Just had a power cut (70 minutes not too bad a fix time for an 11kV
fault I suppose)
OK, I did have to get up to silence the UPS (also shut down a couple of
unnecessary PCs while at it to extend runtime)
But most annoying is the 15 minutes of alarms sounding due to failed
batteries, then sounding again for another 15 minutes when the power
comes back ...
People seem to think this is normal/acceptable ...
Some people must have had batteries replaced as I only heard one alarm
last night, when we had a power-cut this summer (windows wide open)
there were several to contend with.
I phoned-in the outage after giving it a couple of minutes (and a couple
of failed attempts at cutting back in) the 105 reporting line
automatically got my postcode from somewhere and recorded the outage,
which appeared on WPD's fault map shortly afterwards, just had a call
back from them to apologise ...
It isn't all that benign an environment especially for the external bell
battery in the external sounder. Being on float charge ready to go kills
them in 3-5 years. Worth testing before swapping out though.
The problem in alarm panels is the battery is never cycled - just
trickle charged 100% of the time. After a number of years the battery
still has the correct terminal voltage but cannot provide the
instantaneous backup power when the mains fails. As a result there is a
glitch in the power to the alarm panel and the alarm is triggered. The
backup batteries still can supply enough power for the sounders/lights
for a long time etc. Checking the battery afterwards will again show the
correct terminal voltage. You may only see the problem for the first
time is when the mains power fails.
Is this a pet theory or can you provide links to this battery behaviour?
Sounds unlikely to me.
Um, you do know that the external sounder and strobe are additionally
powered by a second non-rechargeable battery? Usually lithium I believe.
There’s no need to postulate a “sluggish battery” theory. The voltage from
the duff battery is just too low to run the system when there’s a power cut
and this triggers the tamper circuitry.
My bell box has a rechargeable battery that is also trickle charged in
my wired system via the alarm panel. My bell box also has red flashing
LEDs so a self contained non-rechargeable battery may not last that long.
The tamper circuit in a bell box is often a spring loaded mechanical
switch which when the box is removed from the wall is activated.
The bell box battery doesn't power the alarm panel which, in my case,
has its own loud sounder.
Once triggered by a power cut the alarm can be disabled and run normally
from the old battery which you suggest has a voltage too low to run the
The bell box is normally powered from the panel to keep rechargeables
charged or flash pretty lights. It also "holds off" the sounder
and/or strobe. If the panel voltage drops to low, the sounder and
strobe trigger and run from the battery in the bell box. Once this is
triggered I'd expct it to run for several minutes before trying to
reset and retrigger if the panel volts are still low/abscent. You
don't want a scrote to be able to shut up the sounder simply by
feeding it volts from another source...
And hopefully the cover... this should trigger a full tamper alarm
condition. Which may or may not cause the sounder/strobe to be
Voltage too low when the sounder and strobe are drawing power...
Commonly adjustable from 1 minute to 20 minutes (the legal maximum).
Some also limit the number of re-triggers, while more basic ones don't.
They can do that, but usually the wiring comes through the wall from
inside the house, so they'd need to be inside or they'd have to pull the
box off the wall or open it first (either of which triggers them
immediately). Far easier and quieter to squirt foam inside.
Usually it would directly trigger the sounder/strobe of the bell box,
without needing the alarm panel to signal one and then it'd continue to
run on its internal battery. It would also give a tamper alert at the
panel, which may well no longer have a sounder/strobe connected, but as
its going off anyway, that doesn't matter.
I thought the bell box battery is only there so it will sound if it or the
wiring to it is tampered with. Under other circumstances, the panel
supplies the ringing volts to it - either from mains or its battery.
*Lottery: A tax on people who are bad at math.
Dave Plowman firstname.lastname@example.org London SW
Self Actuating Bells do not ring when voltage is supplied from the panel.
The 12v from the panel stops them ringing and charges the battery.
When an alarm condition exists the 12V is removed and they bell rings
until it comes back or the battery goes flat or if its a modern one a
Only cheap and nasty DIY alarms need 12V to operate the bell.
If you have one then get rid of it as it takes seconds to cut the wires
and then there is no external bell.
The old fashioned strobes may be powered from the box, mine are, they
take too much power for the battery.
Its not the same 12V that operates the rest of the box.
But if you remove the 12v the sounder sounds?
No expert on alarm internals since I've only ever followed a wiring
diagram. But with your method, it would mean the sounder no longer works
if the internal battery is kaput?
Thing is the bell box should be situated where it's difficult to get at.
Which means changing it or the battery equally difficult?
And anyone who feeds it with accessible cabling should be shot. ;-)
*Never miss a good chance to shut up.*
Dave Plowman email@example.com London SW
That's why you can test them.
Its how all the bell boxes on banks work so it can't be the worst way.
No banks use one that rings when it is powered by 12v in an alarm condition.
I have actually installed some for Chubb alarms but it was a long time
ago and I quit as I didn't like the training bloke as he was getting
things wrong about mains and didn't like being corrected.
It was my first proper job before I moved the GEC telecomms.
The battery is fine for years.
Not all buildings have somewhere difficult to get to.
crowbar, rip box off wall cut wires and one that needs power from the
panel goes quiet.
When chubb fitted real bells they had a special box specifically made to
stop expanding foam from getting in. They were the triangular ones. I
won't comment on what they did as they may still use them for all I know.
Both SABs and SCBs should still ring if ripped from the wall.
It would also set of the internal alarm and any other bell boxes.
It's also a good bet that it would also set off an alarm signal to a
BTW cutting the telephone wires on a proper monitored system also
actives the monitoring company.
Mind you opening one door without first closing another door on an unset
alarm can also set of a silent alarm in some houses BTDTGTTS. The police
were there in about 3 minutes.
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