What Level of Diagnostics is required for Machine Safety Systems?

When designing machine safety control systems, what level of diagnostics is appropriate? This seems to be an issue that causes confusion and inconsistency throughout the industry.

For example, if the safety system is being designed to Safety Category 3 (according to AS 4024.1501), the requirement is as follows:

“Whenever reasonably practicable the single fault should be detected…… some but not all faults will be detected”

Not surprisingly these requirements have led to many interpretations of what diagnostics should be implemented for Category 3. The application that causes most confusion is when the safety system is monitoring multiple guard doors. Can these guard doors be connected in series? If so, how many? What criterion needs to be considered?

Up until now there hasn’t been any appropriate guidance on how wiring guard doors in series degrades the level of diagnostics and what level is acceptable for the Safety Category.

New machine safety standards have now been developed to assist the designer. ISO 13849.1 is a standard that provides a method for the designer to quantify the diagnostics of their safety system, using a measure called Diagnostic Coverage (DC). Each Safety Category will have specific requirements for the DC and the designer will know exactly what level of diagnostics is required for their system.

ISO 13849.1 is a current international standard that can be sourced for your reference and this international standard will be adopted into AS 4024 in the next revision. This will provide better guidance for safety system designers in the Australian Standard.

Are Safety Categories Obsolete?

Have you heard that Safety Categories are a thing of the past? That changing international standards have rendered Safety Categories redundant since the start of 2012? And that all new safety control systems for machinery must be designed to Performance Levels (PL) or Safety Integrity Levels (SIL)?

If so, here is some more information on the matter:

Yes, international standards have now moved to probabilistic methods, with two current standards as the options:

1. IEC 62061 – Highly mathematical method where safety control systems are designed to a Safety Integrity Level (SIL)
2. ISO 13849.1 2008 – Method based on the architecture of Safety Categories where safety control systems are designed to a Performance Level (PL)

However, Australian Standard AS 4024.1501 is a current machine safety standard where safety control systems can be designed to a Safety Category. So if you are comfortable using Safety Categories, you can continue to use this standard.

It is true that AS 4024.1 will eventually be updated to reflect current international standards, but AS 4024.1501 will remain unchanged for at least the next 3-4 years.

In my opinion, for simple safety systems, (i.e. systems using devices such as safety relays), Safety Categories is a good option that will result in a high level of integrity. If software is being designed, I would recommend following the software lifecycles available in IEC/AS 62061 or ISO 13849.1.
However, be aware that at some stage in the future our Australian Standards will transition to these probabilistic methods, but not in the immediate future.

Is Work Health and Safety Harmonisation Still Alive?

In July 2008, an agreement was reached between the Commonwealth and all states and territories within Australia except Western Australia (WA) to begin the process of harmonising Australia’s work health and safety laws. This new model was aimed at reducing red tape and compliance costs for businesses that operate in multiple states. The umbrella laws would also provide workers with equal levels of safety across the nation as well as recognising their licensing and training nationally.

Fast forward to 2013 and what is the state of play? We now have Queensland, Northern Territory, New South Wales, ACT, South Australia and Tasmania all adopting the harmonised model. This leaves just WA and Victoria out of the loop.

WA are still committed to the process and have completed their draft WHS which is expected to be implemented in 2013. WA has also started public consultation on the Regulations in August 2012 and will be working through this process in 2013.

This leaves Victoria. The last communication on the WorkSafe website stated that Victoria would not adopt the national legislation. The reason for this stance is an expected $3.4 billion cost over five years with no perceived benefit in terms of safety levels.

So what does this mean for harmonisation? Do you believe it will still be successful without the involvement of Victoria? Do you believe that Victoria will eventually join the rest of the states and adopt national safety legislation or stay with their own set of rules?