This guide is developed by the Signaling Protection and Communication Section. It provides technical information on basic fire alarm systems with a focus on early-warning smoke detection devices installed in duct applications. Duct mounted smoke detectors are designed to provide a specific type of protection that cannot be duplicated by any other type of system. However@ there has been a tendency to misapply these devices attempting to use them as a substitute for an early warning smoke detection system. This fact@ coupled with new methods of detecting smoke in ducts@ has prompted the writing of this industry guide.