Introduction to Systematic Capability
Systematic capability (SC) is a crucial concept introduced in the second edition of IEC 61508:2010, a standard for the functional safety of electrical/electronic/programmable electronic (E/E/PE) safety-related systems. SC is defined as a measure of the confidence that the systematic safety integrity of an element meets the requirements of the specified Safety Integrity Level (SIL). This measure is expressed on a scale from SC 1 to SC 4 (IEC 61508-2:2010, Clause 7.4.2.2).
What is Systematic Safety Integrity?
Systematic safety integrity refers to the probability that an E/E/PE safety-related system will perform its specified safety functions under all stated conditions within a specified period (IEC 61508-4:2010, Clause 3.5.4). While hardware safety integrity addresses random hardware failures, systematic safety integrity focuses on failures due to design errors, incorrect usage, software-induced failures, and other non-random factors. These failures are mitigated through rigorous processes and measures applied throughout the system lifecycle (IEC 61508-2:2010, Annex B; IEC 61508-3:2010, Annex A).
Why is Systematic Capability Important?
The introduction of SC in IEC 61508 Edition 2 addresses inconsistencies in how systematic integrity was applied in Edition 1. SC provides a standardised way to assess and ensure that systematic safety integrity is maintained across different elements and systems. This helps in achieving higher confidence levels that safety functions will perform correctly, reducing the risk of systematic failures (IEC 61508-2:2010, Clause 7.4.2.2).
Routes to Achieve Systematic Capability
IEC 61508:2010 outlines three routes to achieve systematic capability:
Route 1S: Designed in Accordance with IEC 61508
This route involves designing elements and components according to the requirements specified in IEC 61508:2010. It includes applying measures to avoid and control systematic faults as detailed in various tables and annexes of the standard. When these measures are appropriately applied, the element achieves a systematic capability corresponding to the SIL level intended (IEC 61508-2:2010, Clause 7.4.2.2).
Route 2S: Proven in Use
This route is based on the proven-in-use concept, where the systematic capability is derived from the historical performance data of an element or component. It requires careful consideration of any modifications and the context of the new application to ensure that systematic failures are not introduced (IEC 61508-2:2010, Clause 7.4.10).
Route 3S: Pre-existing Software Elements
This route applies to pre-existing software elements and involves specific requirements for software systematic capability as outlined in IEC 61508-3:2010. It includes measures to control software-induced systematic failures and ensure that the software meets the required SIL level (IEC 61508-3:2010, Clause 7.4.2.12).
Assessing Systematic Capability
Step-by-Step Assessment Process
- Identify the Element: Determine the element or component to be assessed for systematic capability.
- Select the Route: Choose the appropriate route (Route 1S, Route 2S, or Route 3S) based on the element’s design, usage history, or software nature.
- Apply Measures: Implement the necessary measures to avoid and control systematic faults as specified in IEC 61508:2010 (IEC 61508-2:2010, Clause 7.4.6 and 7.4.7).
- Evaluate SC Level: Assess the systematic capability level achieved by the element. This involves verifying that all required measures for the intended SIL level are met.
- Document and Justify: Record the assessment process, including any deviations and justifications for not implementing certain measures.
Example Assessments
Lets have a look at some examples of how Systematic Capability is used in practice.
Example 1: Same Manufacturer Elements
An input subsystem uses two pressure transmitters from the same manufacturer in a 1oo2 voting arrangement. Both transmitters have an SC of 2. Despite meeting the hardware fault tolerance for SIL 3, the systematic capability remains at SC 2 due to the likelihood of common systematic failures (IEC 61508-2:2010, Clause 7.4.3.3).
Example 2: Different Manufacturer Elements
An input subsystem uses two pressure transmitters from different manufacturers in a 1oo2 voting arrangement. Each transmitter has an SC of 2. The diversity in design and manufacturing reduces the likelihood of common systematic failures, allowing the combined SC to be increased to 3, meeting the requirements for SIL 3 (IEC 61508-2:2010, Clause 7.4.3.3).
To summarise. Systematic capability is a vital aspect of ensuring the functional safety of E/E/PE safety-related systems. By understanding and applying the routes to achieve SC, organisations can enhance the reliability and safety of their systems, meeting the stringent requirements of IEC 61508:2010. Over the next few weeks we will look more in depth into the different routes and how an organisation can show compliance with the requirements of the standard.