Forum Topic

Monday 19 May 2008 10:53:10 am

PFS（probability of failing safely）is one of the most important parameters of the SIS(safety instrumented system), which is related with economy of factory. It does not matter if safe failures are detected or not .both will cause a shutdown. Then it will waste the money. So I think it is very important, But I don’t find the formula of PFS in the IEC61508. I just find the formula in the Control Systems Safety Evalution and Reability. Besides this ,where can I find PFS formula in other way, then I can compare those.

Saturday 07 June 2008 1:49:35 pm

There are books available that list PFS formulas. But you should not use these standard formulas. You should learn to make your own reliabilities models with markov, fta, or other modeling tools.

Monday 14 July 2008 9:54:46 am

I just have a question. When we calculate the level of SIS, we use the concept of PFDavg which means the average of PFD happens during the whole process, so, but I can’t find the concept of PFSavg that use in SIS. Does it exist?

Wednesday 16 July 2008 6:41:42 pm

61508 and 61511 do not talk about this. They do no care about spurious trips.

ISA S84 and its technical reports do talk about PFS.

But just because it is not in a standard it does not mean it is not important.

Thursday 17 July 2008 5:03:35 am

First of all,thank you for help me .I find that there is a new concept (STL) in your web, which will be

more convenient to calculate the PFS. Can you mail me more

information about STL. Thank you very much!my e-

mail:2002043127@163.com