IEC 61500-2009 pdf Nuclear power plants – Instrumentation and control important to safety – Data communication in systems performing category A functions

5 General requirements 5.1 Principles of selection of data communication techniques and equipment The communications equipment shall meet requirements for systems performing category A functions. NOTE To ensure acceptability for nuclear applications one of the following principles for selection of data communication techniques and equipment can be applied: • use of protocols implementing safety features; • use of industrial standard protocols with added safety layers; • use of protocols where higher protocol layers implementing unsafe or not needed functionality are removed or replaced by ones with reduced and safe functionality. The hardware and the software shall be qualified, see Clause 9. 5.2 Functional requirements Generally each data communication channel is part of an overall system providing services of information gathering and presentation, control or protection of the nuclear power plant. Equipment providing cyclic data over a communication channel shall not depend on the receipt of acknowledge messages from the receiver for continued operation. Communication channels shall not be allocated dynamically during the run time of the system but shall be statically allocated and predefined by design. All messages of application software shall be transmitted periodically within a pre-defined variation of cycle time. Messages should have fixed length predefined by design. The communication system shall enable messages from instruments or other outstation equipment using a communications channel to be sent and received within a specified time frame, together with data integrity status information (if implemented). The data communication network topology and media access control shall be designed and implemented to avoid CCF of independent systems or subsystems (see 8.3).
6 Physical separation and isolation 6.1 Electrical isolation The electrical isolation of systems performing category A functions connected by communication channels to other systems shall be considered in accordance with IEC 60709. The degree of electrical isolation will depend on the station power supply voltages present, national practice, and plant-specific requirements. NOTE A method of achieving a high degree of electrical isolation is by means of optical fibre connections or opto- electronic isolators. Appropriate isolation shall be demonstrated between data communication equipment and connected equipment. This shall be sufficient to prevent faults of the connected equipment and cables from affecting the operation of the data communication equipment adversely. Connected equipment includes sensors, contacts, power supplies and other communication equipment.