I am currently working on a remodel on a residential property with multiple buildings. The main house was built pre-1906 with tube and knob system (2 hot and neutral). The building I am working on is a cottage located about 100 feet away from the main house where its electricity originates. The house's main panel has 4 double pole breakers, each feeding a sub-panel.
On the service are the three cables as expected. The three lines that feed the cottage travel through a plastic conduit up the side of the building, across to opposite end on the house and up to a gooseneck and back down another plastic conduit into the ground then ground where it travels under a swimming pool over to the sub-panel in the cottage. At the gooseneck there is an additional cable (green insulated which must be a ground).
The owners have been having problems with the electrical for some time and I have found numerous issues with the wiring and made those changes. I have replaced the sub-panel with a newer model. The 4 that enter the building are connected to appropriate spots, (the two hot cables to a 100 Amp circuit breaker, neutral to the neutral bus and the last to the ground bus. The neutral is un-bonded.
When tested the 2-poles test correctly but I also have 120 across the neutral and ground. I decided to add another grounding rod at the cottage to be sure that the sub-panel is in fact grounded but still have the same test results and when I use a plug tester on the receptacles I all three lights light up.