These requirements cover electrically operated single and multiple station smoke alarms intended for open area protection in indoor locations of residential units in accordance with the National Fire Alarm Code, NFPA 72, smoke alarms intended for use in recreational vehicles in accordance with the Standard for Recreational Vehicles, NFPA 501C, and portable smoke alarms used as "travel" alarms. A single station smoke alarm, as defined by these requirements, is a self-contained fire alarm device that consists of an assembly of electrical components including a smoke chamber, alarm sounding appliance, and provision for connection to a power supply source, either by splice leads or a cord and plug arrangement or containing integral batteries. Possible accessories include a supplemental heat detector, terminals for connection to a remote audible signaling appliance or accessory, and an integral transmitter to energize a remote audible signaling appliance. Multiple station units are single station smoke alarms that are either interconnected for common alarm annunciation or connected to remote thermostats. These requirements, where applicable, also cover all remote accessories that are to be connected to or are intended to be used with a single or multiple station smoke alarm. See 33.2.1. This standard does not cover the following: a) Smoke detectors of the non-self-contained type that are intended for connection to a household or industrial system control unit. These are included in the Standard for Smoke Detectors for Fire Protective Signaling Systems, UL 268. b) Mechanically operated single and multiple station fire alarm devices that are specified in the Standard for Single and Multiple Station Heat Detectors, UL 539. c) Heat detectors [except for the requirements in the Fire Test (Heat Detector), Section 73] incorporated as part of a single station smoke alarm assembly whose requirements are covered in the Standard for Heat Detectors for Fire Protective Signaling Systems, UL 521.