Impulse and 60-cycle characteristics of driven grounds

I. Introduction Driven grounds are important in electric power transmission and distribution. In fact, they comprise one of the essential elements in the art of lightning protection. Yet, to this day, the value of protection derived from grounds under actual operating conditions of lightning discharge is difficult to state in full quantitative measure. And the reason for this situation lies partly in the lack of fundamental knowledge of the impulse characteristics of driven grounds. In part, the difficulty also is due to the complex factors that inherently make up driven grounds and ground systems. A comprehensive survey of principles and methods on driven grounds was issued in 1918 by the Bureau of Standards.¹ Further contributions have appeared since, some presenting new developments and findings,² others dealing on theoretical aspects of the problem,³ and a third group bearing on related questions.⁴ In recent years, progress has been centered on the immediate field of application. Here the emphasis has been to obtain effective service with such methods of grounding as lend themselves particularly to economical installation. For instance, a common practice with some utilities nowadays is to drive rods to considerable depths, even down to bedrock, so as to attain the lowest measurable resistance. A recognized practice for securing low-resistance grounds is also to place a sufficient number of electrodes in parallel (multiple grounds). Still another expedient is that of reducing the resistivity of the soil immediately surrounding the electrode by suitable treatment with common salt (NaCl) or other conducting solution. All these developments have been based largely either on 60-cycle values or on closely similar methods of testing. And from the measured 60-cycle values plus what experience would ensue there have been established present practices.