The first architecture commonly used across the telecom industry is the -48V system. Originally designed to transmit and receive telecom signals and used to power telephony equipment, 48V was chosen as the voltage to be high enough to reduce losses across distances, not too high as to be lethal, and be suitable for battery backup. The reason for the -48V (as apposed to positive) is to reduce the galvanic effect on cabling in damp conditions with respect to GND. If the cables carries +48V, the copper cables would suffer galvanic erosion.