Negative DC voltage causes of using & -48 V DC:
(i) Positive voltage courses comparatively more corrosion in metal then Negative voltage.
(ii) Negative voltage is safer for human body while doing Telecom activities.
(iii) Thundering may cause positive voltage in the equipment circuit. In the case negative voltage (Lack of electrons) neutralizes positive charges and protects producing heat.
(iv) Negative voltage is safer for long telephone line for transmitting power trough it.
(v) A standard lead-acid battery provides 6V. This number corresponds to 8 of those, which is pretty standard. It's the highest, safest compromise voltage to run over long wires.Negative voltage is used so that leakage currents to ground caused by moisture do not electroplate away the copper in the wires.