I'm in the process of upgrading a bunch of switches in my house to TP Link Kasa smart switches. I just opened a switch box and I don't understand the wire configuration in it, and was hoping to get some explanation about what's going on.
When I turned off the breaker switch that I thought was controlling the power to this switch, the switch did stop working as expected. But when I tested the terminals on my switch with a multimeter on AC voltage setting, I was surprised to see that the display showed me ~120V. I turned off the breaker switch for the adjacent room and redid the test, and this time the display showed me ~0V.
To test this further, I separated the three black cables to test them individually, and the results are as follows:
Black 1: 0.025V Black 2: 1.608V Black 3: 118.2V
Another interesting observation, after separating the black cables, the light switch in the adjacent room stopped working.
Any explanation about what's going on here would be much appreciated!