Question for any electricians...

wooky

wooky

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Martyn
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Our upstairs lighting ring main has on it the upstairs and downstairs mains powered fire alarms on it , which are interlinked.

The fire alarms have been playing up recently and they've been changed for new ones as they were due for change, but that's an aside.

I noticed a problem with the system; when I turn on the main bedroom light the power to the fire alarm system goes off and this is also the case if I take the bulb out the light fitting in the main bedroom; the problem doesn't repeat if the lights are turned on in bedroom 2 or in the bathroom and same for removing the bulbs from their respective fittings.

Cabling in the ceiling rose and switch looks tickety boo, so anyone got any idea where the problem lies cos I'm stumped....
 
Who changed the fire alarms? If it was an reputable company then call them and tell them of the problem. If you changed them then call an electrician if you do not know what you are doing. Simples!!
 
Problem was there before the change...
 
In which case the question should be Who fitted the alarms in the first place? Sounds like they've been bodged in and need sorting. Pronto!

*Must get a couple of PP9s - smoke alarm batteries get changed at commercialmas as a matter of course and it draws ever closer...*
 
wooky said:
Problem was there before the change...
Click to expand...

You have still failed to answer who fitted them though. If they are run on the mains then the new ones should have been fitted by a NICEIC registered electrician.
 
Sounds like what should be the neutral connection to the alarm actually goes to the bulb connection which connects to the switch.
With the light off, the alarm gets its neutral connection through the bulb. Remove the bulb and it loses power.

Switch the light on and that point which was effectively neutral as far as the alarm is concerned is now live.

If both the live and neutral of the alarm are connected to live, no current flows so the alarm loses power.

I suspect it was always like this and it just needs a neutral wire moving from one side of a bulb connection to the other side. Power most likely comes from the ceiling rose in your main bedroom.

Have a look at this:
daves-ceiling-rose.jpg


Power to the alarm should come from the centre contacts (live) and the right contacts (neutral). I think the neutral wire has been connected to the contacts on the left by mistake.


Steve.
 
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Steve, that diagram is poetry in motion. If all were like that!
 
Col said:
Steve, that diagram is poetry in motion. If all were like that!
Click to expand...

Probably less than 10% of ceiling fittings I have seen are actually wired like that!

In the OP's case though, I think the principal is the same. Alarm neutral connection going to switched live instead of neutral.

You sometimes see a similar thing happening with old cars where a rear light cluster has lost its ground connection due to rust. The indicators work o.k. until the brake lights come on. One side is o.k. but the other side goes dim. This is because with just the indicators on, the indicator bulb is finding a false ground return through the brake light bulb on the other side. Once power is applied to this bulb, there is no longer a return path for the indicator and neither will light.


Steve.
 
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Very true. Had that problem on a Standard eight and a Ford Anglia.
 
Good diagram...trouble is on light fittings installers have used twin and earth to go to the light switch and used the black wire as the switch wire from the switch back up to the light fitting and not red sheathed it and causes all kinds of confusion when the diy'er decides to put a new light fitting up
 
30 minutes this afternoon switching around the neural cables in the ceiling rose and everything's sorted, my good lady can now have the house lit up like Heathrow Airport without disturbing me from a slumber on the couch....
 
A satisfactory outcome then.

Yay!
 
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