A correlator-based current measurement system for transmission line pole footings

The ground wire on top of a H.V. transmission line poses a unique problem when measuring the resistance of pole footings. With a multimeter most of the current is diverted away from the footing and a simple clamp-on type current meter is impractical for poles ranging in diameter from 8 to 32 in. In addition, clamp-on meters typically fail below a few milliamperes but sub-milliampere measurements are required for accurate monitoring of pole footing resistance. A correlator-based sub-milliampere current measurement system using giant magneto-resistive (GMR) magnetic field sensors is reported in this paper. A correlator-based phase insensitive algorithm was used with injected currents at 94 Hz. Experiments conducted on a pole in the laboratory indicate measurement of current values exceeding 1 mA with a standard deviation of less than 2%. Measurements were also performed with current values as low as 0.2 mA with a standard deviation of 16%. Details of the system, correlator algorithm, and results are presented in this paper.