Glossary · Functional safety
Common-cause failure (CCF)
Also known as: CCF, common cause failure
German: Ausfall aufgrund gemeinsamer Ursache (CCF)
In functional safety, a common-cause failure (CCF) is the failure of multiple elements resulting from a single shared cause. It can defeat redundancy, because both channels fail at once.
- Functional safety
- Systems engineering
In one sentence
A common-cause failure (CCF) lets one shared cause disable several channels at once and can defeat redundant safety designs.
Example
Both channels of a door interlock are wired in the same cable along a conveyor, so one crushed cable disables them together, a common-cause failure that the redundant design was meant to prevent.
How it applies
- Functional safety: ISO 13849-1 requires measures against CCF for Categories 2, 3 and 4 and assesses them with a scoring method (separation, diversity, protection against overvoltage and environmental influences, competence). IEC 61508 and IEC 62061 use a beta factor.
- Integrated systems: Shared power supplies, shared cabling, shared software libraries and shared environmental conditions are typical common causes. In an integrated system, check them across machine boundaries.
- Technical documentation: Record the CCF measures and their justification. Maintenance instructions must preserve them, for example by forbidding re-routing of separated cables.
- Evidence: A redundancy claim is only as strong as its CCF analysis.
Common-cause failure vs. single failure
A single failure affects one element and is tolerated by redundant designs. A common-cause failure affects several elements at once from one cause, so redundancy alone does not help.