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Article | REF: SE4058 V3

Probability of failure of the safety function

Author: Olivier IDDIR

Publication date: September 10, 2024

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3. Typology of failure rates

3.1 Definitions

IEC 61508-4 defines a failure as the cessation of a functional unit's ability to perform a required function. It also considers that failures can be of two types:

  • dangerous: a failure that has the potential to place the safety-related system in a dangerous state or make it impossible to perform its function;

  • in safety: a failure which does not have the potential to place the safety-related system in a dangerous state or make it impossible to perform its function.

These failures are then divided into two categories:

  • detected: failure detected by diagnostic tests, periodic tests, operator intervention (e.g. physical inspection and manual...

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Typology of failure rates
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