I just had a near-disaster while trying to install new smoke detectors. My house has AC hardwire connections for the smoke detectors. I wasn't sure which circuit the smoke detector AC connection was on, but after going through the breakers one at a time I found that they were on the same breaker as the dryer for some reason.
I verified that the circuit was off using two methods:
- While the circuit was on, the smoke detectors all showed a green LED indicating AC power. After shutting off the breaker, the green LED went off on all smoke detectors.
- While the circuit was on, my non-contact voltage detector went off when I held it near the hot wire connected to one of the smoke detectors. After the circuit was off, the voltage detector gets no reading from the hot wire.
I started with the smoke detector I'd used the voltage detector on. I unplugged the old smoke detector and checked to make sure the new smoke detector fit. I discovered that the new detector wouldn't fit because it has a bulge that would need to stick up into the junction box, but there was no way to make it fit because the wires in the box were so long (roughly 8 inches from the ceiling + wire nuts and another 2 inches to the AC connector) that they filled up the whole box. I decided to trim the wires back a few inches.
I grabbed my wire tool and cut the red interconnect wire first. Then I did something which in retrospect was very stupid: I cut the hot and neutral wires at the same time, thinking I was safe because the circuit was definitely off.
As soon as the cutter made it through the rubber insulation, it bridged the connection between hot and neutral and EXPLODED. A huge shower of sparks burst from the wires and I fell off the ladder screaming. By some miracle, I escaped unharmed and the sparks didn't set anything on fire.
I'm guessing the only reason I'm not in the hospital or dead right now is because I was on a fiberglass ladder and the wire tool has an insulated rubber handle. You can see here where the arc burned through the metal and lightly singed my finger.
After I recovered, I checked the hot wire with the voltage detector (still no reading), checked the other smoke detectors to make sure they weren't getting power (still no green light), and finally went outside to check the breaker (still off).
What went wrong here?
Edit: I keep thinking I figured out what circuit these damn things are on and then finding that the wires are still hot. There have been moments where it seemed like the most plausible explanation was that gremlins were switching the smoke detectors to different circuits at random.
I discovered a compounding factor. In the bedrooms with recessed ceiling lights, if the lights are off, my voltage detector doesn't read anything from the smoke detector wiring. If the lights are on, the NCVD detects current all three wires (hot, neutral, and interconnect). That might be the explanation for some of my confusion. However, my multimeter doesn't show any current flowing through the wires even when the lights are on (I double-checked it against a regular outlet to verify it's working), so maybe the NCVD is wrong.