Why Cpt is so important? In the process industry when we calculate the Probability of Failure on Demand (PFDavg) of a Safety Instrumented Function (SIF) we use equations of the following type: “Cpt” parameter is one of the most important and often is not paid much attention. “Cpt” (Proof Test Coverage) parameter is defined …
Dangerous failures of the SIS When calculating Probability of Failure of a Safety Instrumented Function (SIF), the most important are the dangerous failures, as we see in the following equations of 1oo1 and 1oo2 architectures.The value Lambda DD is the rate of dangerous detected failures, and Lambda DU corresponds to the undetected ones. We can …
The importance of diagnostics in the SIS The diagnostics in an Safety Instrumented System (SIS) are of crucial importance because they are the key to reduce the rates of undetected dangerous failures and, therefore, to reduce the probability of failure on demand (PFD / PFH) and increase the SIL. We can see it in the …
The importance of common cause failures When designing a control system, we should paid special attention to common cause failures, that is, to the factors that may cause the simultaneous failure of several components or redundant channels. It is an even more important aspect in the case of safety instrumented systems (SIS) and is considered …
What is a Safety Instrumented Function? The safety instrumented function is a control loop in a process or machine whose objective is safety. SIF is its acronym in English. In the following image we see the most common simplified representation of the SIF. The integrity and performance of the safety instrumented function depends on a …
SIL requirements – Systematic Capability, Failure Probability and Architectural Constraints. The designer of the safety instrumented function must verify that the 3 SIL requirements of the IEC61508 Standard are met. Each requirement will meet a certain maximum SIL level. The final SIL of the SIF will be the lowest of the three and must be …
What is a safety PLC? Let’s try to explain it in a simple way for non-experts. The fundamental difference with a general purpose PLC is summed up in one word “Diagnosis”. In addition, there are differences in terms of internal architecture, software and firmware, and certification for applications where compliance with a certain SIL level …
