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Insulation resistance works in parallel too25, 50 and 100 megohms. What's the total?
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The question at the end: 25, 50 and 100 megohms. What's the total? The answer is in the notes below.
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Circuits are wired in parallel, so when you test more than one at a time the insulation resistance values combine in parallel as well.
Three circuits reading 25, 50 and 100
1/25 + 1/50 + 1/100 = 0.04 + 0.02 + 0.01 = 0.07 RT = 1 ÷ 0.07 = 14.29
A sanity check on length — conductor resistance, not insulation
Three conductors of 10 Ω each in parallel -> 10 ÷ 3 = 3.33 Ω Double the length, so each becomes 20 Ω -> 20 ÷ 3 = 6.67 Ω For conductor resistance, doubling the length doubles the result. Insulation resistance goes the other way: a longer cable has more insulation in parallel, so doubling the length roughly halves it.
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