Workplace safety through machine design

Safety applied without reference to workforce productivity and the operability of the machines, plant and equipment can lead to dangerous outcomes. The law has long been exhorting the importance of eliminating risk and if that is not possible risks should be minimised through engineering means. Those engineering means are not described in legislation, except the guarding hierarchy which defines the guarding types and the order in which they should be examined and employed, so far as is reasonably practicable. This hierarchy is central to many engineering solutions for risk minimisation of machines, and here the law has got it just about right.

A permanently fixed (welded) physical guard must be applied if possible, says the law (including the new harmonised laws now enacted by NSW, Qld, the Territories and the Commonwealth). This, of course, can only happen if access is never required for operation, maintenance or cleaning. A guard, which is not removable, isolates the hazard from us, thus reducing the probability side of the risk equation ( risk = severity x probability) to a very low level regardless of the severity of the injury that might result if we were to come into contact with that hazard.

Machine safety standards

Most machines require us to have access to the dangerous parts for operation, maintenance or cleaning. In this case, says the law, we must have an interlocked physical guard. An interlock is typically a switch which turns off the hazardous energy when we open the guard. It may even prevent us from opening the guard until the hazardous machine functions have stopped. But how do we design the interlocking system so that it is guaranteed to function, so that the switch and control system can’t fail and let us into a machine which is still running? The law has nothing to say about these things, but it does point us to Australian standards, via the codes of practice. The prime standard in this regard is AS 4024.1-2006 Safety of Machinery with its suite of technical and machine-specific standards. This standard covers all the fundamental requirements of machine safety from risk assessment via safety control systems, interlocking, emergency stop, unexpected start, guarding design to a complete set of ergonomic considerations. It provides detailed guidance on how to design safety control systems which have decreasing probabilities of dangerous failure as the risk increases.

Is it reasonably practicable to interlock every guard that has to come off a machine for operation, maintenance or cleaning? Absolutely not. So where to from here? Well, the law quite sensibly leaves this decision to you. If you decide that it is not reasonably practicable to weld or interlock a guard, then you can now use a guard that can only be removed with the use of tools, and the standard AS 4024 supports this further by saying that the tool should not normally be available to the operator. However, please make sure that you have understood the “not reasonably practicable” part and can defend that decision if an incident were to occur. There are five tests for reasonable practicability, defined in the OHS Act (and in the harmonised laws) and each one should be considered in the context of that risk. Cost is at the bottom of that list, but can be a valid argument only if it can be clearly demonstrated that the cost is grossly disproportionate to the risk.

Human factors

Many guards are rarely removed. For instance, the idler guards on a very long conveyor, and if that is the case, an argument may be legitimately raised to bolt these guards only. But in this case, we are now dependent on human behaviour to control the risk. That is, the human must follow some administrative control like a lockout, tagout and test procedure before removing the guard, to ensure there is no hazardous energy left in the machine and it cannot start unexpectedly.

Sadly, these approaches are often applied to guards that are removed frequently, and over time diligence wains, complacency creeps in and terrible injuries can result. This is simply because humans are relatively unreliable when it comes to performing repetitive tasks. Accidental and occasionally deliberate departures from procedure are made. The legal hierarchy of risk control tells us to minimise risk by engineering means first, before relying on human behaviour via administrative controls and PPE. The clear message here is that the risk assessment team has to tread a careful line analysing the situation in detail, before deciding on bolted guards.

The law has allowed for the fact that physical guards may not always be reasonably practicable. In these cases, a human-presence sensing system can be used. These include devices such as light curtains or pressure mats that detect people entering a hazardous area and shut the machine down. Why is a presence sensing system at the bottom of the legal guarding hierarchy? Simply because many machines can eject objects at a very high velocity, and a presence sensing system will not contain those hazards even though it may detect the object movements before it strikes you.

However, there are many instances, for example palletising machines, where presence sensing is the suitable solution. Goods can enter and exit the machine freely by careful muting of the presence sensing device. However, if a human tries to enter such a machine, the machine shuts down - controlled by an AS 4024-based control system matching the level of risk.

Productivity and safety

If we diligently follow the requirements explained so far, we could be ignoring the most dangerous and often subtle issue. That is, the tendency of humans to want to do things quickly and easily. A safety system which has not taken this into account can, and frequently does, motivate people to bypass or defeat the system which is slowing them down or making their job more difficult. They are not usually doing this with malicious intent, or to harm themselves, but with the laudable motive of maximising output. Often, safety designers cleverly attempt to make things extraordinarily difficult to defeat only to find that the defeater has been even more highly motivated to find even cleverer ways to get around it. A recent German study showed that around 40% of installed metal working machine safety systems were tampered with in some way. It is a global problem. Therefore, we must have the productivity and operability of the plant we are trying to make safe at the forefront of our thinking. We must discuss the possible safety solutions with the people who operate, maintain and clean the machines. We must solicit their input and arrive at clever solutions that make the machines safe by design but not harder to use and do not reduce the productivity of the process. There is no one-size-fits-all solution to this problem. The optimum, safe and productive design should be determined on a case-by-case basis. Finally, after all this has been done, there is usually some residual risk left. As long as all the engineering-based risk reduction solutions (as far as reasonably practicable) have been considered, only then can the administrative controls and personal protective equipment be applied to control the final risks. These controls can be essential in many environments (noise, falling objects, ejected substances), but we must recognise that they are totally reliant on the predictable behaviour of humans to be effective. This requires total focus from the management of the company and their OHS teams.

*Frank Schrever, Director, Machine Safety by Design. Schrever has managed a number of subsidiaries of multinational companies, establishing the Pilz subsidiary in Australia in 1998 and managing it for 12 years. He has been the IICA representative on the Australian standards committee for ‘Safety of Machinery’ SF 041 reviewing AS 4024 and other machine-specific standards since 1999. He is now chairman of this committee and a member of the ISO technical committee for combining ISO 13849 and IEC 62061.