BS EN IEC 60709:2019 pdf free download – Nuclear power plants – Instrumentation, control and electrical systems important to safety – Separation
5.3.2 Criteria
Components and/or cables not classified in safety class 1 become associated circuits in one or more of the following ways:
a) electrical connection to a safety class 1 power supply without the use of an isolation device;
b) electrical connection to an associated power supply of safety class 1 systems without the use of an isolation device;
c) proximity to safety class 1 circuits and equipment without the required separation (physical distance or barriers);
d) proximity to associated circuits and equipment without the required separation (physical distance or barriers);
e) sharing a safety class 1 or associated signal without the use of an isolation device. Associated circuits shall comply with one of the following requirements:
f) they shall be uniquely identified as such or as safety class 1 and shall remain (traceable to the associated safety class 1 division), or be physically separated to the same extent as, those safety class 1 circuits with which they are associated. They shall be subject to the requirements placed on safety class 1 circuits.
g) they shall be in accordance with f) above from the safety class 1 systems up to and including an isolation device. Beyond the isolation device, such a circuit does not belong to safety class 1 provided that it does not again become associated with a safety class 1 system.
h) they shall be analysed or tested to demonstrate that safety class 1 circuits are not degraded below an acceptable level.
Associated circuits and isolation devices shall be subject to appropriate qualification. This qualification shall show that the higher classified circuits will perform correctly when the associated circuit or isolation device and its cables are subjected to electrical conditions for which the higher classified circuit should function correctly. Where an associated circuit is connected to a device/system not belonging to safety class 1 without isolation， that device/system not belonging to safety class 1 shall also be subject to this appropriate qualification. Associated circuits need not be qualified for performance of function, since their function does not belong to category A/components do not belong to safety class 1. Isolation devices for I&C circuits shall be in accordance with IEC 62808. Application of the associated circuit concept on a wide scale may lead to a broad combination of circuits of different safety classes provided that the general safety principles of physical separation are maintained. For example, cabling of differing safety classes need not be separated from each other within a safety group if the safety functions of the higher category can be performed by a redundant safety group that is separated from the safety group that contains the associated circuits. Separation inside the electrical or I&C cabinets should not be necessary if the components belonging to the lower classified circuits are qualified following the rules for the qualification of the higher classified circuits.