The requirement for independence as mentioned in the first subsection, implies that the fire and gas detection system comes in addition to systems for management and control and other safety systems. The process safety system can interface with other systems if it is not adversely affected as a consequence of system failures, failures or individual incidents in these systems.
Process safety functions should be identified and implemented using a systematic approach based on an established risk-based methodology.
The requirement for two independent safety levels as mentioned in the third subsection, implies that the process safety levels shall be protected against dependent failures, so that a single failure does not lead to the failure of both safety levels. For example, the overpressure protection should consist of two independent protection levels, of which one is normally a pressure safety valve (PSV).
The overpressure protection should be designed in accordance with the API Standard 520/NS-EN ISO 4126 and the API Standard 521.
As regards instrumented/electronic safety functions, the IEC 61508 and 61511 standards should be used.