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A systematic failure is a failure due to a systematic cause that may be attributed to a human error or misjudgment in the specification, design, manufacture, installation, operation, or maintenance of the item. A software bug is a typical example of a systematic fault. After the error is made, the systematic cause remains dormant and hidden in the item. Examples of systematic causes are given in Example 3.12.

A systematic failure occurs when a certain trigger or activation condition occurs. The trigger can be a transient event that activates the systematic cause, but can also be a long‐lasting state such as environmental conditions, as illustrated in Example 3.14. The trigger event is often a random event, but may also be deterministic.

A systematic failure can be reproduced by deliberately applying the same trigger. The term systematic means that the same failure will occur whenever the identified trigger or activation condition is present and for all identical copies of the item. A systematic cause can only be eliminated by a modification of the design or of the manufacturing process, operational procedures, or other relevant factors (IEC 61508 2010). A systematic fault leading to a systematic failure by the “help” of a trigger is shown in Figure 3.10. Systematic failures are often, but not always, random events, but it is the trigger that is random, whereas the item failure is a consequence of the trigger event.

Figure 3.10 A systematic fault leading to a systematic failure.