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Safety Systems Must Be Designed For Productivity I don’t care if your safety system is CAT 4, PL e or SIL 3, if it significantly interferes with the use of the machine then it’s unsafe. Anyone who works with machinery has seen safety systems that are designed as an afterthought in an ad hoc fashion. For example:

So how do we avoid these common issues? Guidance is now at hand with the new Australian Standard for Interlocking Design and Principles, AS 4024.1602:2014.

This standard has a method to identify if the proposed safety system will create a motivation to defeat. Firstly the designer must identify the modes of operation, for example common modes would be; normal operation, manual operation, cleaning, maintenance, etc. The designer then needs to identify what tasks are performed in these different modes of operation.

The method will then assess if the safety system allows the task to be performed in the mode. If not, then a redesign of the safety system is required to allow for this activity.

If the safety system does allow the task to be performed, the designer still needs to analyse if the safety system interferes with this activity. For example there might be motivation to defeat the system because of these typical reasons: If motivation to defeat is discovered then design measures that will eliminate or minimize this motivation must be considered. For example, providing a transparent guard to allow the required visibility to perform the task. If there aren’t ways to minimize motivation for defeat then the standard recommends measures that can be used to make defeat difficult. For example, selecting highly coded safety interlock devices that are difficult to defeat.

Design of interlocking systems to reduce motivation for defeat has always been a consideration in the safety standards but now a formalised method is available for use. It is hoped that safety systems will be designed with the operation of the machine in mind so we can avoid non-productive safety systems that encourage defeat and create unsafe practices. Labels: CAT 3, CAT 4, Electrical Engineering, interlock switch, Machine Safety Systems, Risk Management, safety, Safety Integrity Levels, SIL 3



