The calibrated risk graph method as per IEC 61511 defines SIL-a for safety functions with a required risk reduction factor lower than 10 (i.e. below SIL-1). SIL-a functions may be implemented in the BPCS as ‚low integrity safety functions‘, but shall be subject to Functional Safety Management, including:
Safety and quality management Independence of safety and control functions Documentation control Management of change IT-Security Periodic testing