I am trying to wrap my head around some excerpts from:

  • CCPS Layer of Protection Analysis
  • CCPS Guidelines for Independent Protection Layers and Initiating Events
  • IEC 61511

I am trying to understand how to apply Approach B and use two functions in the BPCS (e.g. a control loop and an alarm) as independent Protection Layers (IPLs) in a LOPA, when the CPU is shared.Guidelines are very vague about it.

What parameters should be looked at, to decide to apply Approach B?

Note: I understand that, if the guidelines are not clear to me, I should not apply Approach B. The request comes from a client who is trying to consider as many safeguards as possible before having to update their BPCS and SIS.


You are allowed to claim 2 BPCS IPL's but not from the same BPCS, eg you will need a PLC and a DCS with an IPL in each to claim both of them, plus they must be adequately independent.

Additionally, you will be capped at a RRF of 10 for each, giving 100 for both, but that could be the difference between a SIL 3 SIF and a SIL 1 SIF in the SIS, potentially.

  • $\begingroup$ Thank you for the input - yeah, if they are in separate logic solvers and all initial and final elements are independent, then we do not have this issue. The client question is really about the possibility of having two functions in the same LS $\endgroup$ – laureapresa Apr 13 at 8:30

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