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Old 30-06-2006, 21:26   #1
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Grounding the Shield on Power cables from Chart Plotter

I am in the middle of installing my power cables for my Furuno Chartplotter and Fax 30. Both cables are twisted pair, shielded with a woven metal shield.The schematic in the manual notes that BOTH ends of the shield must be grounded. However they don't say where or how.

So do I ground them to "boat ground" or to the same point as the cable's ground wire??

Does anybody have any thoughts on how best to capture that shield so I make a good connection??

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Alan Perry
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Old 01-07-2006, 16:32   #2
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Shields and grounding

If a shield is part of a transmission line which depends upon the characteristic impedance to be uniform between source and load then it must be connected as per a schematic at both ends.

When a shield is present as an attempt to reduce susceptiblility of the wires encased to interference or in an attempt to reduce radiation from the wires encased then termination of the shield at both ends of the cable will be effective only if all the electrical characteristics are predetermined between the shield and each wire enclosed. This you cannot reliably do at the power source end of your wires. You should not make a shield connection to the battery negative at the battery end or power distribution end in order to prevent either power currents or noise currents from passing through the shield. If you make a shield connection to the negative at each end you cannot guarantee this.

So, with a Furuno supplied power cable having a connector at the chart plotter end that, when connected, has continuity between the shield and the negative wire made internally to the chart plotter circuit board do not make a shield to negative connection at the other end of that wire.
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Old 01-07-2006, 16:57   #3
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Rick; thanks for the info, So then where do I connect it? Or do I need to connect it at all?

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Old 01-07-2006, 19:06   #4
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Start off with NOT connecting it. If you have a major interference issue, (which is rare) then you use the sheild to an earth, but that is a longer story, as how you use the shield can be varied as Rick has alluded to.
The shield is in effect a faraday cage. A farady cage does not have to be earthed at each end to work. It is designed to sink radiated interference to earth. In most circumstances, this can be just as effective being connected to the case of the unit, which is usually the outside case of the plug.
As most of the interference tends to be generated by SMPS and LCD display's, the very weak interference field tends to remain within about 3ft of the device generating it. So the Shield doesn't have to extend all the way to the DC supply, (batteries) which of course would be close to impossible for most anyway.
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