The 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...

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userbinator 19 hours ago | parent [-]

Supplying (very limited) power, data (full-duplex!), and signaling over a single twisted pair was certainly a concept far ahead of its time.

	▲	gumby 4 hours ago \| parent [-]
		The system is so robust that for a long time in remote locations there was only one wire, with the return line simply being the top wire of a barbed-wire fence. Couple that with a party line and you could service a lot of remote customers for a very low cap ex.