Machine Safety & Residual Risk

Residual risk is a term used for the past several years referring to a level of risk for a given hazard after applying protective measures (risk reduction measures). ANSI B11.TR3; 2000, ANSI B11 – 2008, ANSI / ISO 12100-1:2007, ISO 14121:2007, …..to mention a few, all tend to be harmonized on this definition. However, tolerable risk and acceptable risk are two additional terms also in the mix. Tolerable risk is generally defined as that risk which is accepted for a given task and hazard combination [hazardous situation] after applying risk reduction measures. The extension of this process introduces acceptable risk as the level at which further risk reduction will not result in significant reduction in risk or that additional expenditure of resources will not result in significant advances towards increased safety.

In my opinion, the reality of this concept is that there is a very strong reluctance in industry to ever document two words in the same sentence – “risk” and “acceptable”. Residual risk, on the other hand, seems to imply that after risk reduction measures have been applied to a known hazard, some level of reduced risk still exists. It’s generally understood that it’s virtually impossible to drive risk to a zero value. With that said, does this remaining component of risk need to be “accepted”? Or, does it still need to be addresses? It’s kind of like the comparison to a zebra. Is a zebra white with black stripes or is it black with white stripes?

In the final analysis, how much does this level of granularity really matter? I believe that the goal is to achieve operator safety. So, shouldn’t we all follow the following steps?

1. Perform a thorough risk assessment
2. Identify the hazards
3. Apply risk reduction measures
4. Document the process

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