Why is telecommunications DC power −48 V?

Telecommunications standardised on a negative 48 V bus with positive earth, which reduces electrolytic corrosion of exposed and buried metalwork. ETSI EN 300 132-2 defines the normal service voltage range at the equipment interface as −40.5 V to −57.0 V, the upper figure derived from 24 lead-acid cells at 2.35 V per cell plus regulation allowance, the lower from cell end-of-discharge voltage plus distribution drop. Outside that range, from 0 to −40.5 V and −57 to −60 V, equipment must survive without damage but service may be interrupted.