# How is the PFDavg calculated for IEC 61511?

The most common method for calculating PFDavg in a safety chain is to use the Reliability Block Diagram (RBD) approach. Here's what it's all about.

## What is Reliability Block Diagram (RDB)

It is nothing more than a diagram that provides the relationship between component states and the success or failure of a given system function.

So let us take a classic safety function consisting of sensor, logic solver and final elements:

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The probability of on demand failure is then calculated by independently considering the PFDavg for the three subsystems, and then adding them:

## Calculation of the probability of failure of subsystems

So now the question for us is, how do we calculate the probability of failure of the various subsystems?

Here we are helped by IEC 61508-6 which provides formulas for calculating various configurations.

In fact, the result of the calculation for each individual subsystem depends on:

• Architecture, and therefore any redundancies
• The dangerous failure rate of each component
• The effect of the diagnostic function, if any
• Common cause failures
• The frequency and effectiveness of the diagnostic proof test
• The frequency and effectiveness of the proof test
• MRT (Mean Repair Time)

It should not be forgotten that the calculation of PFDavg is not the only parameter to be considered in determining the SIL level of a safety function: in fact, architectural constraints and systematic capacity play a very important role.

# What ISO 26262 says about Fault Classification

ISO 26262 key-points The ISO 26262 series of standards is an adaptation of the IEC 61508 series of standards needed to address the specific needs of the road vehicle sector. Some of its key-points are: Provides a reference for the automotive safety life cycle and supports the adaptation of activities to be performed during the […]

# The brand new ICSA Certification by ISASecure for IIoT devices

What is the ICSA Certificate The IIoT Component Security Assurance (ICSA) certification was inspired by recommendations published in the joint ISA Global Security Alliance (ISAGCA) and ISA Security Compliance Institute (ISCI) study. The ISASecure IIoT Component Security Assurance (ICSA) is a security certification program for IIoT devices and IIoT gateways. ICSA certification applies to IACS (Industrial Automation […]

# Field returns for IEC 61511

Difference between Prior Use, Proven in Use and Route 2H Here are the differences between the three types of field returns: Proven in Use (“PIU”) is a way to demonstrate a device’s ability to avoid systematic failures. This concerns IEC 61508 and so this mode of analysis is used by manufacturers Route 2H is used […]

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