Mystery cable in receptacle box

[ndrose]

I am replacing some of the old receptacles in a 1940s house. Armored BX cable, two conductors, cable is the ground. Mostly pretty straightforward.

One box contains a problem. Two cables coming into the box. Black and white from one cable attached to the receptacle. Other pair taped up.

The receptacle works fine—that pair is ok. But I wanted to know what the taped pair was. Tested for voltage.

Known hot to known neutral (just to be sure): 110V
Mystery black to known neutral: 0V
Mystery white to known neutral: 0V
Known hot to mystery neutral: 110V
Known hot to mystery black: 55V

Not sure how to interpret that. I had expected it to have fed something decommissioned downstream, and that it would be 0 to everything. That’s obviously not the case. But I don’t know what it is.

Thanks for any help

 

[ndrose]

My speculation so far:

The mystery cable can’t be a source since there’s no voltage to ground from either conductor.

The white conductor must have a path to ground, and the black conductor a partial path to ground.

If the cable went downstream to a decommissioned fixture, maybe whoever disconnected whatever it was left the conductors free in the box, and just taped up the source end, in the box I’m examining. Suppose the white is in full contact with the downstream box, the black in poor contact. The metal boxes and BX armor would provide a path to ground from there.

 

[Megawatt]

My speculation so far:

The mystery cable can’t be a source since there’s no voltage to ground from either conductor.

The white conductor must have a path to ground, and the black conductor a partial path to ground.

If the cable went downstream to a decommissioned fixture, maybe whoever disconnected whatever it was left the conductors free in the box, and just taped up the source end, in the box I’m examining. Suppose the white is in full contact with the downstream box, the black in poor contact. The metal boxes and BX armor would provide a path to ground from there.

It sounds like just a cable that’s not supplying any loads anymore.