IEC 61508 vs. IEC 61511 Requirements

IEC 61508 is the basic reference for Functional Safety that settle the minimum safety requirements of Safety-Related Systems to be applied in each specific industrial sector. Specifically, the process sector safety instrumented system standards refer to IEC 61511.

Difference between IEC 61508 e IEC 61511 standards

IEC 61511 is used by integrators in processes and systems. It applies to safety devices integrated in a safety system, which comply with IEC 61508.
The requirements remain the same for both IEC 61508 and IEC 61511 (quantitative, qualitative and semi-quantitative).

IEC 61508 recommends the requirements to be applied to:

• New devices
• Proven In Use method
• FVL (Fully Variable Language) Firmware/Software

IEC 61511 rather defines:

• Prior Use method
• Integration
• LVL (Limited Variable Language)

Proven In Use method (IEC 61508)

The “Proven In Use” method may be an alternative to Route 1H (FMEDA analysis).

The architectural constraints for the proven in-use analysis are reduced. The standard gives confidence to devices proven by the market. Proven in use requires high documentary evidence.

The proven in-use approach requires a sufficient number of hours of product operation, revision history, non-compliance analysis, and fault reporting. It is typically used by manufacturers with a consolidated tool in known processes.

The field data to be used in “Proven In Use” must come from a stable design, in which no changes or updates have been introduced for an extended period of time.

Proven In Use (IEC 61508) vs. Prior Use (IEC 61511)

According to IEC 61508, the Proven In Use concept is based on the manufacturer’s design (e.g. temperature limit, vibration limit, corrosion limit, desired maintenance support) for your device.

According to IEC 61511, the Prior Use concept concerns the performance of the device installed within a process sector application in a specific operating environment that is often different from the manufacturer’s design base.

Ultimately, a product that is not SIL certified, for which neither the classic FMEDA methodology nor the Proven in Use has been applied, can be used for safety applications when the integrator has a high level of expertise with the device and there are no non-conformities relevant to safety in order to guarantee the Prior Use in accordance with IEC 61511.