It has to do with what direction the electrons are moving. In a positive charge they want to move to the outside of the conductor. With a negative charge they move inward. With things like CAT5 or simple RCA cables you have a positive and negative charge moving down the same multi conductor cable. To keep the signals from interfering they are encased in a shield. This is done to reduce interference from the cable itself. Say you have 100' of CAT5 your are running through the ceiling of a building, you only need 80 for the run, you get to the destination and roll u the remaining cable and tie it in a big loop. If that cable where not shielded that big loop would cause interference in itself, the resistance would go up and the data loss would probably be affected because the server would have to push harder on the data.
Now in an automotive single conductor situation you don't have to be that protective of signals. There really isn't any multi signal wires or long overlapping runs. Yes you will have positive and negative running side by side, but at the low amperage they operate there is no electro magnetic interference created. As a general rule EMI will increase with amperage in the same cable size, increase the cable size and the EMI goes down.
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