Mushy,
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BS 5839: Pt.6 grades fire detection systems from Grade F up to Grade A. Generally speaking, the greater the fire risk and the more demanding the application, the more comprehensive the system needs to be.
Grade F - System of one or more battery powered smoke alarms (and heat alarms if required)
Grade E - System of interlinked mains powered smoke alarms (and heat alarms if required) with NO stand-by supply. The interlink can be hardwired or radio-interlinked. Please note that Safelincs Ltd will not distribute Grade E alarms, as we only recommend alarms with backup batteries.
Grade D - System incorporating one or more interlinked mains powered smoke alarms (and heat alarms if required), each with an integral stand-by supply. The interlink can be hardwired or radio-interlinked.
Grade C - System consisting of smoke detectors and sounders (which may be smoke alarms) connected to a common power supply, comprising normal mains and stand-by supply, with central control equipment
Grade B - Fire detection and alarm system comprising fire detectors (other than smoke alarms), fire alarm sounders and control and indicating equipment to either BS EN 54-2 (and power supply to BS EN 54-4), or to Annex C of BS 5839: Pt.6
Grade A - Fire detection system incorporating control and indicating equipment to BS EN 54-2, and power supply to BS EN 54-4, installed to BS 5839: Pt.1 with some minor exceptions