I was replacing a duplex outlet in my basement and discovered 1) a circuit carrying 30V and 2) testing 120V between two bare "ground" wires from separate circuits.
Two pairs of black/white/bare wire enter an outlet box and are pigtailed to an outlet. I traced it using a Breaker Finder to Circuit 4 (15A), which I turned off at the breaker. I tested with a non-contact tester and found voltage. I shut off power at the main, disconnected the pigtailed wires, then turned it back on at the main (with power from Circuit 4 still off) so I could find out why there was voltage.
I discovered that Pair B from the bottom of the outlet box is Circuit 9 (15A), and it carries 30V of electricity. Pair A comes from a ceiling light in my laundry. Pair B comes from outside where it is attached to a GFCI with three pairs of Romex entering the box.
Here is a table showing voltages when measured from each wire to each other wire, testing both common and red (?) when using a multimeter.
Outlets nearby are correctly wired with a socket tester. Home built in the mid 80's.
My questions are, "Why does the circuit measure 30V?", "Why would bare wire to bare wire from two separate circuits measure 120V?", and finally "Is this safe?"