Ground resistance measurement with alternative fall-of-potential method

An alternative fall-of-potential method is presented to measure on-line and in situ resistance of ground electrode systems in low-voltage distribution networks. It provides simultaneous resistance measurements of both equivalent ground electrode system of a substation and ground electrode of a premise. The method does not require usage of any auxiliary electrode, but employs two existing ground electrodes in the distribution network. The method has the advantage that it uses moderately high currents than the classical fall-of-potential method, and results are independent of test electrode positions. The method is especially useful in urbanized districts where it is difficult to apply the classical fall-of-potential method. Also, a capacitive potential probe was successfully tested in place of the current electrode used in the fall-of-potential method.

I. Introduction

The best assurance for the effectiveness of a grounding system may be verified by periodically measuring its ground resistance. There are several methods for measuring resistance of a ground electrode system. Among them, as verified in many field tests, the fall-of-potential method is widely applied for almost all types grounding systems [1]. Fig. 1 shows the test setup for measuring the resistance of a ground electrode system. The method uses a current electrode (CE) located at a distance X from the center of the ground electrode system and a potential probe (PP) at a distance Y. Assuming uniform soil conditions, the principle of measurement is basically passing current through the remote CE and measuring the voltage between the ground electrode system and the remote PP maintaining the distance , and keeping PP aligned with SE and CE . The resistance of a grounding system becomes the ratio of the measured potential difference between the grounding system and PP, and the injected test current . Fig. 1. Principle of fall-of-potential method. Fig. 2. Low-voltage distribution system with n-premises supplied in TN-S form.