IEC 60744:2018 pdf download – Nuclear power plants – Instrumentation and control systems important to safety – Safety logic assemblies used in systems performing category A functions: Characteristics and test methods.
1 Scope This document provides requirements and recommendations for the design, construction and test of safety logic assemblies used in safety systems to perform category A safety functions (in accordance with IEC 61 226). Safety logic assemblies include logic such as the hardwired logic assembly interfacing computer-based systems to switchgear, actuators or contactors to provide trip or engineered safety feature actuations. Safety logic assemblies are significant parts of a safety system and may include voting logic between redundant channels. This document provides a general description of safety logic assemblies for safety actuators control. The principles to meet dependability objectives are presented. The main features relating to the design requirements are described and explained. Various tests and their requirements are given in order to validate the design (including the qualification tests), the manufacturing and the correct installation on site. Annex A (informative) gives a list of possible applications of safety logic assemblies. Annex B (normative) suggests a list of possible hardwired technologies with their respective requirements to design safety logic assemblies. Annex C (informative) gives explanations on dependability and its attributes to improve reliability and to reduce the final risk which compromises the safety and the availability of the NPP. The scope of this document does not address the design of a protection system, it covers only the technological and architectural solutions required to design a safety logic assembly. The design of safety systems using safety logic assemblies is covered by IEC 61 51 3. The detailed and specific functions implemented in a safety logic assembly strongly depend on the design of each reactor and are not addressed in this document. As this document is focused on I&C part of the system, the final voting logic made with power breakers is excluded from the scope of this document.
5 Safety logic assembly – Principles and description 5.1 Safety logic assembly The protection system is generally designed with software based technology to perform safety functions with digital means. Usually, it has multiple redundant and sometimes diverse divisions to, among other things, meet the single failure criteria, achieve the target reliability and allow on line testing and maintenance. The outputs from the multiple divisions are subject to some additional processing, made by a safety logic assembly, before sending the final command signal to an actuator. This is dependent on the design but usually includes some form of voting to deal with divisions that may have failed or may be in bypass for periodic testing. Annex A gives more examples of possible functions performed by a safety logic assembly. As the final processing is simple (sequential and / or combinatorial) and important to safety (direct command of safety actuators), the technology to design a safety logic assembly shall be highly dependable: reliable and safe. Annex C gives clarifications on concepts related to dependability. Safety logic assemblies using a software based technology are not addressed in this document since computer based systems and software development are adequately covered in other standards. Therefore, in this document a safety logic assembly performs Category A logic functions to send the direct command signal to safety actuators without using a software based technology. The safety logic assembly, as part of the protection system, shall be designed in accordance with IEC 61 51 3.