How to Select the Correct Safe Guard

When should you install a fixed guard as opposed to an interlocked guard?

Is a bolted guard a permanent fixed guard?

When is it acceptable to replace physical guards with light curtains?

These are common questions people have when selecting the appropriate guarding for their machinery. Guidance on selecting the appropriate guard is available in the Work Health and Safety legislation, section 4 of the Code of Practice, "Managing the Risks of Plant in the Workplace" explains this process. This is based on clause 208 of the Work Health and Safety Regulations (Current legislation for all states and territories except for WA and Victoria).

Here's what the code of practice says about selecting your safe guard:

If access to the area of the machine is not needed during operation, maintenance or cleaning then a permanent fixed safe guard is required. What is a permanent fixed safe guard? The code of practice states this guard is welded or incorporated into the body of the machine, thus a bolted guard is not a permanent fixed safe guard.

If access to the area of the machine is require during operation, maintenance or cleaning then an interlocked guard can be used. This guard will have a safety control system that will cease any relevant hazardous energy to the machine when the safe guard is not in a closed position.

If it's not reasonable to use a permanent fixed safe guard or interlocked safe guard then a fixed safe guard can be used. A fixed safe guard can be removed and replaced with the aid of a special tool, such as a coded spanner or Allen key. Thus a bolted guard would be classed as a fixed safe guard.

If none of the above physical guarding options are practicable, then a presence sensing system, such as light curtains or laser scanners can be used. A common example of this would be a conveyor that transports goods into a robotic cell, if a physical guard was used then the goods would be blocked in entering the cell whereas a light curtain will allow the goods to enter the cell in a safe manner.

The most common query people have about the above guidance is: When should I use an Interlocked Guard or a Fixed Guard?

The interlocked guard is the first option when access is required because a safety control system is protecting the operator from the hazards on the machine. When using fixed guarding we are relying on human behavior to ensure three things:

1. The hazardous energy has been isolated before the safe guard is removed
2. The hazardous energy will remain isolated while the safe guard is removed
3. The guard is replaced before the hazardous energy is resupplied to the machine

Thus two considerations should be made when deciding if a fixed guard is appropriate:

1. Frequency of Access - The more frequently we rely on human behavior, the more likely the process will fail. If access is required multiple times a week or during normal operation it would be recommended to use an interlocked guard
2. People performing the task - For access through a fixed guard the operator must be trained on the isolation procedure of the machine, this training must be refreshed and documented. If this knowledge can't be relied on then an interlocked guard should be used.

How do you future-proof your safety systems?

Looking through machine safety standards there is plenty of guidance for the early phases of machine safety system life cycles, by this I mean you can find good guidance to explain the following activities:

• Select the required integrity level; CAT/PL/SIL
• Design the safety system
• Verify the system design
• Validate the safety system

But what guidance is available for the operation phase of the safety system? Safety systems can be operational for 10 to 20 years, sometimes even longer! Is it reasonable to expect application parameters won't change the requirements of the safety system over that extended period of time?

Requirements can change dramatically over the life of a safety system for example here are some parameters that could affect the suitability of the current safety system:

• The uses of the machine 
• Speed of throughput
• Frequency/duration of safety demands on the system
• Stopping times of the equipment

The need to design systems to take consideration of the above changes is becoming more prevalent. Functional safety standards such as AS 62061 mention these factors as prompters for safety system modification, but how can you reliably identify these parameter changes?

Relying on manual monitoring of the safety system parameters causes extra work and is susceptible to human complacency/error.

With the ability to have high levels of data sharing from modern safety systems to standard control systems, it is possible to create this parameter checking as an automated function of the control system. Thus if the use of the machine is changed in a way that effects the safety system's suitability, this will be flagged by the control system and initiate the appropriate modification process.

The most common example of the above concept is Stopping Performance Monitoring (SPM), which is a requirement out of IEC/TS 62046. SPM should be performed when presence sensing systems such as light curtains, safety mats or laser scanners are used as a trip device and the stopping performance of the machine can be subject to deterioration, due to wear of brakes, valves, etc. SPM could be achieved by the machine control system monitoring the stopping performance of the machine and comparing this result to the calculated stopping time used for the safety distance calculation of the presence sensing system. Once the calculated stopping time is exceeded the control system could initiate a safety stop, provide information to the operator of this condition and not allow operation until the system is restored to its acceptable state.

Preventative warnings could be provided by the control system as the stopping performance approaches the calculated stopping time, thus the braking system can be repaired in upcoming scheduled maintenance. Downtime is then avoided and the level of safety is maintained.

Require more information about how modern safety systems with increased integration can assist?  Craig may be able to assist you with the above mentioned issues, so please reach out via email - cimrie@nhp.com.au. Craig has been a Safety Specialist with NHP Electrical Engineering Products since 2007. He is also a committee member at Standards Australia and is a TUV Rheinland certified Functional Safety engineer.

Craig Imrie

PL and SIL merger cancelled. What does it mean for AS 4024.1?

For those who are designing machine safety control systems to achieve international standards you may have been aware of the process in place to merge the two current standards. This would result in a new standard, IEC/ISO 17305, which would merge the methods of Performance Levels (PL as per ISO 13849.1:2015) and Safety Integrity Level (SIL as per IEC 62061).

This process was seen as a positive step for machine safety designers as we would finally have one unified standard that everyone would design their systems to, instead of the confusion of having multiple standards running concurrently. However this merger process has now been cancelled without a guarantee of when or if the process will be restarted.

So what is the relevance of this to Australian Standards?
Our Australian Standard, AS 4024, adopts directly from international standards and as stated in AS 4024.1100 the future direction of the control system section was dependent on the merged standard:
It is envisaged that on completion of the work of Joint Working Group 1 of ISO/TC 199 and IEC/TC 44, combining ISO 13849-1:2006 and IEC 62061, the resulting unified Standard will replace both
Parts 1501 and 1503 in the next revision of the AS 4024.1 series

So what does this mean for the future direction of AS 4024.1? Well that seems to be up in the air at the moment.

Potentially the next revision of AS 4024.1 will see Safety Categories disappear and Performance Levels remain. Another option may be Safety Categories remain as an option for safety control systems that consist of simple devices, such as safety relays, safety contactors, safety valves, etc.

There may be advantages of this second option for the following reasons:

• The Australian industry has much more familiarity and knowledge of Safety Categories compared to PL or SIL
• Safety Categories provide a simple method to design safety control systems
• When applied correctly Safety Categories provide adequate risk reduction
• International systems using PL will still be designed to a Safety Category architecture

What do you think? 
If you have a comment or opinion on what the future direction of safety control systems should be in AS 4024.1 please leave a comment below.

Feedback from the industry is essential so the committee can ensure the standard reflects the industry’s needs.