A spark while installing a smoke alarm

Question

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.

Answer 1

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.

Answer 2

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