What is a Safety Instrumented Function?

The safety instrumented function is a control loop in a process or machine whose objective is safety. SIF is its acronym in English. In the following image we see the most common simplified representation of the SIF.

safety instrumented function

The integrity and performance of the safety instrumented function depends on a large number of factors, and it is measured by the so-called “Safety Integrated Level” (SIL) which are covered by various international standards such as IEC-61508 (for all industries), IEC-61511 (for the process industry), IEC-62061 (for machinery safety), IEC-61513 (for the nuclear industry) or ISA-84.

Some of the main factors that influence the performance of the SIF are the following:

  • The technology used: the quality of the components and the manufacturer, the safe and dangerous failure rates, the capacity of automatic diagnostics of the components, etc.
  • The architecture used: component redundancy, common cause failures, etc.
  • The response time of the components, the time to be repaired and restoration time to normal operation.
  • The activities throughout the life cycle of the safety instrumented function such as periodic tests, documentation of failures and other actions, SIL level verifications, etc.

In the following image we can see a more detailed representation of the SIF where you can see many other elements that make up the safety function.

safety instrumented function

It is very important to consider everything around the sensor to work properly, such as an adequate connection to the process, a correct measurement technology in each case, or other aspects of the design such as wiring and interface components with the safety PLC.

Logic solver

The logic solver can be a PLC, a relay system or an electronic system in general (programmable or not) but must meet a series of requirements to be used in an safety instrumented function. In this article we talk for example about the safety PLC. The design should take into account both hardware and, software or firmware, as well as external factors such as cybersecurity.

Final elements

In the safety instrumented function the final elements are usually the weakest link in the chain for different reasons (mechanical elements and in direct connection with the process). It is very important to select well the construction materials, as well as all the components and a correct execution of the mounting on site.

Other elements

There are many other elements and external factors that greatly influence the performance and integrity of the safety instrumented function such as external temperature, vibrations, electromagnetic interferences, if there is dust in suspension (especially if it is corrosive), power supplies, operation and maintenance tasks, etc. All these factors are in the category that we call common cause failures and that must be analyzed in detail in order to minimize their impact on the performance of the SIF, i.e., to avoid the degradation of the required SIL level.

Watch this video about the basics of Functional Safety.

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