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I have measured the capacitance between wires number, say, 1 and 3. With the wiree #2 just floating, the result was 390 pF. But after i have grounded the wiree #2, the capacitance was 220 pF. What is the reason of these results? The cable is FFC few meters long, so the wires just run in parallel.

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  • \$\begingroup\$ Do you really believe your question can be answered with the information you supplied. Do you have a schematic? How are the wires related to each other? Are there only capacitors involved or other components? etc., etc. \$\endgroup\$ – Barry Mar 28 '14 at 20:59
  • \$\begingroup\$ What is the physical arrangement of the wires? If wire 2 is partially shielding between 1 and 3 that would a sensible result. \$\endgroup\$ – Spehro Pefhany Mar 28 '14 at 20:59
  • \$\begingroup\$ What type of meter measured the capacitance? Ditto the above comments. \$\endgroup\$ – Andy aka Mar 28 '14 at 21:01
  • \$\begingroup\$ Folks, the physical arrangement of the wires was already specified in the question title, but the poster has now added an additional description of the cable. \$\endgroup\$ – Chris Stratton Mar 28 '14 at 22:07
  • \$\begingroup\$ The description of the wire arrangement is pretty ambiguous. Without some sort of diagram, and a rigorous definition of what "grounded" means here, I don't see how it can be answered. \$\endgroup\$ – Phil Frost Mar 29 '14 at 2:54
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Lets look at a schematic:

schematic

simulate this circuit – Schematic created using CircuitLab

As you can see L2 is now directly grounded instead of beign grounded through C20. If you do your math you can calculate C20 from those 2 measurements. I will leave the rest of it for you to do, have fun.

P.S. This is the representation using concentrated elements, you can create \$\Pi\$ or \$T\$ equivalents and connect them in a cascade to have a better representation of the circuit.

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