Typology of failure rates
Probability of failure of the safety function

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...