Wired smoke alarm: current on red line ?

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Old 03-13-16, 10:21 AM
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Wired smoke alarm: current on red line ?

Hi,

I'm replacing the wired smoke alarm units in our house, just the units, not the wires. I was using my non-contact tester and noticed the red line also has current going through it. Is that normal?

I assumed that line was just for the alarm to send out a blip to the others when one detected smoke, not a continuous current like it appears mine to be doing. If you knew the quality of contractors the prior owners always used, you'd understand the concern.

Thanks for your help.
 
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Old 03-13-16, 10:25 AM
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You can't use a non contact checker to test for power on the red wire. Since there is power on the black wire you could also see it on the red wire. It transfers between the wires inductively.

If you are replacing existing smoke detectors...... be sure to change them all so that they are compatible.
 
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Old 03-13-16, 11:01 AM
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I just wasn't seeing anything on the neutral line (as I shouldn't expect), and the signal for current was very strong on the red which I didn't expect. (BTW, in this house, black doesn't always mean hot, thanks to the before mentioned contractors)
 
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Old 03-13-16, 11:25 AM
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If your white wire is neutral as it should be..... you won't see any voltage indication on it.

With a non contact tester you aren't checking for current. You're checking for dangerous voltage.
 
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Old 03-13-16, 11:59 AM
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I was using my non-contact tester
The manufacturers come close to lying when they write "voltage detector" on the packaging. It actually detects the presence of an electromagnetic field which may be caused by conditions other then a meaningful voltage. Best is to use an analog multimeter. While a cheap analog multimeter is good a cheap digital multimeter can give misleading readings for the same reasons that make non contact testers even less useful for real testing so best to use an analog.
 

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Old 03-13-16, 06:22 PM
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the same reasons that make non contact testers even less useful for real testing so best to use an analog.
But, they do make nice toys. I heard several models were recalled not long ago.
 
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Old 03-14-16, 12:56 PM
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So, is everyone saying I need a multimeter because there should be voltage on the red line and I need to get a reading on the actual voltage for some reason?
 
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Old 03-14-16, 01:03 PM
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No.... you should find voltage only on the black wire. If you use a non contact tester you may see an indication of power on the red but technically there would be very little. The red line is only a control line between units to operate the audible.

If you were to use a voltmeter.....you should measure 120vac between the black and white.
You would probably see a much lower voltage between the red and white.

If the units were working.... and if one smoke detector triggered caused the other ones to sound.... your wiring is fine.
 
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Old 03-14-16, 07:39 PM
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Depending on smoke detector model, that red line may be normally high, low, or floating. I believe many are floating.

The issue with non-contact tester is it can falsely detect floating wire next to hot wire as being hot due to induction. It is very low current, so it only appears on floating wire. For neutral wire, it is connected to ground at main breaker panel and voltage occurring from induction will be just absorbed.
If you measure voltage between red wire and neutral with a multimeter you should get 0V with analog meter and may bet some voltage much lower than 120V with digital multimeter (probably somewhere around 20 to 50V). The reason for this is digital multimeter has much higher internal resistance than analog and it is capable of detecting even low current voltage.
 
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Old 03-15-16, 12:28 PM
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Thank you, that makes a lot of sense. My old multimeter broke, and with my non-contact I never felt a real need till now to replace it. I really just use the non-contact, like everyone else I assume, to re-confirm I killed breakers and test hots from neutrals. Never had a problem till this somewhat unique problem occurred.
In the mean time, to test the non-contact, I put it up to the wire for my 1920's or earlier chandelier and discovered that any metal part on the chandelier was making it go crazy, beeps at 6 inches from the metal. I'm assuming there must be a short somewhere, but could it have something to do with the hots and neutrals being reversed, since that's a common problem? I wouldn't expect that from a lamp I picked up at Walmart, but since it's Edison-era I'm not sure if that would have somehow been the perfectly acceptable for the INTENDED neutral to not be properly insulated. Nonetheless, a re-wiring is in its future.
 
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Old 03-15-16, 01:29 PM
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We have had a poster who got excited and worried because their non contact tester chirped when aimed at the ceiling. Yes you can use for a quick non critical check to see if a breaker is off or if a line is hot, just don't trust any reading or non reading.
 
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