Common Cause Failures
An event or mechanism that can cause two or more failures (basic events) simultaneously is called a common cause, and the failures themselves are called common cause failures. Because common causes can induce the failure of multiple components, they have the potential to increase system failure probabilities. The elimination of these common causes can appreciably improve system reliability.
Designers must recognise the failure sources that are responsible for common cause failures and implement specific solutions to deal with them. A list of frequently encountered causes, which are not in any specific order, follows:
• Mechanical Causes:
◦ Abnormally high or low temperature.
◦ Abnormally high or low pressure.
◦ Stress above design limits.
◦ Impact.
◦ Vibration.
• Electrical Causes:
◦ Abnormally high voltage.
◦ Abnormally high current.
◦ Electromagnetic interference.
• Chemical Causes:
◦ Corrosion.
◦ Chemical reaction.
• Other Causes:
◦ Earthquake.
◦ Tornado.
◦ Flood.
◦ Lightning.
◦ Fire.
◦ Radiation.
◦ Moisture.
◦ Dust.
◦ Design or production defect.
◦ Test/maintenance/operation error.