Safety Interlocks – How to choose yours

There are various parameters (safety, environmental, controls, response time, etc.) that need to be considered when selecting the proper safety interlock for the application.

It is important to use well tried safety principles when designing and selecting the components for the machine. The parameters to consider include but limited are:

• Environment: the environment in which a safety interlock is used should heavily impact the type of interlock used and its ratings. Ensure that when an environment requires wash down to be performed regularly that interlocks being used should not have cavities that can create hygienic issues. Dust, temperature and humidity are as well important factors to consider when selecting the interlock.

• The stopping time of the hazardous motion, is an important factor in selecting the appropriate safeguarding device most suitable for the application, when a safety interlock without guard locking is used. The stopping time of the hazardous motion specifies the safety distance needed between the hazard and the safeguarding device/access guard. The quicker the stopping time of the hazardous motion the closer the safeguard device can be placed to the hazard and the longer it is the further the stopping time needs to be, refer to ISO 13855 for calculations.

• Incentive to defeat, when defeating interlocks is a concern on a plant floor, there are various means to minimize their incentive to defeat.
  • Talk to the personnel defeating the interlocks to understand their reasons as it could be relating to
    • maintaining machine uptime,
    • Door miss alignment leading to interlock channel miss match, need to perform a task under partial energy that was not in the risk assessment, etc. ) assess a remediation approach for it.
  • If a task is deemed to require partial energy it is recommended to first perform a risk assessment to understand the tasks, hazards and risk reduction measures.  
    • Then design and implement a manual suspension mode with other alternative control measures.
    • This mode should only be accessible to trained and qualified personnel, to perform some specific tasks under manual suspension, through partial energization, energy limitation and the use of alternative control methods. 
  • If a task is deemed not to require partial energy, a way to reduce defeating would be, to increasing the coding of the interlocks actuators are some of other remediation path to consider. Implement interlocks or trapped keys with higher coding, mounted with secure means that are hard to defeat; 

• When selecting the interlock for an application, it is important to consider the risk level for the hazard, which correlates to the performance level required. Machine safety applications with high risks, require interlock solutions and safety functions, that can achieve higher reliability and vice versa for lower risk ones.  

• If the application requires whole body access, applications where personnel can be fully within a safeguarded zone, here are some of the many safety measures that need to be considered,
  • Implement protective structures that prevent personnel from being stuck within the safeguarded space,  
  • Using interlocks with slide bolts that can be locked in the open state,  
  • Implementing personnel keys to ensure that personnel have to remove a key to gain access,  
  • Adding escape releases for the guard locking interlocks to allow personnel exit  

In summary, these are some of the parameters to consider when selecting the proper interlock for your application. It is as well important to inspect the interlocks on a periodic basis to ensure that there are no degradation in the interlock’s components, that could result in its failure. See the article on Inspection of Machines for more info.