Smoke Alarm hard wiring problem

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Old 05-02-16, 12:08 PM
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Smoke Alarm hard wiring problem

I am putting in two new hard wired smoke alarms with battery backup and when I try and connect them to the wiring, they activate and will not stop. So after scratching my head and poking around, I've found that the voltage reading between hot and neutral is just 20 volts. The voltage between neutral and ground is 120 and between hot and ground it is 98 volts. It's the same on both alarms which are on the same circuit.

So I'm thinking this means I have an open connection in the circuit somewhere. But do these readings mean I have an open in the hot or neutral or ground? And what's the best way to track it down? The home is super old and I'd like to try and figure out a faster way of tracing the circuit rather than open dozens of outlets and light fixtures to try and spot a loose connection somewhere.

Any suggestions?
 
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Old 05-02-16, 01:49 PM
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What is the wiring method used?
 
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Old 05-02-16, 02:20 PM
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Not sure I understand the question, but the 120v harwired smoke alarm has a connector with 3 prongs- hot, neutral and interconnect(black , white, red). Typical residential alarm
 

Last edited by pcboss; 05-02-16 at 03:27 PM. Reason: interconnect was ground.
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Old 05-02-16, 03:11 PM
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How about the wiring in the walls? How many alarms are there? Are they all interconnected? Are the units compatible per manufacturer specs (not all of them can interconnect)?
 
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Old 05-02-16, 03:51 PM
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The wiring is the box for the alarms have red, black and white as well.

I know that the two that I'm trying to replace are on the same circuit but not sure if there are other alarms or lights or anything else on the circuit or not. It's a large old place so I haven't been able to track it all down easily.
 
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Old 05-02-16, 04:38 PM
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Normally power goes to black and white and remote alarm connection to red. Is that what you have.
 
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Old 05-02-16, 05:27 PM
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Yes, I just read the instructions and agree that black is meant for hot, white is neutral and red is to interconnect. (Not what I previously had posted)

I think that may have solved the issue. I assumed the wires were hot, neutral and ground, but I was wrong. The interconnected wire may be interfering with another fixture. I'll try to adjust and be back with an update
 
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Old 05-02-16, 05:57 PM
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The interconnected wire may be interfering with another fixture
Different makes and models often don't play well together.

Tip: If it isn't green or bare it isn't ground.
 
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Old 05-03-16, 09:35 AM
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Yes you found it -- red is interconnect. The plastic residential smoke detectors are not grounded. You splice the ground wires in the box, but they do not connect to the smoke detector.
 
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Old 05-03-16, 02:35 PM
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Well, that was it. I just hooked up the black and white wires only and left the red unconnected. Attached and it's working. I'm not going to bother at the moment with tracking down the source of the 20 volts on the red wire. I'm sure there is some other old smoke alarm burried in an attic or somewhere else that isn't compatible with my kiddie smoke alarm and that's the red wire issue. For now they are working as standalone alarms and I'm good with that. Thanks to all for your help!
 
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Old 05-03-16, 03:29 PM
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The alarms are of limited use if one does not alert to a fire in another part of the building.

The 20 volts may just be phantom voltage.
 
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Old 05-04-16, 06:39 PM
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I know that the two that I'm trying to replace are on the same circuit but not sure if there are other alarms or lights or anything else on the circuit or not.
If the detectors are or were interconnected, they'd all have to be on the same circuit.
 
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