BS IEC 62808:2015 pdf download-Nuclear power plants — Instrumentation and control systems important to safety — Design and qualification of isolation devices.
5.3 Actuation priority Where plant equipment that is controlled by a Class 1 system is also controlled by a lower class system, devices are provided which ensure priority of the Class 1 system actions over those of the lower class systems. Failures of, or normal actions by, the lower class system cannot interfere with the Category A functions under plant conditions requiring success of those Category A functions. The equipment performing the priority function is classified as Class 1 . The circuit that provides the required isolation could be within the same system, or may be in other systems. Failures and mal-operations in the non-Class 1 systems cannot cause a change in response, drift, accuracy, sensitivity to noise, or other characteristics of the Class 1 system which might impair the ability of the system to perform its safety functions. Where plant equipment that is controlled by a Class 2 or 3 system is also controlled by signals from a lower class system, failures, or normal actions by the lower class system cannot prevent the higher class system from performing its function. Where signals are extracted from Class 2 or 3 systems for use in lower class systems, isolation may not be required; however, good engineering practices are followed to prevent the propagation of faults. In cases where Class 2 systems need to take on the aspects of Class 1 systems due to the functions performed (i.e. Category A functions), isolation is used. 6 Isolation device design requirements 6.1 Requirements on isolation device application 6.1.1 Isolation device power Isolation devices are classified as part of the safety system and are powered in accordance with the criteria of IEC 61 51 3 if a power supply is necessary for the function. The power supply of the isolation device shall not be required for the device to perform its isolation function.