20 volts in smoke alarm signal wire, why
I am trying to find out why I have 20 volts in the signal wire of the tiny smoke alarm system that was newly installed for an addition I built for some customers.
My best guess is that magnetic induction is the cause but the electrician swears this is not the problem. He says he has installed too many smoke alarm systems for this not to have happened already.
The manufacturer says 20 volts is too much and that amount of voltage will set off the system with a false alarm, which was happening already and thats why I/we started troubleshooting the system.
The wire for the smokes is your standard 14/3 conductor and it runs approx 25 feet from panel to end of the line, with one other smoke on the circuit.
I’ve checked the voltage and it’s 120 volts on the circuit, and now the electrician wants me to check the continuity. I should have done that if I wasn’t a novice. So, I dont’ think there is a short/arc.
But if anyone has run into this before I am ready to hear the cause.
Thx, clark
Replies
what do you mean by "signal wire"? - if you have the basic interconnected hard wired smoke detectors with battery back-up, then you should have a "hot", neutral, common and ground - the ground will probably just ground the box and not the device, and the smoke should have three wires - black for hot, white for neutral, yellow/red for common (carries the signal to active all the smokes in the circuit)
What are you testing it with? And WHY are you testing it?
My guess is that you are using a digital voltmeter. They are very high impedence sand will read phantom voltage due to capacitive coupling from other lines.
Is there a problem with the system?
Is this with all units removed?
Yes, the smoke alarms (and they have the Carbon Monoxide sensor/alarm option too) are from your local neighborhood Home Dpt. and the wiring is just like you described: black, white and yellow (the signal wire).
I removed the 2 smokes, and at the farthest wire from the panel, tested the signal wire/yellow with my fluke digital meter (not their most expensive model) by touching the digital meter to the yellow and the white/neutral. When I did that, I got 20v on the signal wire/yellow.
Went down to the other smoke and/or wires and got the same 20 volts. (I disconnected that alarm before testing the signal wire)
The homeowner was telling me that as soon as the system was installed a few weeks ago, it started to alarm/go off.
Maybe there is C.Monox in the house, haven't checked that yet, but I doubt it.
We replaced the smokes with new ones recently, and when we checked the signal wire this time before connecting the new ones, that's when we noticed the phantom 20volts in the signal wire.
Currently, the alarms are just connected to the hot/neutral with the signal wire disconnected and dont seen to be causing any trouble/false alarms.
They are blinking and/or showing that they are hooked up properly too, from their blinking light signal.
Hope this info helps
thx so much
My smoke detectors flash or blink only when the 9 volt backup battery is getting weak. They also chirp briefly until I replace the battery. Is the factory supplied battery connected ?
Edited 7/19/2005 10:43 am ET by arrowpov
Well, I will check that, but they have never "chirped" when I have been there working/testing the system. I know what you mean; that chirping noise when the battery runs low.
The homeowner says the alarm went off a couple times and I think, but cant prove, that the phantom 20 volts is the cause of the alarm.
I still think that the measurement is phantom voltage.Give some details of the system.How many units? Do the units have an indicator by ligh or sound if it is fire or smoke?How often do they go off?How many units are there and what else is on the circuit? I assume that you have 14-2 (or 12-2) to the first unit and then 14-3 to each additional unit. Are there anyother things that the 14-3 goes to?What happens if you disconnect the signal wire for each unit? That will indicate if one of them is going off and which one.Go to Radio Shack and get a 100,000 (100k) ohm resistor and connect it from the signal wire to neutral with out anthing else connected to the signal wire. Then measure the voltage across it.
That dang chirp ! !
My Mom called me the other day. A bird had gotten into her chimney. She's 76, lives alone, four hard-wired smoke detectors with battery back-up. Yep, that bird kept her up all night.
Chirp
Chirp
Chirp
Smoke detector batteries.
The kicker is that I spent almost $50 for lithium batteries last time I changed them. Then, when she had a little trouble with her water heater, (output line developed a leak in the crawlspace - her water wasn't getting very hot) the local handyman she called told her she needed a new water heater, and we were going to have to relocate it from the attic into her MBR closet. And since I'm here, we need to change the batteries in your smoke detectors ! He had her buy alkaline 9v and he took my lithium batteries with him ! !
I fixed the under-the-house leak in 15 minutes and told her to NEVER, not NEVER call that jackleg again. I didn't know about the batteries until later.
Chirp
Chirp
Greg
Edited 7/20/2005 2:32 pm ET by Greg Gibson
The system is just two smoke alarms. The one in the furnace room is the closest to the breaker then the 14/3 wire goes to the sunroom/addition where the second and/or final smoke is located. The seconc wire is about 20 to 25 feet away.
All the wires that go to the addition do come to a logjam area where holes were drilled through the rim joist from the basement to get into the sunroom. So I think that if I have to run a new wire thats the spot I should avoid (to avoid any interference).
Today, I disconnected all the wires and tested for continuity on all the wires. Thankfully I did not get a noise out of my digital meter. Its good to know that a screw/nail is not the problem.
So, I think the voltage is still a phantom voltage.
In the box with the second smoke alarm the circuit continues on to just the fan unit half of a split system air conditioner. Its just the fan and control so i don't think it draws a lot of amps.
I bet I am forgetting to tell you something.
Oh, I checked the batteries and they are connected and both units are new ones so I don't think the bat. are old/bad.
Thanks again, clark
This may be so far out in left field that its laughable but just for giggles .... is it possible the 20 volts is the sum of the two backup batteries? Two 9 volt batteries under no load and a voltmeter with a slight tolerance problem could easily read 20 volts.Sorry, I'll go back to sleep now.
The batteries would be DC. He seems to be reading 20 Volts AC, right?
-- J.S.
Good point, but I just re-read the whole thread and he never said AC or DC.