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What’s the Purpose of a Functional Test Plan?

So your company has decided to follow ISA S84 / IEC 61511 and you find yourself trying to decide if your current test plans are “good enough” as-is. 

This raises the question of what makes a functional test plan “good”? 

When I ask this question to practitioners I get a whole host of answers:

  • The test plan must be easy to follow
  • The test plan should include references to all supporting information (P&ID, Loop Sheet, Cause & Effect)
  • The test plan should have clear pass / fail criteria
  • The test plan should written such that a junior instrument technician could run the test
  • The test plan should written such that a senior instrument technician could run the test
  • Should include device calibration
  • Should NOT include device calibration
  • Should include leak testing of valves
  • Should NOT include leak testing of valves
  • Should test the entire SIF from sensor through the logic solver to final element
  • Should test SIF in pieces (sensor only, logic solver only, final element only)
  • Should include testing of all diagnostics
  • Should NOT include testing of all diagnostics

While some of the answers are things one would definitely want their test plans to address, some of the requirements conflict with one another. So let’s step back and ask what is the primary purpose of a functional test plan per the requirements of S84 / IEC61511? I’ll even refine this question further – define the primary purpose of a functional test plan in four words.

If we were together when I asked this question, this is the awkward pause moment of the conversation. 

The primary purpose of a functional test plan is to “detect dangerous undetected failures”.  Keeping this concept in mind, it becomes much easier to answer the question of what makes a functional test plan “good”.

For additional information, read the white paper titled Roll Out and Maintenance Integration of SIS Proof Test and Inspection.



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