Why is telecom DC power −48 V, and why do substations use 110 V or 125 V?

[why is telecom -48V · why substation 110V DC 125V DC] Telecommunications standardised on a negative 48 V bus, positive earth, because it reduces electrolytic corrosion of buried and exposed metalwork. ETSI EN 300 132-2 sets the normal interface range at −40.5 V to −57.0 V, derived from 24 lead-acid cells at 2.35 V per cell on float with allowance for distribution drop. Substations use higher voltages, commonly 110 V or 125 V DC, with AusNet reporting 125 V as the most common across its fleet, because tripping and protection circuits are long, and higher voltage reduces conductor size and voltage drop over distance. Rail signalling adds its own conventions: 50 V DC for signalling logic, 24 V DC for electronics, 12–15 V DC for level crossings.