BS IEC 62808-2015 pdf free download – Nuclear power plants — Instrumentation and control systems important to safety — Design and qualification of isolation devices.
5 General principles for isolation devices
5.1 General The requirements for the application of isolation devices are in IEC 60709. Clause 5 is included as a summary and provides additional requirements for the isolation devices. The word “shall” identifies the additional requirements. Isolation devices used in interfaces between I&C systems important to safety or between channels within a system important to safety may have an impact on the integrity of the overall design and in particular, on defence in depth. When used, they may be relied upon to provide electrical isolation between redundant safety functions or safety functions in different layers of defence in the overall architecture. In general, the introduction of such interfaces between systems should be considered carefully based on the principles and approaches outlined in IEC 61 51 3. A systematic analysis of failures at system and overall I&C architecture level is required. Functional inter-dependencies are introduced between systems due to signal interfaces and their associated failure modes shall be considered carefully.
Where signals are transmitted between a Class 1 system or equipment (performing CategoryA functions) and systems of a lower class, the transmission of these signals are throughisolation devices that are included within the higher class system. When failures or conditionsare present at the output terminals of the isolation devices (which are connected to the lowerclass system) the safety action of the Class 1 system or sub-system to which the isolationdevice is connected cannot be affected.As an example, a circuit performing a Category Afunction may be monitored by a lower class circuit utilizing a relay coil in the Class 1 systemand the relay contact in a lower class system.
lsolation devices are to be used where signals are transmitted between independent Class 1systems and between redundant equipment channels of Class 1 systems.
Where signals are transmitted from Class 2 or 3 systems for use in lower class systems,orbetween independent subsystems in these classes，isolation devices may not be required;however, good engineering practices are followed to prevent the propagation of faults. Incases where Class 2 systems need to take on the aspects of Class 1 systems due to thefunctions performed, isolation is applied.An example of this is a Class 2 system performing aCategory B function in support of a Class 1 system performing a Category A function toprotect against the same fault.
Temporary connections for maintenance to systems performing Category A functions withoutisolation devices are only permitted provided that they are connected to only a singleredundancy at any given time，that they are disconnected after use,and that the system iscapable of withstanding a fault introduced through failure or use of the connection.
NOTE This standard discusses isolation devices as stand-alone devices which are separate from the equipmentperforming safety functions. The isolation device may be part of a module or equipment that performs a safetyfunction. in other designs, the isolation device may be contained in several moduies (e.g.one part handling rapidtransient overvoltages and the other static voltages). This standard is also applicable to these design variations.
5.2 lsolation characteristics
The isolation device shall be capable of providing isolation against the following failureconditions:
a) short-circuits between terminals or to ground;b) open circuits;
c) application of the maximum AC or DC potential that could reasonably occur, considering
potentials and sources available in both the Class 1 and non-Class 1 systems; and
d) electromagnetic and electrostatic interference.
lf the equipment can generate other signal types in fault conditions,such as a square wave orother form of oscillating signal in fault conditions,the isolation device shall be capable ofproviding isolation against such signals.BS IEC 62808 pdf download.