TABRA: A task based approach to risk assessement in Machinery

Last edit: 01/08/2023

When applying ISO 12100, we tend to focus on the hazards that result form situations an operator or the maintenance team can incur to; in North America they tend to focus on the hazards associated with the tasks a person may be dealing with. In essence, the majority of the European risk assessment procedures are Hazard Based, while in USA and Canada they are Task Based. That is the meaning of the acronym widely used in USA: TABRA. The reference standard is ANSI RIA TR R15.306.

The Task Based Risk Assessment concept is mentioned in the new edition of the EN ISO 11161, the reference standard regarding the safety of the integrated machinery systems (IMS); it is mentioned together with the Hazard based one, meaning both are important.

Task Based Risk Assessment is a formal process of identifying the hazards associated with each task to be performed, assessing the risk, and providing the safety controls to manage the risk. The key steps can be summarized as follows:

  • Identify the groups that interact with a machine;
  • Determine the tasks performed on the machine;
  • Identify the hazards of performing these tasks;
  • Score risks using a risk scoring system;
  • If necessary, reduce the risk.

Once the initial risk is assessed, it needs to be evaluated.
If the risk is at a tolerable level, it is possible to accept the outcome or attempt to lower the risk even more, depending on the cost and efforts of the mitigation.
If the initial risk analysis yields an unacceptable risk level, then risk mitigation must take place in an iterative manner, until the risk is reduced to a tolerable level.

Hazards shall be eliminated or the risks associated with the hazard shall be reduced according to ISO 12100 by the three-step method:

  • inherently safe design measures
  • safeguarding and span-of-control
  • information for use

Both normal and atypical tasks should be listed based on the experience of the personnel and the anticipation of what could occur. This process also allows to consider the different classes of stakeholders that interact with the machine and other elements that can affect the work environment (e.g. shift work, different phases of the production cycle). Projecting future scenarios is especially important for the machinery that is not currently in use or has been recently commissioned.

Our experience is that a Hazard Based Risk Assessment tends to produce a large excel sheet that leads to minimise the solutions to be used to reduce the risk. A Task Based Risk Assessment normally uses Risk Charts whereby each chart represents a risk incurred by a person and the way to reduce it. The Risk Charts tend to be more specific and usable by the Development or the Engineering team who must apply safeguards to the machinery and by the operator who must perform the required task.

Task-based machine risk assessment yields results that are easy to quantify and prioritize according to their score. The scoring system enables the plant personnel to focus on tasks with the most severe hazards to achieve practical risk reduction.

In conclusion, Task Based Risk Assessment is an alternative way to perform a risk assessment that can complement the Hazard Based one, normally used in Machinery Safety in Europe. It is a good way to manage risk associated with critical work that is scheduled to be performed in a critical area. This provides the systematic approach to ensure safety while performing critical work and thus helps prevent accidents.

As for all Risk Assessements, it is a team work: for an effective TABRA, you need to involve colleagues from different disciplines to contribute to the identification of the hazards, the risk assessment and the decision on control measures.

Safety in Collaborative Robotics
There is no “Collaborative Robot”. That is one of the first statements you hear from people working in Collaborative Robotics. The reason is because...