Hi, I'm still replacing light switches in my old, strangely wired house. So I have two switches that kept reading zero volts when I measured between the black and white wires (these are single-pole switches). So I measured from the black wire to the ground and got 124 V. And I measured from the white wire to the ground and got 124 V. I think this means they're both hot and when I measure between them they're cancelling each other out. Is this normal? Safe? Can I just replace the switch or does they need rewiring so only one wire is hot? Thanks!
A switch leg has a hot wire, and a switched hot wire in it. No neutral. If the switch is on, the two terminals of the switch are connected to eachother, so no voltage difference. When the switch is off, or the wires are disconnected from the switch, only one should be hot, the other should have no voltage over ground.
In a switch leg, the hot and neutral go directly into the fixture box, the neutral is hooked to the fixture's silver screw or white wire, and the hot is connected to one of a pair of wires that goes to the switch box. The switch box just has two wires (one hot and one switched hot), and back in the fixture's box, the switched hot gets attached to the fixture's gold screw or black wire.