This International Standard establishes requirements for the design, analysis and qualification of isolation devices used to ensure electrical independence of redundant safety system circuits, or between safety and lower class circuits, as specified in IEC 60709. This standard includes guidance on the determination of the maximum credible fault that is applied to the isolation devices. The maximum credible fault can be used as a basis for the test levels used in testing based on other standards (e.g. IEC TS 61000-6-5 or IEC 62003). This standard does not address safety or CCF issues due to functional inter-dependencies and possible interferences or CCFs that may result from signal exchange or sharing between systems or sub-systems. It also does not address design or qualification issues related to digital or programmable logic in isolation devices. For isolation devices containing digital or programmable logic, additional design and qualification requirements must be considered; these requirements are outside the scope of this standard.