My smoke alarms went off yesterday. Verified there was no fire and tried to reset the units with no luck, they would keep alarming. I ended up disconnected them from the hardwire and they would stop alarming. I emailed the company (USI Eletric) and they gave me some tips to troubleshoot with no luck. As soon as I would plug the detector back into the hardwire they would alarm. While troubleshooting I found if I disconnected the interconnect wire (red wire) they would stop alarming. I called and talked to an engineer at USI and he recommended I measure the DC volts between the neutral (white) and interconnect (red) he said if I get a reading of 5-9 volts that would trigger the detector to alarm. I measured and found that it was 10.9 volts. I’ve isolated the wire that gets the DCvolts to be one that goes from the upstairs detector to the downstairs. (the red wire would drop straight down the wall from upstairs to the ceiling of the downstairs detector) What can be causing the red wire to be getting DC volts? Thanks for the help!!

  • Shouldn’t the interconnect wire be reading 0V if nothing is connected? Nov 24, 2022 at 0:26
  • no ... the voltage reading is only valid if measured in a circuit ... disconnected wires are not part of the circuit, so any reading is possible
    – jsotola
    Nov 24, 2022 at 0:38
  • Since you've been in contact with USI's support, what do they suggest for troubleshooting where this voltage is coming from?
    – FreeMan
    Nov 28, 2022 at 14:07

2 Answers 2


If this one detector is outputting 10V on the interconnect wire while alarming: yeah, that’s what it does. The voltage on the interconnect wire triggers the rest to go off so everybody in the building hears it. If it’s alarming without the presence of smoke, replace it.

If it’s outputting the voltage without alarming, it’s defective. You shouldn’t attempt to repair it. Replace it.

  • The interconnect wire is showing 10V with nothing connected. Shouldn’t it be reading 0 and only jumps up if the detector is activated? Nov 24, 2022 at 0:27
  • @SamMcCullough -- is that 10V DC or AC, and have you ensured that absolutely nothing is connected to the interconnect wire? Nov 24, 2022 at 1:45
  • It’s 10V DC this when the breaker was on but nothing was connected just measuring between the neutral and interconnect wire. If I turned off the breaker is would drop to around 2V DC. Nov 24, 2022 at 1:57
  • @SamMcCullough -- I'd double check for anything connected to it then Nov 24, 2022 at 23:11

First, if your interconnect is hooked up at the panel it will always have voltage on it when the breaker is on. Second, if you accidentally crossed the interconnect and line wires on one of the detectors it will cause voltage on the line constantly. Third, it's possible one detector has gone bad. Removing them one at a time to see if the voltage drops will isolate the bad one. Fourth, check to see if your neutral line got crossed somewhere.

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