Definition:
Common Cause Failure (CCF) is the simultaneous failure of two or more components or systems due to a single shared cause or event, rather than independent random failures. Common cause failure could be a bad weld on multiple valves from manufacturing due to low quality weld rod
Key Points:
- Common causes can include environmental factors (temperature, humidity), human error, design flaws, or systemic issues like shared utilities.
- CCF undermines redundancy and reduces the effectiveness of fault tolerance.
- Quantified in reliability models using a Beta Factor (β)
- Comes into play in the PFDavg calculations with more than 1 instrument, such as 2oo3.
Example:
A SIF with 2oo3 coincidence logic uses SC 2 pressure transmitters. These instruments have projected scenarios that a common cause could come into play. Thus in the 2oo3 PFDavg calculations the Beta factor is included.
In 1oo1 scenarios it is not.
See also: Beta Factor
Cited Source:
- IEC 61508-4:2010, Clause 3.6.16.