This International Standard provides the functional requirements for the alarm systems in the main control room of nuclear power plants. It gives definitions of the terms used for alarm functions. It also establishes the human factors requirements and the design guidelines for alarm presentation for the main control room of nuclear power plants. NOTE The alarm functions can be implemented in a dedicated system (alarm system) or preferably be an integrated part of the main control room HMI (Human-Machine Interface) system. It specifies the alarm functions including those for the selection and definition of original alarm signals, alarm signal processing (e.g., event sequence processing, static and dynamic prioritisation), alarm display processing (e.g., alarm suppression) and the use of associated display devices (e.g., Visual Display Unit (VDU), conventional alarm fascia, mural display), with acknowledge and reset sequences, and other related matters. Under abnormal conditions or plant transient conditions in the nuclear power plant, many alarms occur simultaneously. For this reason, the alarm functions of the main control room of nuclear power plants require special considerations for human factors engineering and system configuration, to avoid operator misunderstandings and to provide the operator with adequate information. Therefore, the scope includes special alarm functions based on human factors for monitoring and operation of nuclear power plants. It does not cover specific alarm systems, such as the fire alarm and security alarm systems. The object of this Standard is to establish a common international understanding of the underlying functional design basis of alarm systems for control rooms, covering the corresponding functional requirements, the human factors requirements and design guidelines for the alarm functions and alarm presentation for the main control room of nuclear power plants. This Standard therefore aims to give guidance to reduce problems which have been experienced in the past: omission of important alarms, delay in detecting important alarms, increased workload that may affect the performance of other operational activities, inattention to frequently activated alarms known as ‘nuisance alarms,’ and confusion associated with the misunderstanding of the relationships among alarms and of the importance of alarms.