Experimental Investigation of the Electromagnetic Signal Attenuation Characteristics of the Partial Discharge Laboratory at Jadavpur University

Abstract

Electromagnetic interference is becoming an increasing concern, because of the high intensity of surrounding electromagnetic waves, mainly arising from communication signals and also due to widespread use of equipment that operates at radio frequencies. As a consequence, sensitive data acquisition equipment suffers from erroneous results. Operating such instruments in a suitable shielded environment can significantly reduce this electromagnetic interference. But to achieve good shielding in practice, construction-related problems are to be faced, especially in large spaces, where a single metal plate cannot cover the whole area. Unless special care is taken, electromagnetic waves can penetrate through the gap in the joints and defects like drill holes reducing the shielding effectiveness. Also, a single layer of shielding is not always effective as the quality deteriorates drastically even due to minor constructional defects as mentioned above. This paper describes real-life experiences, step-by-step, in the shielding of a spacious insulation diagnostic laboratory (with a target of at least 55 dB signal attenuation), firstly using a good conducting material, using two different methods for joining the sheets, and ultimately constructing a second layer of shielding using a magnetic material. To study the attenuation behavior of the laboratory with respect to electromagnetic waves, a device for the relative measurement of surrounding electromagnetic signal strength is developed. The signal levels are measured initially outside and then at different places inside the shielded laboratory. The results presented in this paper show (1) the variation of attenuation characteristics inside the shielded laboratory due to different methods adopted for joining the shielding sheets using a good conducting material, (2) the effect of a second shielding layer in the form of a box that was constructed using a magnetic material and placed inside the laboratory and (3) the improvement in attenuation behavior after the actual construction of the second layer of shielding using a magnetic material.