Measurement of ground potential difference at power substations

A method of obtaining a record of ground potential difference (GPD) values in substations where a remote ground and an oscillograph are available is presented. Typical recorded values of GPD range from under 100 V to a maximum of 450 V RMS. These measurements suggest that the values used for ground mat design should be considered by the telephone company engineer as conservative and that additional amounts conservatism are not necessary. The telephone company engineer should determine how the power company has calculated the requested data so that a proper judgment can be made. A history of recorded GPD values will aid in determining needed protection. Significant savings may be realized by avoiding excessive conservatism. Since the theoretical calculation of GPD results in a typical large value of voltage, the associated zone of influence will be large. Within that zone, all telephone grounds will be removed, causing greater lightning damage to the outside telephone plant. Therefore, the probability of lightning damage as compared with the likelihood of GPD-caused damage must be considered.<>