I&C systems important to safety may be implemented using conventional hardwired equipment, programmable digital equipment or by using a combination of both types of equipment. This document provides requirements and recommendations for the hardware aspects of I&C systems whatever the technology and applies for all safety classes in a graded manner (as defined by IEC 61513). The requirements defined within this document guide, in particular, the selection of preexisting components, hardware aspects of system detailed design and implementation and equipment manufacturing. This document does not explicitly address how to protect systems against those threats arising from malicious attacks, i.e. cybersecurity, for programmable digital item. IEC 62645 provides requirements for security programmes for programmable digital item for all their development phases and on-site operation. Pre-existing items may include microcontrollers or HPDs and, where firmware or programming files are deeply-embedded, be effectively "transparent" to the user. In such cases, this document can be used to guide the assessment process for such components. An example of where this approach is considered appropriate is in the assessment of modern processors which contain a microcode. Such code is generally an integral part of the "hardware", and it is therefore appropriate for the processor (including the microcode) to be assessed as an integrated hardware component using this document. Software which is not deeply-embedded, as described above, is developed or assessed according to the requirements of the relevant software standard (for example, IEC 60880 for class 1 systems and IEC 62138 for class 2 and 3 systems). In the same manner, HPDs which are not deeply-embedded, as described above, are developed or assessed according to the requirements of the relevant HPD standard (for example, IEC 62566 for class 1 systems and IEC 62566-2 for class 2 and 3 systems).