How are Safety Instrumented Systems derived?
Generally the significant Hazards for a process are identified by the duty holder in a Hazard & Risk Analysis. The method and approach for conducting a HRA is documented in both standards. It is the first stage of the lifecycle as defined in IEC61511. The HRA is used to derive the Safety Requirement Specification.

What is a Safety-Related or Safety Instrumented System (SIS)?
A system comprising of hardware, software and human elements, which is required to carry out one or more Safety Functions, where failure of a Safety Function would give rise to a significant increase in the Risk to the safety of people.

How does product conformance relate to specific Safety Integrity Levels (SILs)?
A Safety Integrity Level applies to a Safety Function to be carried out by the SIS. A SIL is not directly applicable to individual products or devices. Device manufacturers should not claim "... this product is a SILx device..." or "... our product is certified as a SILx...". Manufacturers should identify that their product is suitable for use in a Safety Function up to SILx, when applied in a specific manner and arrangement, typically referred to as the "approved methods of use". If a product is applied outside of its "approved methods of use" the design and reliability information for the product is invalidated.

What are typical applications requiring SIS?
- Burner management
- Emergency shutdown systems
- Fire and gas systems
- Turbine Controls
What information should I expect from a supplier of devices or sub-systems?
When devices or sub-systems are integrated into a SIS it is necessary to take into account the contribution that these elements will make toward the performance of the system in relation to Safety-Integrity. Specific information relating to design and reliability of these devices or sub-systems is required, this should include some or all of the information summarised below:-

- Specification covering functional, interface and
  environmental aspects.
- Estimated failure rate (due to random hardware failure) for
  each failure mode.
- Diagnostic coverage and diagnostic test interval.
- Hardware fault tolerance
- Information needed to identify hardware and software
  configuration
- Information needed to enable the derivation of the Safe
  Failure Fraction
- Documentary evidence of validation.
- Safety Integrity Level capability.

What is the difference between Risk and Safety Integrity?
Risk is a measure of the probability and consequence of a specified Hazardous Event occurring. Safety Integrity applies solely to the Safety Instrumented System, other technology safety-related systems and external risk reduction facilities. It is a measure of the likelihood of those systems satisfactorily achieving the necessary risk reduction in respect of the specified Safety Functions.