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I came across a technical specification which mentioned "The IPS (Instrumented Protective System aka ESD) shall not have a direct connection to the PCN (Process Control Network)." That means that any connection of the ESD system shall be via the DCS controller. The PCN is a network where other subsystems such as MOV systems, TGS systems, CCTV systems etc also reside on. Is this requirement mandated by IEC61511 or is it this particular specification's requirement?