New series provides guidance on Common Cause Failure (CCF)

What is a common cause failure (CCF)? CCF is a term that has much more significance in the new AS 4024.1 series. A good description of CCF can be found from a very trusted reference...TV! If you have ever watched Air Crash Investigation then you may be familiar with the 'Swiss Cheese Model'.

From this Swiss cheese model, it can be seen that if the holes in the multiple layers of cheese line up then a single path through the layers can exist and thus the safety critical system has failed. CCF is an example of this occurring in machine safety systems. If we think of a dual channel system; a CCF would be both channels failing at the same time due to a common event, for example:

• Two independent switches on a guard failing because the ambient temperature is above their rating
• Two independent safety channels having erroneous signals induced on them from the same source of electromagnetic noise
• Two mechanical switches on a guard fracturing due to the one impact event of that guard door

The above explanation shows how threatening common cause failure can be to a safety system. We will generally design machine safety systems with two channels when the risk of the application is high; this provides redundancy so the system can tolerate a fault on a single channel. However if the system hasn't been designed to avoid CCF, then there is a real chance that a certain event will defeat both channels and the system will fail.

For this reason, whether you are designing systems to categories (AS 4024.1501) or performance levels (AS 4024.1503) CCF should be a consideration for any multiple channel architecture. Unfortunately the previous version of AS 4024.1 didn't have any usable process to avoid CCF, however this has been rectified in the new version of the series released in 2014.

AS 4024.1503:2014 Annex F contains a test for common cause failure that can be used to determine if the safety system has been designed to avoid CCF to an acceptable level. I would recommend any machine safety system designed to architectures cat. 2, 3 or 4 should be analysed with the process from Annex F. This is the only usable guidance in the AS 4024.1 series for designing safety control systems to avoid CCF.

Other parts in the 2014 version of AS 4024.1 also provide guidance on designing to avoid CCF for common safety functions. For example, AS 4024.1602:2014 Clause 8.3, provides excellent guidance on how to prevent common cause failures in interlock guard functions.

This is an example of the improvements that have been made with the new 2014 version of AS 4024.1 series. We will be exploring some of the new features of AS 4024.1 in many of the safety blog topics this year. If you missed our last topic Safety Systems Must Be Designed For Productivity, be sure to to check it out as this topic explored how the 2014 version of AS 4024.1602 can help you design interlock guards that operators won't defeat.

Don't be a Robbo or Danny boy, protect yourself from common cause failure.

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:

• 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

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.

What’s the hold-up with the new Conveyor Safety Standards?

For those not aware, the process of replacing the Australian Standard for Conveyor Safety, AS 1755, is well underway. This standard will be replaced by the following set of safety standards:

• AS 4024.3610 – Safety of Machinery, Conveyors, General requirements
• AS 4024.3611 – Safety of Machinery, Conveyors, Belt conveyors for bulk material handling
• AS 4024.3612 – Safety of Machinery, Conveyors, Light duty belt conveyors
• AS 4024.3613 – Safety of Machinery, Conveyors, Screw conveyors
• AS 4024.3614 – Safety of Machinery, Conveyors, Mobile and transportable conveyors

The General requirements standard, AS 4024.3610, will replace the bulk of AS 1755. The major differences will include:

• Updated references to current international and Australian standards
• Updated control system requirements to allow for new design standards and current technology
• Increased information on guarding options and methodology. This information should be used within the requirements of any state legislation (ACT/Regulations) or codes of practice.
• Increased attention on the lifecycle activities of the equipment, such as risk assessment, installation,  commissioning, decommissioning, etc.

This General requirements standard will now be complemented by specific conveyor type standards, which will allow for more detailed guidance and compatibility of safety control measures. For example, with the AS 4024.3611 standard there will be improved information on aspects of belt conveyors for bulk material handling such as:

• Hazard list specific for this type of conveyor
• Safety considerations specific for this type of conveyor
• Inclusion of current industry practices
• Statistics on incidents involving belt conveyors for bulk material handling

For those who have been paying attention, there has been a significant delay since the public comment period of AS 4024.3610 and AS 4024.3611, which ended in February 2014. So why has it taken so long to publish these standards?

To avoid confusion, the new standards will only be released once all of the sections are ready for publishing. This allows AS 1755 to be fully superseded in one step.

So in conclusion the new set of standards will provide improved guidance for conveyor safety and the process is still moving forward, however the new standards will only be available once they are all ready to be published.

Unsure what all this mean for you? Want an overview of the different machine safety standards in Australia and how these tie in with OH&S requirements? Want to help on knowing how to select the applicable standard when designing a safety system? Well register and come along to the information session below.

NHP Electrical Engineering Products in conjunction with EngSpace are holding a Safety Standards session with guest presenters Frank Schrever, Chairman - Standards Australia Committee and Amy Whykes, Product Engineer - Safety & Sensing at NHP.