Which Measure (Rationalized or Annunciated) is More Important?

Apr 14, 2020

[:en][vc_row][vc_column][vc_column_text]Get your priorities (distribution) straight

A very common question is posed during alarm management training. Does the recommended alarm priority distribution of ~5% / ~15% / ~80% for high / medium / low priority alarms apply to the rationalized alarm priority distribution (as configured in the control system) or to the annunciatedalarm priority distribution (those alarms actually presented to the operator)? Is one priority assessment more important than another?

Alarm priority, which is the relative importance assigned to an alarm to indicate the urgency of response, is determined during the rationalization process. The effectiveness of alarm priority depends on the distribution of alarm priorities; higher priorities should be used less frequently (Ref ISA-18.2). If too many alarms are high priority, then effectively none of the alarms are high priority.

The ISA-18.2 / IEC 62682 standards provide recommended metrics for the annunciated alarm priority distribution as shown below:

The guidance in ISA-18.2 / IEC 62682 on rationalized priority distribution is much less precise.

This essentially says that a good rationalized priority distribution is one that leads to meeting the target annunciated priority distribution. There is no direct correlation between the rationalized and annunciated priority distribution, so defining rigid requirements for rationalized priority distribution is not useful. Instead good guidance for the rationalized priority distribution might be that there should be fewer high priority alarms than medium priority alarms, and fewer medium priority alarms than low priority alarms. Obviously, you are not likely to hit the target annunciated distribution for high priority (~5%) if > 50% of your rationalized alarms are high priority. Therefore, it is still important to review the rationalized priority distribution to assess whether rationalization was performed correctly.

Another interpretation of what is written in the standards is that the annunciated priority distribution is more important as a KPI than the rationalized priority distribution. Not all alarm management practitioners agree with this interpretation. Some believe that rationalized priority distribution is more important. To comply with the 5 / 15 / 80 % distribution targets, these practitioners assign a numerical score to each alarm during prioritization, where the score is calculated based on summation of different weighting factors (e.g. severity of consequences in environmental, in personnel safety…). The alarms are listed in order of score, and the 5% with the highest score are assigned to the high priority. The annunciated priority distribution is then accepted however it turns out.

[/vc_column_text][/vc_column][/vc_row][:zh][vc_row][vc_column][vc_column_text]No! They are not Inherently Safe!

A collaborative robot is intended to work “collaboratively” with a person. i.e. share a common workspace. It is force and speed limited by design to minimize any potential hazard. Collaborative robots fit the application where the task cannot be easily or cost effectively automated. They are easy to deploy, program and repurpose. Collaborative robots are new to everyone including the standards agencies.

A hazard and risk assessment is required that assesses the robot and the environment that it is deployed in. Just as any other robot, things such as collisions, speed, type of end effector and worksite need to be evaluated. Collaborative robots have their own sorts of collisions and hazards. They may not be as severe, but they still exist.

This all comes down to risk and the amount of risk that you are willing to accept! The diagram below shows the high-level steps for doing a Hazard and Risk Assessment. When following the steps, if you assess the risk and find it to be acceptable (your companies acceptable risk norms) then you are done. No need to add any risk reduction.

The next best approach is to determine if protective measures other than a Safety Function can reduce the risk to an acceptable level. If not, then you must assign a SIL and implement a safety function that will provide the required risk reduction.

exida can effectively train your team to perform machine hazard and risk assessments to identify all possible hazards and estimate the risk for each hazard. Specifically, exida coaches you through the process of evaluating the risk, developing and implementing risk reduction options. exida can also educate your team in multiple approaches to SIL target selection. These are just some of the things exida does to ensure you are on the right path![/vc_column_text][/vc_column][/vc_row] [:]

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