Why “Negative” 48VDC?

Why “Negative” 48VDC?

Why does the telecom industry use “negative” 48VDC for power? This is a common question that seems to come up. The answer is found in the electroplating process. When two materials are at different potentials in an electrically conductive environment, molecules migrate from the positive node to the negative node.

A telecom cabinet is electrically tied to an earth ground rod. If the negative terminal of the power system was connected to earth ground, and a wire with a positive potential is near the earth ground or cabinet, leakage currents would allow copper molecules from the wire to migrate to the earth ground rod or cabinet. This process is compounded by wet corrosive environments which are common for outdoor cabinets. In extreme environments the copper losses become significant and can cause the small wires (typically 24awg) and connections to fail.

The solution is to shift the earth ground to the positive terminal. This means all voltages in the cabinet are at a lower potential than earth so the direction of the molecular migration is reversed and deterioration of the phone lines is no longer a problem.

A much more definitive study of the issue can be found in this paper written by Professor Richard Levine of Southern Methodist University – Why Positive Power Ground in Telephone Subscriber Loops?”

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