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House alarm is going off despite it's non-functioning why is it going off now?

My house alarm went off yesterday, and it was in the off position.
The alarm was fitted before I moved in, and hasn't been active or working. There is power going into the system, but I thought the system was disconnected.
After a year, I have switched off all the power to do house work over a period of days, and we've had a few power cuts in the area. The alarm never sounded.
I've read that if you cut the power, the alarm would go off because it relies on the internal battery and thinks it could be tampering (how much of that is true, I don't really know?).
Anyway, the alarm has been switched to the off position and the alarm suddenly went off. So my missus turned it to the on position and it switched off for about a minute, before she stepped back into the sensor range and tripped the alarm?!..
So now, after all this time, the alarm decides to work properly?.. I resetted the alarm, and found that I could manually set off the alarm to test it?!..
After a few tries, the alarm hasn't sounded for the last 12 hours..
Any ideas?..

UPDATE: 24-06-2015

Thanks for the answers you provided.

It's been 9 days, and there hasn't been any other alarm fault since?.. So whether it's the battery in the unit or the external noise box... it doesn't seem to be the case as it should be going off ... bit of a strange one? maybe it's a spider in a sensor?.. Thanks for the comments anyway and will see what happens in the future

16/08/2016

Still nothing? No alarm since last year.. how weird?.. Thanks again

3 Answers from MyBuilder Electricians

Best Answer

I think you will find that the sysytem is still powered. The panel battery is probably not holding a charge and you may have a backup battery in the external sounder that can on occasion back feed the panel.
Switching off the mains and reinstating will automatically put the sytem into an alarm state as before the sytem died it was in a fault condition (ie battery fault)
I would suggest that you get an electrician to test the mains input in the panel to see if you have 240vac there.
If you need to kill the system, the 240v and panel battery need to be disconnected, with the external sounder battery removed too.

2015-06-19T08:45:02+01:00

Answered 19th Jun 2015

If you turn the mains power off to your alarm, the back up battery kicks in.

depending on the size and condition of the back up battery, and what it is powering will determine how long it will last before theres insufficient power to maintain the system and the external siren fires (has its own back up battery.

Modern bellboxs should stop after 15 minutes unless activated again.

Turning the power back on, you may be surprised to hear that some alarms actually arm themselves when power is restored after having no power at all. Sounds like that may be what happened in your case.

Finally the last part is unclear, reset and nothing else happened, if the alarm was set it should alarm when a sensor is activated, however if the back up battery isn't in good condition then the alarm panel may have the lights on but it behaves strangely and not as you might expect. This is usually the case for the entry level panels which have no protection to stop the battery from running completely flat.

2015-06-28T14:10:03+01:00

Answered 28th Jun 2015

there is a battery in the external alarm box

2015-06-19T08:45:02+01:00

Answered 19th Jun 2015

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