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Hossenfeffer 03-18-2013 04:54 PM

9V battery alarm any fire hazard?
I have installed several 9 volt and buzzer type water alarms (very similar to smoke alarms, I suppose) in a critical crawl space. However, to maximize the coverage, I added several more sensors to each....merely by attaching wires to pairs of copper strips and placing these under carpets and on concrete floors where water from leaks would accumulate....then running these back to barrier strips with jumpers to which the power wires have been attached from the 9 volt alarm circuit.

The sensors all work fine, but is there any possible fire or heating risk from these sensors under carpet and so on? Of course the circuit will not be closed unless of moisture present and then the alarm will sound....but just want to be sure the senors sitting connected to 9 volt battery but not closed are not going to be a problem.

I posted this in both appliance and electrical groups as it doesn't really fit anywhere.

Thanks for any thoughts.

gregzoll 03-18-2013 06:05 PM

Only if there was a short, to cause a run away condition.

av-geek 03-26-2013 08:04 AM

These sensors are carrying very low current (usually in the milliampere range), and won't be carrying any more than 9 volts of course because that's the maximum voltage of the power source. Most alarm systems only carry enough power through the sensor wires to trigger the alarm. The actual alarm sound is generated by an internal amplifier in the alarm box itself. The sensors only activate a switch to sound the alarm. If the wires were damaged or shorted, most likely the only thing would happen is the alarm would sound, and then you would be seeking the reason for the alarm to go off, and thus the problem would be corrected in short order. (Most alarm system are what is called "normally connected" so if a sensor malfunctions or is defeated by a crook, and alert would be produced)

9-volt "transistor" batteries as they were once called, do not have a large amount of power in them. It's typically in the neighborhood of 300-500 milliamp-hours. You could probably short the terminals on the 9 volt battery together, and it would merely get a bit warm to the touch, but not quite hot enough to cause a fire.

Ocelaris 04-02-2013 07:56 AM

I'm going to have to disagree. If you take a 9v battery and touch it to steel wool, it will ignite... now this may be a bit of a stretch. But I would say that you need to abide by standard low voltage saftey standards. The idea of running a wire under a carpet or anywhere you might step on or get rubbed is a bad idea to me. If you're going to put sensors, make sure that only the sensors meant for abrasion by stepping on etc... are under those areas. Don't run wires anywhere except in walls or protected just like high voltage. Look at security system wiring, they don't run wire under carpets etc... I'll say that your chances of having a problem are probably very low, but what is the power source for the 9v? i.e. what is the maximum amount of current that it can push, because in a failure that is what I would plan on existing under your carpets. Sorry for being downer, but I would plan for the worst circuit conditions, and run all the wiring like it was 120v except for where you need the sensors to exist out in the test conditions.

Think of it like a car battery, it's nominally 12v, but it's the amperage that will kill you. If you have a 9v battery backup in addition to the a/c power, assume that all those amps could possibly dump into the 9v wire as well. That's how I would plan/protect myself.

ddawg16 04-02-2013 11:10 AM

As long as the current from the battery is current limited, no issue.

In a worse case scenario, you want to be able to short the wires just before they go into the carpet and there should not be any overheating of the wire.

I personally would install a 0.1A fast blow fuse inline with the battery. If you are in fact operating in the ma range, this will give you plenty of protection.

Thurman 04-02-2013 07:37 PM

You're idea may work in theory and practicality but--I would venture to say that IF (Lord forbid) there was a fire within the structure and these strips were found YOU would be under investigation. Installing some type of "home made" device such as this and not being UL/FM listed is just asking for trouble IF there was trouble such as a fire.

Ocelaris 04-03-2013 09:45 AM

Absolutely Thurman, the fire marshall will pinpoint anything not to code and blame that, and your insurance will not pay. Everything has to be "to code" or you're basically throwing away your insurance policy. A coworker's neighbor house caught fire due to faulty wiring. The insurance company was ready to cut a check when the fire marshall pointed out he had run an extension cord in the wall to power his TV, which was responsible for the fire. The insurance company voided his check and didn't pay a dime. With this said, make sure any wire you run is at least CL2 or CL3, and run according to code for low voltage. Can't run it anywhere in the same holes as high voltage, should have it's own penetrations through drywall etc... no under rug business IMO. If you have to run under a rug, use something more durable than lamp cord, zipwire etc...

If you're going through all this trouble to detect fire/water etc... you might as well hook it up to an alarm system and pay for someone to monitor it. i.e. if you're not home, those alarms don't do much good. There are a lot of off brand monitoring companies who will monitor your alarms for relatively cheap. And there's an entire forum to DIY alarm systems. I'd check there for ideas on running wire:

av-geek 04-05-2013 08:30 AM

Just a thought on the decision for this...not discounting what people said here but I was looking at one of these water alarms recently in a radio shack, and discovered how they work.

The water senors are quite simple actually....They are merely a circuit board with bare traces on them. If water touches the circuit board, it will short out the traces on it, and trip the alarm.

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