Machine Guarding: Tolerable vs residual risk
Why do so many colleagues in manufacturing choose to avoid the terms – tolerable risk and residual risk? If there’s one thing “integrated safety systems” and requirements for risk assessments have brought to the industrial forefront over the past ten years – it’s risk awareness. It’s now “OK” in conversation and correspondence within companies to use words like; risk, safety, mitigation, hazard, and consequences – all in one paragraph.
So, is there any difference between “tolerable risk” and “residual risk”?
In my opinion, yes! First, let’s start with residual risk. Residual risk is defined by ANSI B11 – 2008 (General Safety Requirements Common to ANSI B11 Machines) as; “risk remaining after protective measures (risk reduction measures) have been taken”. Furthermore, a commonly understood fact is that there will always be some level of residual risk for a given hazard after all risk reduction measures have been applied. In other words, residual risk will never equal zero. Let’s draw this machine safety an analogy with everyday life. Almost daily we get into our automobile and drive some distance to go to the store, work, school, etc. and we have experienced the residual risk associated with this experience. Many risk reduction measures have been applied by; the automobile manufacturer, the State Department of Transportation, the highway developer, the licensing department, etc. and the local police department is the enforcement agency. Therefore, everyone in the automobile has understood the residual risk associated with this experience acknowledging that there still is some risk which means it cannot equal zero.
Now, what is tolerable risk? Tolerable risk is defined by ANSI B11 – 2008 as; “risk that is accepted for a given task (hazard situation) or hazard. For the purpose of this standard the terms “acceptable risk” and “tolerable risk” are synonymous”. The standard goes on to say that; “The expression “acceptable risk” refers to the level at which further risk reduction will not result in significant reduction of risk; or additional expenditure of resources will not result in significant advantages of increased safety”. In my opinion, this definition means that all risk reduction measures have been taken to mitigate the given hazard in order to reduce the risk to a tolerable risk level. My experience is that various companies define tolerable risk levels which are indigenous to; their industry, their business model, the individual hazard, the competency level of employees they are able to hire, the level of local enforcement, and so forth. It’s sort of like the difference in driving a car in Lexington, Kentucky versus Mumbai, India. I would very likely define my level of tolerable risk very differently in Mumbai to the point that I probably would not even attempt to drive an automobile.
Now, is it possible that residual risk and tolerable risk may equal the same level? Of course! However, they are very different in my opinion because of different definitions and because their approaches are very different.
What is your opinion?
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