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My guess was that the equivalent insulation resistance of two cables in parallel is same as the equivalent resistance of two resistances in parallel. But instead it is the sum of the two insulation resistances. What is the logic behind this?

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  • \$\begingroup\$ Welcome. That is often referred by the DC breakdown voltage per mil of thickness. If you want dielectric losses at certain radio frequencies,or digital signal loss or phase-shift per meter, those are all separate issues. Please be specific about your question(s). Your wording is confusing. Two resistances in series is what you might be looking for. \$\endgroup\$
    – user105652
    Feb 6 '19 at 1:40
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    \$\begingroup\$ Let's play a mind game. Imagine that there is a battery connected to the parallel conductors. Now imagine how any leakage current would flow from one conductor to the other. The current has to pass through the insulation of the first conductor, then flow through the insulation of the second conductor. In other words, the insulation resistances are in series. \$\endgroup\$ Feb 6 '19 at 1:42
  • \$\begingroup\$ Thanks @Dwayne Reid But how would the leakage current of one cable pass through the second cable? The leakage current flows radially outward right? \$\endgroup\$ Feb 6 '19 at 1:48
  • \$\begingroup\$ I'm re-reading your question and I may have misinterpreted what you asked. Are we talking about two separate conductors that are connected to two different nodes in the circuit? Or are the conductors connected in parallel with each other? My comment is correct for the first case but not correct for the second case. \$\endgroup\$ Feb 6 '19 at 2:49
  • \$\begingroup\$ The conductors connected in parallel with each other. \$\endgroup\$ Feb 6 '19 at 3:15
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You have to think of the path the leakage current would take, I think the comments you received cover this mostly.

schematic

simulate this circuit – Schematic created using CircuitLab

think of it like this imagine those two pi circuits are the cables, where the capacitors are the insulation at the start and the end of the cable and the resistor is the "normal" path for the current. Imagine you wanted to reach node 1 from node 2 or viceversa from the capacitors you gotta go through one capacitor reach the point where they have a common connection and then go through the other, so there is a path that goes through two capacitors so you can get from 1 to 2. This path would be the case that they appear to be in series and a voltage divider of some sorts will happen between them.

As one of the people in your comments mentioned if they are fully parallel cables so connect 1 and 2 in my schematic to, this reduces the problem of the leakage current only going through 1 capacitor to reach ground.

I have avoided using ground terminals on the circuit to make my explanation a bit clearer and there is purpose of using capacitors as "insulation" as well.

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