UL 864 is the North American standard covering control units and accessories for fire alarm systems, and it is one of a small group of standards in which the reliability requirements of a life safety system govern every technical decision. Equipment complying with it is installed in buildings throughout the United States and Canada under the local building and fire codes, and the certification is a condition of installation rather than a commercial preference. The standard addresses the control unit, the initiating device circuits, the notification appliance circuits, the power supply and the standby battery, and it sets requirements for the supervision of every circuit so that a fault is annunciated rather than concealed, for the operation of the system under abnormal conditions including the failure of the mains supply, and for the behaviour of the equipment when it is subjected to the electrical and environmental conditions of a building. The electromagnetic compatibility requirements are part of that assessment.