An SIS is intended to perform specific control functions to prevent unsafe process operations when unacceptable or dangerous conditions occur. Because of its criticality, safety instrumented systems must be independent from all other control systems that control the same equipment, in order to ensure SIS functionality is not compromised.
Safety Instrumented Systems (SIS) are designed to manage unsafe processes by preventing operations when hazardous situations arise.  To guarantee their effectiveness, SIS must operate independently of any other control system managing the same equipment.