This publication addresses a safety concern within the protection system for nuclear power plants that might result in unacceptable consequences for certain combinations of common cause failures and postulated initiating events, especially in case of programmable digital protection systems. When this situation is encountered, a diverse actuation system is often provided to back up the reactor protection system. The publication identifies and discusses common criteria for the design of diverse actuation systems at nuclear power plants (NPPs) with the aim of developing a consensus on the adequate level of diversity in the reactor protection systems. It relates to IAEA Safety Standards Series No. SSG-39, Design of Instrumentation and Control Systems for Nuclear Power Plants, and provides specific details for utility engineers, operators, researchers, managers, and personnel responsible for all aspects of design and implementation of instrumentation and control systems of diverse actuation systems for NPPs. It will also aid Member States to support assessment of diversity in I&C architecture as a defence against common cause failures.