Hi Thomasz, funny you should bring this up as I am currently in the middle of designing a SIL rated loop with this exact same scenario. I was chatting with another one of our engineers this morning and he said he had previously used generic data (Exida SERH) for the Contactor, and said the SFF is usually around 60%. We will be running the calcs along with others over the next couple of weeks so will report back with which data we use ... Just in addition to this we will wire the N/C aux contact of the contactor in series with the reset of the PILZ Safety Relay to inhibit the SIF being reset if the Contactor fails (welded contacts etc). This should provide better diagnostic coverage since the existing VSD pre dates Safe Torque-Off functionality!

Hi Sergio, great question. Personally this is not something I think you could came claim credit for twice in a LOPA, i.e. as a BPCS layer and Safety layer (if that’s what you are meaning) as there are too many points of CCF (the same initiator and same final element). It would be best to have a completely Independant second loop for my safety layer if you want to claim credit for an additional layer of protection. For safety system trips I would always design the trip signal to be repeated to the BPCS via software link (or if ifs a hardwired trip, using a second contact off the trip amp), however this would be for indication purposes only, Also note that whenever there’s an interaction from between the SIS and the BPCS the designer must be very careful to ensure that any BPCS failure does not have any affect on the SIS. So personally I would stray away from the idea you mentioned as I don’t see it serving much additional benefit from a risk reduction factor point of view. It would be best to go for two PT’s for two completely independent channels and ideally with separate final elements, although on some legacy plants sometimes the final elements are shared. In this case there must be a reduction in your IPL credit claim on the BPCS layer … This is just my opinion, would be good to hear what the others in the group think.

Hi Thomasz, I am not entirely sure I am reading your question correctly but are you referring to Functional Safety Audits? Independent FS audits should check the adequacy of company safety procedures against the requirements from the standard. Audits are also used to check the company procedures are actually being followed by the project …

@Tomasz Barnert great stuff mate. I will be there so will be great to meet you!

@Rob Connors this would be great also as I am likely to miss the sessions if during working hours …