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:
- Machines where the operator needs to bypass the safety system to set-up or clean the machine
- Machines where guards don’t allow the visibility required for the task
- Safety procedures that are time consuming and become ignored
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:
- The task can be performed much quicker if the safety system is defeated
- The safeguard restricts visibility or audibility required to perform the task properly
- The safety procedure requires much more physical travel
- The safety system restricts movement and adds difficulty in performing the task
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.