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.
Lets look at a schematic:
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.