Redundancy
Definition
The provision of additional components, systems, or means beyond the minimum required for functional capability, intended to maintain system function when one or more components fail. Active redundancy means all redundant items operate simultaneously; standby redundancy means backup items are activated only when the primary fails. Redundancy improves system reliability but increases cost, complexity, and maintenance burden.
Context
A key design strategy for achieving required system reliability and availability levels. Redundancy decisions are driven by criticality analysis, SIL requirements, and availability targets. In reliability modeling, redundancy transforms series reliability (all must work) into parallel reliability (at least one must work), dramatically improving system-level reliability.
Examples
- A safety instrumented system rated SIL 2 uses 1oo2 (one out of two) redundant pressure transmitters — either transmitter can independently detect an overpressure condition.
- A 2x100% (two trains, each at full capacity) pump configuration provides active redundancy, allowing one pump to be maintained while the other carries the full load.
Applicable Standards
- IEC 61508
- IEC 61511
- IEC 60300-3-1
Related Terms
The reliability of a system composed of multiple interconnected components, calculated from the individual component reliabilities and their functional configuration (series, parallel, complex). System reliability accounts for the way component failures combine to cause system failure, considering dependencies, common-cause failures, and load-sharing effects that may not be captured by simple series-parallel models.
Reliability Block Diagram(RBD)A graphical method for modeling system reliability by representing components as blocks connected in series, parallel, or complex configurations that reflect the functional relationship between components. Series connections mean all components must work for the system to function; parallel connections provide redundancy where only one of multiple components needs to work. RBDs enable calculation of system reliability from component-level reliability data.
Safety Integrity Level(SIL)A discrete level (SIL 1 through SIL 4) for specifying the safety integrity requirements of safety instrumented functions (SIFs). Each SIL level corresponds to a target probability of failure on demand (PFD): SIL 1 = 0.1-0.01, SIL 2 = 0.01-0.001, SIL 3 = 0.001-0.0001, SIL 4 = 0.0001-0.00001. Higher SIL levels require more rigorous design, testing, and maintenance.
AvailabilityThe proportion of time that an item is in a state to perform its required function under given conditions, assuming that the required external resources are provided. Availability combines reliability (how often it fails) and maintainability (how quickly it can be restored). Inherent availability considers only corrective maintenance; operational availability includes all downtime including preventive maintenance and logistics.
Where this appears in Reliatic
This concept is used in the Reliability Analytics module.