Safety integrity level is define as a relative level of risk reduction provided by safety function or specific target level of risk reduction. it is IEC 61508 STD.
Applicable to all sectors of industry
IEC61511=Applicable to process sector
IEC62061=Applicable to machine sector
Safety requires protection against dangers for …Human,machine, process, environment.
Dangers that can result because of function faults must be prevented before they occur.