Why is 48V a telecommunications industry standard operating voltage? There are several reasons why this is the case. First, it is considered a “compromise voltage” because this voltage level is high enough to enable relatively low signal loss for transmissions over large distances and yet low enough to be considered a “safe low voltage.” Most common safety regulations internationally consider levels below 50V to be safe.
Another advantage of this voltage level is that four 12V batteries connected in series create a 48V DC backup power source, usable in the event of electrical power outage. Many central stations have elaborate arrays of 48V battery banks which may occupy even an entire room.An 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 the human body while performing telecom maintenance activities. This negative polarity prevents metal from copper wire to migrate, which is the cause of rapid corrosion.
Telecommunications stations have taken on new more compact forms with the evolution of communications technology, and yet they must remain backwards compatible from the negative 48V standpoint. It remains the standard even today and will continue to be in the foreseeable future.
So what happens in locations where space for equipment is limited and in a given station there is not enough space to accommodate multiple batteries? After all, emergency backup capability is indispensable or a power outage would disable the entire sector served by said station. This is where the DC-DC converter comes into play. A remote substation can be made to operate without electricity on a single 12V battery for long periods of time if its power demands are modest. Figure #1 illustrates the hookup scheme which can be achieved with a 12V to 48V DC-DC converter using only a single battery.
Today’s high power DC-DC converters are available with wide input range capability. This ensures that its negative 48V DC output level does not decrease or become noise degraded as the battery voltage transitions from its fully charged to its fully depleted level. Many DC-DC converters for telecommunications have all-aluminum housings with anodized finishes which make them inert and corrosion resistant, ideal for the telecom application.