There’s a lot that exida does behind the scenes to advance the industry and benefit you. We try to make sense of data that we collect and put it to good use. One thing we found is that human activities play a role in functional safety, often causing the intended safety performance to be less than the ideal prediction. This is due to something we call the Site Safety Index™, or SSI.

The SSI is a number from 0–4 that indicates the level of site activities and practices that contribute to the safety performance of safety functions on the site. Table 1 details the interpretation of each SSI level. SSI 4 implies that allrequirements of IEC 61508 and IEC 61511 are met at the site, and therefore there is no degradation in safety performance due to any end-user activities or practices. SSI 0 implies that not much proactive work is done to maintain the safety functions.

Several factors have been identified which impact the SSI. These focus on the quality of:

  • Commissioning and Safety Validation Testing
  • Proof Test Procedures, Planning, and Documentation of Tests
  • Failure Diagnostic and Repair Procedures
  • Modification Procedures (Making changes is the scariest part of functional safety)
  • Device Useful Life Tracking and Replacement Procedures
  • Decommissioning Procedures

Table 1: exida Site Safety Index Profiles


Description of Maintenance Levels


Perfect: Repairs are always correctly performed, testing is always done correctly and on schedule, formal procedures exist and are used for all repair and testing, checklists and event logs are up to date, equipment is always replaced before end of useful life, equipment is always selected according to the specified environmental limits and a thorough process compatible materials analysis. 
Note: This level is generally considered impossible, but is retained in the model for comparison purposes… a level to strive for.


Superior: Close to SSI 4 except for some lapses in one or two areas, or rationale for occasional “judgment calls” due to expediting the work.


Typical: Repairs and testing are usually performed correctly and mostly on schedule, most equipment is replaced before end of useful life, some procedures exist, and they are usually followed. (This is the assumed SSI in a FMEDA, unless further support can be shown.)


Good enough: Repairs and testing are performed, sometimes on schedule, but rely more on personnel experience instead of formal procedures; some equipment is replaced before end of useful life if it’s easy and convenient.


Not so good: Only minimum work is done to keep system running; periodic testing is not done, equipment is not replaced until failure, a chaotic work environment.

If you have any doubts about field personnel affecting the process, here's a quick story: A pressure transmitter manufacturer was getting a high number of field returns from a few customers. The complaint was for inaccuracies and/or non-functioning devices. The evaluation and investigation revealed that maintenance technicians were cleaning out a pressure diaphragm hole with a screwdriver, regularly puncturing or damaging the diaphragm. Of course, this would probably not happen at your site… would it?

Improving your SSI is not rocket science.  You can start with these steps:

  1. Write down what you do, then expand and correct it to provide formal procedures;
  2. Train the maintenance folks on the procedures as well as the equipment;
  3. Develop “management of change” procedures;
  4. Simplify calibration procedures whenever possible;
  5. Refer to the equipment safety manual or user manual in procedures and trainings.

We'd like to hear your stories. What's your SSI? What are you doing about it?

For more information on SSI, see these white papers:

Quantifying the Impacts of Human Factors on Functional Safety

Assessing Safety Culture via the Site Safety Index

Tagged as:     SSI     john yozallinas     IEC 61511     IEC 61508     functional safety  

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