Machinery Safety 101

Q & A: Category 2 and Testing Intervals

Dur­ing the Free Safety Talks that we did with Schmersal Canada and Frank­lin Empire, we had a “hot ques­tion” come up regard­ing Cat­egory 2 archi­tec­ture and the test­ing inter­val require­ment. The defin­i­tion of Cat­egory 2 in 6.2.5 does not include any men­tion of a min­im­um test­ing rate, but 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.


Email Doug dir­ectly.


[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)”,, 2018. [Online]. Avail­able: [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: [Accessed: 22-Nov-2019].

[4]     “Safety Cir­cuit Examples of Safety Com­pon­ents | Tech­nic­al Guide | Aus­tralia | Omron IA”,, 2018. [Online]. Avail­able: [Accessed: 14-May-2018].

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Acknow­ledge­ments: ISO, OMRON, oth­ers as cited.
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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 CATOR 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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