In our (new build) house the three linked, mains (and battery)
powered, smoke detectors were supposed to be fed from a separate
breaker in the consumer unit. Except that they weren't - at some
point during the build the connection was broken and the only
practical way to restore operation was to wire them into the lighting
circuit instead. The electricians assure me it's perfectly OK to do
that, but what are the pros and cons of having a separate breaker for
the smoke detectors?
To reply by email change 'news' to my forename.
I don't think it's a big deal either way, given that there's battery backup.
If the breaker for the lighting circuit trips for some reason, it takes
out the smoke detectors too
If a separate breaker for the smoke detectors should trip, it could go
unnoticed for months. ISTR someone on this group has this system with a
battery-backed emergency light wired into the smoke-detector circuit, so
if ever that light comes on during normal conditions, he knows it would
mean the MCB has tripped - that strikes me as being the optimum solution.
My suggestion to the council that smoke detectors should run (in their
houses) "from the TV socket, as a power failure there would be noticed
faster than any other power failure" fell on deaf ears.
As for the OPs question. A mains powered interlinked smoke alarm set up with
a battery back up should last over a year without mains power and still
function correctly. When the batteries start to lose power then warning
bleeps are given out by the alarms to let you know. These warning bleeps
will happen with or without mains power as it is the battery they test.
Personally I prefer the smokes to be on the lighting circuit.
I believe some interlinked smoke detectors (Kiddie made the one for
which I was reading the manual) require all linked detectors to come off
the same circuit, if you have them both upstairs and downstairs do you
connect them to the upstairs or the downstairs lighting circuit? A
separate breaker solves this problem.
I have half a dozen Firex detectors, upstairs and downstairs. They are
interlinked and need to be fed off a common power circuit. As previously
noted, I use a separate MCB feeding them and a non-maintained emergency
light on the stairs (which is both visible and usefully placed for a
real power cut).
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Well for sure you must have all linked smoke detectors on the same
circuit... but it's no more of a 'problem' than if you have separate
upstairs and downstairs lighting circuits, with the landing light and
hall light connected by two-way switching, which must be the situation
in place in the majority of 2-storey homes.
I agree with what's been said about the advantages of connecting to a
lighting circuit but read the regs and decide.
http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADB1_2006.pdf (para 1.19)
I note that if using a lighting circuit the alarms should be capable of
being seperately isolated from the lighting circuit. Electrically I agree
with that but can't help thinking it largely removes the advantage.
So for once you electricians are right, as long as they incorporate seperate
Personally I would do a hybrid.
- Upstairs lighting
- Downstairs lighting
- Smoke & Hall lighting
If the smoke alarm trips off, only the hall lighting goes with it.
The hall tends to be the one with incandescent light bulbs and more
susceptible to B6 tripping. So when the hall trips off from a blown
all the rooms leading to it (up & down) still retain their lighting
Adding an EmLight to the hall circuit goes that useful bit further.
That meets the regs (314?) re minimised disruption from tripping.
Having an isolator to smoke alarms supplied by a lighting circuit is
useful so the alarms can be isolated when doing Ins Test part of PIR.
Which reminds me to add one at some point.
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