What Does Fail-Safe Design Mean?
Fail-safe design is an approach to building structures, machinery, and equipment in such a way that they can fail without causing excessive damage or putting people at risk of harm.
Preventing failures from occurring altogether would be a preferable outcome. Fail-safe design, however, recognizes that this is an impossible goal - components or entire systems can fail in ways that are difficult to predict or impossible to prevent. Rather than simply attempting to avoid failure, fail-safe design plans for a possible failure by finding ways to minimize its adverse outcomes.
Safeopedia Explains Fail-Safe Design
The aim of fail-safe design is to reduce the probability of malfunctions that affect safety or to minimize the damage or injury caused by a malfunction.
There are two distinct approaches to preventing failure in structures, especially those under dynamic loads: safe-life and fail-safe.
The Safe Life Approach
The safe life approach involves a careful consideration of the typical service load for a system and its components. Key components of the system are then tested to know how long they can hold up without failing under service conditions.
This estimate of how long it will take before the component may fail is then reduced by a factor of safety. The resulting duration is the safe life of the component. The component is scheduled to be replaced before the end of its safe life, thereby significantly reducing the likelihood that it will break, wear down, or otherwise fail.
The Fail-Safe Approach
Rather than preventing failure, the fail-safe approach aims to ensure that the system has enough integrity to remain safe even when one of the components breaks or wears down. Components with multiple load paths are generally fail-safe due to redundancy and because they may contain additional measures for damage control.
The fail-safe method requires periodic inspection in order to identify components that may need to be repaired or replaced.
Fail-Safe Design Techniques
Types of fail-safe design include:
- Redundancies, which essentially build additional capacities into the system that will take over if the primary components fail. For example, a backup system that allows continued safe operation if the main system can no longer function, or multiple load paths so a load can be transferred to another path when one breaks down.
- Intentional weak links, which are cheap and easily replaceable component that can fail first, thereby acting as a shield to prevent damage to more complex or expensive parts of the system. Fuses in an electrical circuit are an example of this, since they break to prevent the overloading or short circuiting an electrical system.
- Physical law, which uses the way certain materials respond to stress and pressure to make components that will fail without catastrophic outcomes. Gas pipelines, for example, are built from ductile materials that tend to bend rather than brittle materials that tend to break.
- Early detection, which makes smaller issues and early warning signs (such as cracks) easily detectable so they can be addressed before they result in more significant risks.