Providing a comprehensive range of sensing devices for use on DC monitored, automatic fire detection systems, this series of smoke and sensing devices has been designed to provide easy installation and maintenance with full detector interchangeability. Particular emphasis has been given to the overall series design, in order to minimise the incidence of false alarms from electrical or other sources of interference. The heat detectors are designed to meet the requirements of BS 5445: Part 2 as recommended by BS 5839: Part 1: 1980, and the smoke detectors are designed to meet the requirements of BS 5446: Part 1 and the draft European standard PrEN54 Part 7. Each detector can be fixed to a mounting base (available separately) which is suitable for two wire circuits using surface or concealed wiring methods. The detector is fitted to the base by a simple plug-in and twist locking action. The design of the base unit ensures ease of installation with easily accessible terminals. Four slotted fixing holes are provided to allow fixing at centres of 50mm and 60mm suitable for conduit boxes. The holes also enable the detector to be positioned so that the LED indicator (illuminated when the detector is triggered) is visible from a doorway, etc.