IEC 61508 is considered a basic or “umbrella” standard for functional safety. It is generic and sometimes even vague. IEC 61508 was intended that various industry sectors provide their own specific standards and guidelines as needed. Here we can see the relationship between 61508 and other standards, such as 61513 for the nuclear sector, 62061 for the machine safety sector, and 61511 for the process control sector.
IEC 61508 Standard
61508 deals with the entire safety lifecycle of safety systems, from cradle to grave. It targets suppliers of safety systems, but is also applicable to some degree to suppliers of equipment used in those safety systems.
Because the number of applications is so varied, such that many different applications could use the same equipment in different ways, only some parts of the Realization phase are applicable to equipment manufacturers.
- Anyone making safety-related electrical/electronic or programmable products for use by others (OEM’s) should follow this standard. SIS and SIF designs should follow the sector standards, if they exist.
- It contains 4 Normative parts (1-4) plus 3 informative or “guideline” parts (5-7).
- Defines the concept of SIL & the Safety Lifecycle.
- Certification is optional; 61508 does not require this. But Assessment against requirements of 61508 is not. 3rd party Certification is valued by end users; it is probably a comfort factor that a 3rd party has assessed the process and product against 61508.
IEC 61511 Standard
61511 is user focused, but it does not assign responsibilities; that is a common issue that must be done for any given project. The same lifecycle and SIL concepts apply as in IEC 61508, but 61511 is in Process Industry language and context.
61511 is performance based rather than prescriptive; the design is based on risk analysis and providing the required risk reduction. Metrics are calculated to prove this out.
There is a focus on the end user application… what the SIF is trying to achieve. But it does not contain detail requirements for embedded software or high level languages like C/C++ (see 61508 for those).