I recently bought this house (built in 2000), so it went through inspection. I assumed everything was fine. I realized the smoke detector in one of the rooms was not showing the green light that the others were showing. I pressed the "test" button and it beeped just like the others, so I thought it may be okay.

More recently I decided to actually take a look at it, and figured out the black wire is completely disconnected. Googling what that wire means gave me different results, but most of the results suggeste the detector would not be connected to the power, which would mean its battery would end very quickly, but I don't see the detector complaining about the battery (it's been a few months since we moved in).

Now I'm completely confused and have no idea if this is actually safe (especially since the home inspector was supposed to have checked this).

Am I safe? Would it be easy to fix this in case there's a problem? The other detectors in the house are not connected like this one.

See the picture here


  • Is there a black wire in the box above the detector?
    – Ed Beal
    Commented Oct 9, 2019 at 18:40
  • 2
    Have you looked up the model online and read the spec sheet to see what the install instructions say? Do that and if you still have questions then update your post and we will see if we can clarify the instructions.
    – Alaska Man
    Commented Oct 9, 2019 at 18:50
  • But, when you test it, do the other detectors also beep? That's how these are wired, a detector in the basement will set off all detectors when signaling an alarm. Commented Oct 10, 2019 at 9:23

2 Answers 2


This is a Firex 4618, interconnected smoke alarm with backup battery. Here is the manual which explains Green Light means its operating on A/C (hardwired) power. Otherwise, its running off the backup battery. It can run on the backup for several years like a battery operated smoke alarm, but the manual states a standard battery should last for 1 year, then when it drops under voltage it will chirp at you.

The black wire (hot) can be connected to a black wire up in the box to run on A/C power again. Often it can pop off especially if someone removed the smoke detector that strained the wires. Those stranded core wires and cheap wire nuts that come with the smoke detector often don't result in a very strong connection.

The fix is to first switch off the power. Hard wired smoke alarms usually share the lighting circuit. Then you can reconnect the black wire on the pigtail to the black wire in the box. When you power it back on, there will be a green light like the others.

  • 3
    In the last two houses that I have had anything to do with the smoke alarms were fitted when the house was built and were on their own circuit with their own breaker at the consumer unit. Don't assume that turning the lighting circuit off will remove power from the smoke alarms - CHECK IT !
    – uɐɪ
    Commented Oct 10, 2019 at 10:59
  • @uɐɪ Amen to that! Never rely on someone telling you which breaker to switch off! Always test the wires you're planning to work with before touching them.
    – Aleks G
    Commented Oct 10, 2019 at 11:08

Yes, you need to be very careful with this, 120V is very dangerous and it will shock you, and especially dangerours for someone who isn't used to working around it. Since the green light will not go off when you flip a breaker, bc the black(hot) wire is not connected. I highly recommend you get a non conctact voltage tester, you can get yhem for 10-20 bucks on Amazon. When you hold it near the circuit, if it beeps at you, then the circuit is still hot. If it doesn't, that means the circuit is powered down and your safe to work on it.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. Commented Oct 11, 2019 at 11:46

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