Why Positive Ground in Telcom Power Systems?

Quote of the Day

Keep the company of those who seek the truth, run from those who have found it.

— Vaclav Havel

Introduction

Figure 1: Positive Ground Reduces Corrosion of
Moist Telcom Wiring.

I have been asked a number of times recently why telecommunications systems are powered using negative potentials like -48 V. The rationale behind this choice has to do with how wires corrode in moist environments.

Today, we use wires with excellent insulation and these moisture-induced corrosion arguments carry less weight.  Back in the old days, however, telecom wiring had cloth-based insulation that would get wet – along with the copper wire. When you mix water and electric fields, you get electrochemical action (i.e. electroplating). While the early systems were configured with negative grounds (i.e. ground as the lowest potential in the system), it was soon discovered that these systems suffered from corrosion. Positive ground (i.e. ground is the highest potential in the system) voltages were found to be effective in combating this corrosion.

Figure 1 illustrates what happens with respect to a standard land-line phone, which consists of a wire pair called tip and ring. If the wire pair becomes moist, the E-fields are directed such that copper will move from the local ground rod to the copper wire. The copper wire is preserved, while an insignificant percentage of copper is lost from the relatively large amount of copper in the ground rod. If either of the wires had a positive potential relative to the ground rod, the positive potential wire would lose copper to the ground rod and would eventually fail.

If you want more details on the subject, see this tutorial.