smoke alarm wiring

5 Apr 2012
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In and around London
United Kingdom
A couple of (hopefully straight forward) questions about connecting mains smoke alarms. I had asked but no response in a thread I made in the alarms forum, so hoping to get a quick reply here.

1) Do smoke alarms not need to be fused? No FCU needed?

2) Building Regs appear to state that there must be a way to isolate the alarms without isolating the lighting? Is this correct? If so then I presume I will need some switched spurs?

Basically I will be connecting the alarms into 2 (or possibly 3) of the lighting circuits. 3 alarms will be radiolinked together and 3 non-radiolinked alarms will be hardwired together along with one of the radiolinked units).

From Building Regs:

The mains supply to the smoke alarm(s) should comprise a single independent circuit at the dwellinghouse’s main distribution board (consumer unit) or a single regularly used local lighting circuit. This has the advantage that the circuit is unlikely to be disconnected for any prolonged period. There should be a means of isolating power to the smoke alarms without isolating the lighting.

Mains-powered smoke alarms may be interconnected using radio-links, provided that this does not reduce the lifetime or duration of any standby power supply below 72 hours. In this case, the smoke alarms may be connected to separate power circuits.
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You can install a none switched spur somewhere to isolate the smoke alarms.
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A light switch will be fine as isolation. I'm not sure what an unswitched spur will achieve with regards isolation?
Yes it provides a fuse, but the alarms will almost certainly be powered from a lighting circuit, or a separate circuit both using 6A MCBs in the consumer unit.

In the event of a serious fault, I would bet that the 6A in the consumer unit will trip way before the 3A fuse does, so there's no point.

I was suprised to see that requirement for separate isolation from Building Regs. It doesn't seem logical to introduce a switch that could be switched off by an unknowing person and therefore disabling the alarms.
I would assume the unswitched fcu is for disconnection during maintenance/ replacing the detector.

You cannot always assume a lighting circuit is fused at 6a.


I would assume the unswitched fcu is for disconnection during maintenance/ replacing the detector.

You cannot always assume a lighting circuit is fused at 6a.

Well, for maintenance/replacement, the alarm just slides out of the base. And the designer would check the CPD value before installing any cabling.

Thanks. That was simple enough. Makes a change. ;)

In any event, I know that Part B says an isolation switch SHOULD be fitted. But this goes against the MIs that I just checked.

For a Kidde battery/mains smoke alarm

It states, there,
AC alarms must be powered by a constant 230V AC, 50Hz supply that is not controlled by any form of switch.

I believe that the MIs take precedence?
Haha...there's always some unecessary complications.

I'll go with the unswitched FCUs. Even though I'm doubtful of the fuse's necessity (the alarms are AICO and there's definitely no mention of a fuse in the instructions or wiring diagrams). But at least it satisfies two requirements (isolation, and prevents accidental switching off).
Although the unswitched FCU only will isolate the line, not the neutral when the fuse is removed. So it depends on the level of "isolation" you require. For proper isolation all live conductors should be disconnected. That is the neutral as well as the line.

Using the fuse to disconnect just the line is actually switching (like a light switch) and not isolation.

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