I just replaced an old dimmer switch that no longer allowed the lights to get to full brightness.
I tested that my non-contact voltage detector lit up and screeched at the faceplate. (Well, it did once I put fresh batteries in it.)
I switched off the power at a circuit breaker, and the lights controlled by the dimmer went out, confirming that I'd found the right one.
But the detector still detected voltage at the faceplate. I assumed that a second circuit must pass through a box. I switched off a second breaker for outlets in the same room. At that point the detector was satisfied that the power was off.
When I removed the faceplate, I was surprised that it contained just the hot, common, and ground for the light circuit. (The dumb switch is single pole, so the common just passes through.) Since I had had to turn off a second breaker, I had expected to see a second hot-common pair passing through the box.
What was the voltage detector detecting if there wasn't a second circuit passing through the box?
Even now, with the switch replaced and the power back on, the detector registers the voltage only at the faceplate. It's not sensitive enough to detect the wires behind the drywall, so it seems as though there really was voltage in the junction box until I closed multiple circuits.
Should I be concerned about a wiring fault?
J-Box: Plastic, residential, single gang.
Non-contact Voltage Tester: Commercial Electric NCS-8904R
Dimmer Switch: Leviton Sure Slide
