Tower Footing Corrosion^★

The measurement of the tower-to-soil potentials provides a means by which the condition of the galvanizing on underground portions of towers can be predicted with fair accuracy.

The electropotential measurement technique has the advantage over the chemical and visual methods of being so rapid that many towers can be checked in the time it takes to examine one by either of the other methods.

The potential measurement gives the over-all condition of the underground portion of the towers instead of the condition at the relatively small areas that it is practical to expose for visual observations or chemical test.

The tower potentials at switching station ends of The Detroit Edison Company's transmission lines are much lower than those that are considerable distances from the stations. Potentials are also low for those towers situated near steel poles set in concrete and grounded individually.

The low potential at the station ends is due in part to the fact that the grounding medium for the stations is massive steel and copper. Similarly the steel poles set in concrete are connected to grounds of either copper rods or pieces of iron.

The disconnection of the overhead ground wire from the station structure usually caused the potential of towers adjacent to the station to increase: In some cases the increase in potential has been great enough to indicate that the towers are adequately protected by the galvanizing remaining on them. In other cases the potential remained low after the ground wire was insulated from the station. This indicated some other means of protection was needed.

Potential gradients do exist at locations other than at station ends. These gradients are .usually downward in the direction of the tower having the greatest loss of galvanizing due to corrosion.

The potential measured is not always the true potential of an individual tower because the overhead ground wire permits nearby towers of lower or higher potential to influence the potential of any individual tower.