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Old 08-08-2009, 01:42 AM   #2161 (permalink)
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Earth shielding at one end is used to eliminate capacitive coupling into a low voltage circuit. The voltage induced into a circuit from a high current conductor is mainly transformer coupling. The only way to eliminate this is to create an opposing field source which cancels out the original. This is done by connecting both ends of the shield to earth. This creates an earth loop current in opposition to the original disturbing field. The value of the field is much smaller than the original but because it is much closer to the low voltage circuit the field tends to cancel out the main disturbing field.

We have been using this technique in telecommunications for years to reduce induced noise into telecommunications cables from power lines nearby. So not all earth loops are bad.

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