Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
I would imagine it has to do with corrosion, if the return is truly at ground
(earth) potential and the signal negative, the signal will not corrode with
-48V but would be chowed quickly with +48V and any moisture at all.