Grounding Electrode Potential Gradients from Model Tests

From these tests on model electrodes in an electrolytic trough certain broad aspects of the problem can be summarized. 1. For small areas and single items of equipment, grounding arrays of from one to a few rods generally cannot be expected to reduce the touch and step potentials to an acceptable value. 2. The control of potential gradients by means of buried wires was explored. From tests on wires with and without rod electrodes atlid at various depths it can be concluded that no one arrangement is better than any other by more than a few per cent. 3. The resistance of a typical transmissiontower footing can be reduced by driving a considerable number of long ground rods. The slope of the tower-potential gradient curve is not essentially changed by the addition of driven rods if the braces are below the earth´s surface. 4. When driven rods are added to a small substation mesh a small reduction in resistance was the only tangible benefit. In actual practice, however, driven rods are generally necessary to stabilize the mesh grounding when the soil is dry or frozen. Model tests are about the only method available to verify experimentally calculated potential gradients for various grounding electrode arrangements. The difficulties in making extensive model tests, however, are considerable as the electrolytic tank must be quite large to maintain an error of less than 5%. This necessitates an outdoor installation subject to weather and other minor hazards.