Random Failures
Random failures occur unpredictably and are typically attributed to the degradation of hardware components due to physical causes such as corrosion, thermal stress, or wear-out. These failures are generally well-understood and happen independently of external conditions.
A random failure is usually permanent, meaning it renders a component or module inoperable, and is often traced to specific devices or loops within a system. Since these failures occur at random times, their likelihood can be statistically analyzed to determine an average probability.
Examples of Random Failure
For example, a lightning strike near a plant can induce an electrical surge that damages a component, such as a transistor in a controller module. This is a classic case of a random failure caused by external stress that is inherently unpredictable.
Another example involves a power supply module failing because the electrolytic capacitor inside it loses its electrolyte over time due to evaporation. This natural degradation process eventually leads to the capacitor becoming an open circuit, preventing the power supply from functioning.
Both cases illustrate how random failures are inherently linked to physical mechanisms and can be mitigated through thoughtful design, such as using higher-integrity equipment, incorporating redundancy, or employing robust materials less susceptible to wear-out.
Mitigation of Random Failures
To address random failures effectively, system designers often adopt strategies that enhance the resilience of the hardware. These include selecting components with higher durability and reliability, implementing backup systems to maintain functionality during failure, and conducting regular maintenance to identify and replace aging components before they fail.
Systematic Failures
Systematic failures, in contrast, result from errors during the development, design, operation, or maintenance of a system. Unlike random failures, systematic failures are not tied to physical degradation but instead arise from flaws in processes, procedures, or logic. These failures are consistent and repeatable under identical circumstances, making them more challenging to predict and characterize statistically.
For instance, a software crash caused by a specific input or a programming error in safety logic that prevents protective functions from activating are examples of systematic failures. The hardware may remain fully operational, but the system fails to achieve its intended function due to errors in its design or implementation.
Systematic failures often have a broader impact than random failures, as they can affect multiple devices, loops, or even entire systems within a corporation. This is because systematic failures are tied to “the way things are done,” such as organizational practices, training, or procedural gaps.
For example, if a safety system fails during a unique operation or when receiving a combination of input data that was never tested, it reflects a systematic issue. These failures can also be transient, appearing under certain conditions but disappearing in others, adding to their complexity.
Mitigation of Systematic Failures
To mitigate systematic failures, organizations must focus on improving administrative controls and monitoring processes. This includes enhancing staff training, refining procedural guidelines, and applying rigorous testing protocols during the system’s life cycle. Qualitative measures, such as life cycle activities, aim to minimize the likelihood of systematic failures by addressing potential weaknesses in development, operation, and maintenance phases. By proactively identifying and rectifying these issues, organizations can reduce the occurrence and impact of systematic failures.
Top References
- Safety Instrumented Systems Verification: Practical Probabilistic Calculations William M. Goble Harry Cheddie
- IEC-61511
- www.exida.com
- https://www.exida.com/Blog/random-versus-systematic-faults-whats-the-difference
- Guidelines for Safe Automation of Mechanical Processes by Center for Chemical Process Safety
- Reliability, Maintainability and Risk by Dr David J Smith
