Q & A: Category 2 and Testing Intervals

Last updated on August 24th, 2022 at 02:36 pm

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During the Free Safety Talks with Schmersal Canada and Franklin Empire, we had a “hot question” regarding Category 2 architecture and the testing interval requirement. Category 2 in [1, 6.2.5] does not mention a minimum testing rate, but [1, 4.5.4], “Simplified procedure for estimating the quantifiable aspects of PL,” adds a requirement for test rate and MTTFD for the Test Equipment. In the short video below, Doug answers that question.

If you have more questions or felt something wasn’t clear in the video, leave us a comment, and we will get back to you!

If you are having problems with developing your safety function, please get in touch with Doug directly.


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[1] Safety of machinery — Safety-related parts of control systems — Part 1: General principles for design, ISO 13849-1. 2015.

[2] “The Bathtub Curve and Product Failure Behavior (Part 2 of 2)”, Weibull.com, 2018. [Online]. Available: http://www.weibull.com/hotwire/issue22/hottopics22.htm. [Accessed: 13- May- 2018].

[3] German Social Accident Insurance (DGUV) – Institute for Occupational Safety and Health (BGIA), “BGIA Report 2/2008e — Functional safety of machine controls — Application of ISO 13849-1”, Institute for Occupational Safety and Health (BGIA), Sankt Augustin, DE, 2008. Available: https://www.dguv.de/medien/ifa/en/pub/rep/pdf/rep07/biar0208/rep22008e.pdf. [Accessed: 22-Nov-2019].

[4]     “Safety Circuit Examples of Safety Components | Technical Guide | Australia | Omron IA”, omron.com.au, 2018. [Online]. Available: http://www.omron.com.au/service_support/technical_guide/safety_component/safety_circuit_example.asp. [Accessed: 14-May-2018].

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6 thoughts on “Q & A: Category 2 and Testing Intervals

  1. Hi Doug – great content. Thank you!

    We are purchasing a machine where the machine builder has included a safety interlock requiring PLd that utilizes a single hydraulic valve to cut off flow to a pair of cylinders. This valve is used in normal operations and cycled 35,000 times/year. The failure of this valve would be noticeable in that the machine would stop functioning. They also plan use this valve in the safety function which is expected to be demanded 300 times per year.

    I have (2) questions:
    (1) Is observation by machine operators considered a valid “test channel”?

    (2) If observation is not acceptable – If a position switch is installed on the valve and monitored by
    an appropriate PLC would I be correct in stating the circuit would meet Category 2 since the correct
    cycling of the valve could be confirmed at a greater than 100x safety function demand rate?

    Thank you!

    1. Hi David,

      Thanks! ? I’m glad you’re finding it useful. Responding to your questions:

      1) Observation can’t be considered a test channel. Humans are far too unreliable for that. ?
      2) This is a reasonable approach to solving this problem presuming that a Cat. 2 structure is sufficient in the application. Also, make sure that you are taking into account the high reliability of most fluidic valves. I have not done any calculations to confirm this for your case, however, I expect that even if you don’t have any reliability data on the valve itself and therefore have to use the default 150 years given in ISO 13849-2 you should find that the resulting active channel MTTFD will be high enough. Do the analysis and see where you come out. Remember that you can chain together sub-systems with different structures to take advantage of the MTTFD/PL achieved by each subsystem.

  2. is {CAT 2 + solenoid latching interlocking switch} equivalent to CAT 3 OR considered control reliable?

    1. Hi Ash,

      No, Cat. 2 + guard locking is not equivalent to Cat. 3. If you review the definition for Category 3, you’ll see that there are two key parameters: DC and the structure, which include two functional channels with cross-checking. Guard locking is normally a separate safety function from guard interlocking, and the requirements for guard locking are different from those of interlocking.


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