Abstract
Different ways of reducing, calculating or measuring the ohmic component, the IR-drop, involved in potential measurements on cathodic protection installations are discussed. The measuring methods that seemed best suited for field usage have been tested in the laboratory as well as in measurements on buried, cathodically protected tank installations. The measuring methods chosen were the well known recorder method and the earth megger method, the later not having been described before. The results obtained with these methods were compared with those obtained with the oscilloscope method.
It was established that both methods when used for measurements on steel structures give too high an IR-drop but that the magnitude of the error was dependent on the quality of the coating. When applied to buried structures having a poor coating the recorder method results are 25–30% higher, while the earth megger results are 10–15% higher, than those obtained by the oscilloscope method. The field investigations also showed that the IR-drops can be quite large, several hundred mV, when the structure to soil potential is measured with the reference electrode on the ground surface.
The new earth megger method proved to be very simple and reliable and the method is well suited to IR-drop measurements on cathodically protected installations demanding a low total current. The accuracy of the earth megger method is not so good when higher currents prevail and in these conditions the recorder method is recommended.
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