


Negative voltage also protects against sulfation on battery terminals. It prevents electrochemical reactions from destroying buried copper cables and rendering them useless if they happen to get wet. Negative 48VDC (-48V), or positive grounded, was selected for use by Bell when it was found to be superior to positive voltage. In fact, -48VDC allows telecom operators to use 12-volt lead-acid batteries wired in series to act as a backup power source in the event of a power failure. Today it is generally accepted by safety regulations and electrical code that anything operating at or below 50V DC is a safe low-voltage circuit, and -48VDC is still the standard in communications facilities serving up both wired and wireless services. The reason Bell selected -48VDC is because it provides enough in power to support a signal, but not enough to be dangerous. In the late 1800’s, most homes of were not yet wired for electricity in fact, communications beat power to the home in much of the United States. The original telephone systems of the Bell Telephone company were powered from a -48VDC infrastructure out of their central office locations.
