alt Hacker NewsThe important part about corrosion protection is that the voltage is negative with respect to the ground. The absolute voltage is more for distance, and low enough to be safe. Nice read for those who are interested: https://pe2bz.philpem.me.uk/Comm/-%20Telephone/Info-901-Tele...
Supplying (very limited) power, data (full-duplex!), and signaling over a single twisted pair was certainly a concept far ahead of its time.