Glossary · Functional safety

Systematic capability (SC)

Also known as: SC, SC 1, SC 2, SC 3, SC 4

German: Systematische Eignung (SC)

In functional safety under IEC 61508, systematic capability (SC) is a measure, expressed on a scale from SC 1 to SC 4, of the confidence that the systematic safety integrity of an element meets the requirements of a specified SIL, when the element is used as described in its safety manual.

Functional safety
Verification

In one sentence

Systematic capability (SC 1 to SC 4) states up to which SIL an element is protected against systematic faults when used as its safety manual says.

Example

A safety PLC certified with SC 3 may be used in a SIL 3 function as far as systematic failures are concerned; its PFH and the architecture of the function still have to be verified separately.

How it applies

Systematic vs. random failures: A SIL has two sides. Random hardware failures are handled with probabilities (PFDavg or PFH) and architectural constraints. Systematic failures, such as specification errors, design faults and software bugs, can't be calculated; they are controlled by the development process and design measures. Systematic capability expresses how well an element is protected against them.
How it is achieved: IEC 61508 offers routes for hardware and software: compliance with the techniques and measures required for the SC (route 1S), evidence from proven in use (route 2S), and assessment of pre-existing software (route 3S). Elements with a lower SC can in some cases be combined with sufficient independence to support a higher level, under the conditions of IEC 61508-2.
In the function: Every element of a safety function needs a systematic capability at least equal to the SIL of the function, unless the combination rules apply. Using an element outside its safety manual, for example with other software versions or configuration tools, can void its SC.
Documentation: Record the SC of each element, the certificate or evidence behind it and the safety manual conditions that were applied. In product documentation, state the SC as what it is, a property of the element, not as the SIL of any function.

Systematic capability vs. SIL CL

Systematic capability is the IEC 61508 measure for systematic safety integrity of an element. The SIL claim limit (SIL CL) of IEC 62061 is the maximum SIL a subsystem may be used for, taking both its architectural constraints and its systematic integrity into account; the SC is one of the inputs that limit it.