Short story: the black wire sparked when installing a hardwired smoke detector as I had left the power on. After properly turning off the power, I completed the installation and now everything works fine. Anything else I should worry about?

Longer story: I was installing a hardwired Google Nest smoke/carbon monoxide detector in my finished attic. More specifically, I had removed all the wire connectors and the black, white and orange wires from the electrical box were exposed. I had removed the old 120V connector and I was installing the new one from Google, by inserting the two white wires (one from the box, one from the Nest 120V connector) into the wire connector and trying to cap them together. I was having some trouble as the white wire from the box was rigid. While I was doing that, the black wire from the electrical box sparked (I believe with the white wire, but I am not sure) and got stuck to the box.

Before the installation, I had turned off the power to the loft (all the loft's lights were off) but apparently the loft's old smoke detector is wired together with the lights in the basement (three floors below) and so I guess the smoke detector still had the power on. Not being an electrician, that is my best explanation for what happened. After I figured that out, I did the following:

  1. Turn off the power to the old smoke detector correctly.

  2. Detach the black wire from the box (its tip was obviously darkened).

  3. Install the Google Nest smoke/carbon monoxide detector.

  4. Turn back the power to the detector (and basement).

  5. Tested the detector.

Now, everything seems to work fine. Nest is not reporting any problems (it tells me if the detector is running only on battery because the power to the detector is out). Later, I will correctly label the breaker on the breaker box.

My question is: because of this incident, is there anything else I have to worry about? My guess is that, because everything now works fine, this incident did not cause (or reveal) other problems (e.g. blown fuses etc.), but I have no idea.

  • A live wire touching a grounded box should trip breaker right away. Hardwire smoke detectors can(should) be on their own circuit. You probably have not found the breaker for the detector(s)
    – crip659
    Jun 15, 2021 at 18:33
  • go buy yourself a lottery ticket, because today is your lucky day ... you could have easily ended up on the floor
    – jsotola
    Jun 15, 2021 at 21:40

2 Answers 2


Check the panel and reset the breaker.

That's all. It tripped for the right reason. But it could have killed you.

Now go and buy a non contact voltage tester.

A good electrician will not necessarily wire all outlets (lights, receptacles etc...) of one room to one breaker.

Especially in the case of smoke alarms there are certain important exceptions that apply, and it is wise to wire them with lighting circuits in frequently used spaces such as dining/kitchen lights.

  • Thanks. I now realize what a tripped breaker means. After the spark, I noticed that the lights in the basement had no power. I checked the panel and everything seemed fine. I had turned off the breaker to the loft and it was still off. I thought that something should have tripped another breaker but did not see it. It is because I thought tripping would shift the lever to the off position. But I just found out that a US breaker has 3 positions: on, tripped and off. Where I grew up (Italy) a spark like this would move the lever all the other way (down, not left), with no intermediate position. Jun 15, 2021 at 19:10

The spark was because of the current inrush. If you do live work (I regularly need to) you get used to it.

But it was only a smoke detector! Yup if you watch closely When installing a computer power supply or phone charger you will see a similar spark as the connection is established.

If you got in the circuit that’s when things can go bad and it is recommended to make connections with the power off.

If you have your smoke detectors on a dedicated circuit you do not want a breaker with a test function. Arcs and sparks turning off the smoke detectors and then no alarms is why smoke detector circuits are one of the circuits that are allowed to have old fashioned circuit breakers.

I recommend all DIY electricians to have at least a non contact voltage detector. This will a Leary you to a live circuit and even a pro model like I use daily is only ~20$ really cheap ones for 5$ online. Test to make sure it’s working with a live circuit, check the circuit then test with the live circuit again. You should get a voltage alert, nothing and a voltage alert if the circuit is dead and the non contact detector is working

  • Granted sparks have many causes, but OP writes the wire got stuck to the box. I assume it got welded, and I doubt it's from the detector's inrush. More like a live-neutral short.
    – P2000
    Jun 15, 2021 at 18:56
  • I did not read the long story but my answer would still be primarily the same to use a voltage detector and this would not happen and the smoke detectors should be on a dedicated circuits they may be.
    – Ed Beal
    Jun 15, 2021 at 19:13
  • I think @P2000 is correct. As I was moving the white wire, the black one was getting tangled up too. The black wire got welded to the box through its tip. Jun 15, 2021 at 19:13
  • Thanks, @EdBeal, I will use the voltage detector next time (or hire an electrician!). Jun 15, 2021 at 19:14

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