Q & A of the Day - Can we use an RCD protecting multiple circuits to also protect a smoke detection circuit?

Question: We are currently involved in tendering various new build housing projects and are looking for your advice as to whether we can install a hard-wired smoke detector circuit lighting circuit, which is protected by an MCB and also an RCD (main switch), which also protects other circuits, i.e. sockets, shower etc (split board). The cables are run less than 50mm from the surface and do not have earth or sufficient mechanical protection.

We have looked through BS5839-6 and are unable to find anything that suggests this is not suitable. However, the attached page from NICEIC's Domestic Electrical Installation Guide suggests, in fact, that this is not suitable. I'm aware that this is NICEIC's interpretation and would appreciate your advice on this.

To be clear, is it acceptable for an RCD that protects multiple circuits (including a regularly used lighting circuit) to also protect a circuit serving a smoke detection system within a dwelling (smoke & heat detectors)?

Or should the smoke detector circuit be supplied from it's own circuit or a regularly used lighting circuit - i.e. no other circuits?

Please click on the link below to download the attachment - 'Domestic Electrical Installation Guide' - (PDF doc - 381kB file size).

Answer: You are correct in your interpretation of BS 5839-6. It is OK for smoke detectors in domestic properties to be wired from an independent circuit, or from a regularly used lighting circuit. If BS 7671:2008 requires that particular circuit to be protected by an RCD, then this is acceptable.

Yes, it is acceptable to protect smoke alarm circuits, either from its own circuit or a regularly used lighting circuit and protected by an RCD, either from multiple circuits or independent of itself - as the smoke alarms will have a standby battery back-up system in place.

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