IEC 61511 defines Random Failure as „occurring at a random time, which results from one or more … degradation mechanisms in the hardware.“ Such failures occur at predictable rates (see bathtub curve) but at unpredictable (i.e., random) times. Systematic Failures are those „related to a pre-existing fault … which can only be eliminated by … a modification of the design, manufacturing process, operating procedures, documentation or other relevant factors". Random failures are often defined as ‚hardware-related‘, whereas systematic failures are ‚due to human error‘. A systematic failure can be eliminated after being detected, while random hardware failures cannot. Implementation of a Functional Safety Management System shall minimise systematic failures.
Examples of random failure are:
Aging or stress failure of electronic components including: Contact failure, soldered joint failure PCB/semi-conductor failure Relay stiction Resistor/capacitor degradation
Examples of systematic failure are:
Errors during the Analysis phase of the SLC (HAZOP, SIL-Analysis, SRS) – see UK HSE “Out of Control: Why control systems go wrong and how to prevent failure” Software „bugs“ Overlooking common cause failure O&M mistakes (SIF bypass, incorrect calibration, etc)