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Old 08-08-2009, 11:14 AM   #2162 (permalink)
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Originally Posted by mcudogs View Post
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.
This could explain why Australian Telecommunication is so poor wherever it is on copper . Full of earth loops, particularly after rain. I would go as far as to say most of the induced noise into telecommunications cables in this country has very little to do with the proximity of power cables and a hell of a lot to do with earth loops often the result of water ingress. It is certainly a common occurrence in country areas.

In the 1980's much research was done into induced noise in sensor circuitry as the popularity of computerised SCADA systems increased. In particular the installation in electrical substations where we experienced substantial issues with induced noise for fairly obvious reasons. One thing we learned is the noise was worst when shielded cables were earthed at both ends, in some extreme cases (long cable runs) it was worse than unshielded cables.
The best solution was twisted individually screened pairs earthed at the computer terminals.
There is another bad thing to do as well (probably not significant in the EV but possibly in telecoms), don't leave unused wires in the same cable unterminated, they work as damn good antenna and amplify the noise level nicely.

Also important that the earth bonding is at the computer end, if it is done the other way nasty little voltages can appear at the terminals and blow things up, can be a little embarrassing.

Last edited by squiggles; 08-08-2009 at 11:18 AM.. Reason: Another point.
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