48V is a telecommunications industry-standard operating voltage. It is considered a “compromise voltage” by being high enough to enable relatively low signal loss transmissions over large distances, and yet low enough to be a “safe low voltage”. (Most international safety regulations consider levels below 50V to be safe. )
An advantage of negative 48V is that four 12V batteries connected in series create 48V DC usable as a backup power source. Central telecom stations are known to have elaborate arrays of 48V battery banks.
One important aspect of telecom power installations is that the polarity of the 48V DC source is setup to be negative with respect to ground. This convention makes the entire telecom system more immune to corrosion and safer for individuals performing telecom maintenance. This negative polarity prevents metal ions from migrating from one metal element to another, which is the cause of rapid battery terminal and copper wire corrosion.
Although many new telecom stations have become very compact, they remain backwards compatible in terms of being powered by -48V. This feature will remain a standard into the foreseeable future.
There are installations where power demand is modest and there is no room to accommodate 4 batteries to provide 48V emergency power. Backup power is vital to maintain the operation of an entire sector served by such a station. A DC-DC converter provides the solution to enable the station’s operation on a single 12V battery for long periods. Figure #1 illustrates the hookup scheme with a 12V to 48V DC-DC conversion from a single battery.
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