependent on the assessment of motivation to defeat. If devices must be replaced over the life of the machine non-removable fixing may not be the most appropriate solution.
A notable point in the latest version of the standard is the explicit exclusion of ‘security torx’ and similarly designed fixings. These are not regarded as non-detachable because suitable tools are readily available. They are however permitted if the pin has been bent so a tool cannot remove it. Similarly, other fixings are permitted where the drive has been damaged preventing removal. Methods of non-detachable fixing explicitly listed include, one way screws, welding, riveting, and gluing of screw threads such that heat or chemicals are required to remove it.
Another approach to preventing defeat is restricting access to the device or actuator. Making access more difficult by mounting it out of reach, shielding it, or positioning it in a hidden location increase the effort needed to interfere and reduce the likelihood of casual or opportunistic defeat.
Physical measures are not the only tool. Clause 8.3 also considers status monitoring and cyclic testing, allowing the control system to detect behaviour inconsistent with normal guard operation. For example, the guard needs to be opened every cycle to remove the product; if the control system does not see this cycle of the interlocking device, this is evidence of defeat. This cycle can be requested by the control system, allowing a cyclic test to be completed. An additional interlocking device using a different actuation principle, combined with plausibility checking, can also make defeat substantially more difficult because more than one condition must be falsified in a credible sequence.
Machine builders and integrators must properly consider motivation to defeat in the design, and if additional measures are required they select them appropriately. Clause 8 guides that process from selecting an interlock with the right level of coding, to how the device is mounted, to checks and monitoring by the control system. It requires the designer to understand the reasons defeat might occur and, where those reasons cannot be fully removed, apply appropriate additional measures.
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