I am trying to calculate the minimum insulation resistance of two parts (a capacitor and box) that are approximately 0.38mm apart or 0.00038m.

I've tried looking at resistivity of air which some people are giving as 1.3 × 10^16 per meter.

Would the equation 1.3x10^16 x 0.00038 = insulation resistance be accurate?

I am guessing no as I imagine the voltage applied and cross sectional area would also have an impact. I need the insulation resistance to be at least 50MOhms @ 50VDC.

Could you please advise.

Thank you in advance.

  • \$\begingroup\$ I don't think you will get reliable results with calculation. Measuring would be my choice. 50 VDC seems a bit low for insulation resistance. And can you make sure the distance stays the same and the humidity of the air? Maybe a solid insulator material would be a better choice with just 0.38 mm space. \$\endgroup\$
    – Arsenal
    Jul 11, 2017 at 9:52
  • \$\begingroup\$ Thank you for the reply. The distance will be the worst case scenario. As for the humidity they are assembled in a controlled environment but imagine there will be small differences. 50VDC is per the customer specification. This is not a high voltage so instincts tells me the gap will be okay but i would like to be sure. I've roughly estimated the crosssectional area to roughly 1.5mm x 1.5mm. \$\endgroup\$
    – R.Joshi
    Jul 11, 2017 at 10:28

1 Answer 1


In electrical engineering we are more normally concerned about the breakdown strengths of air gaps, not the resistance of air.

But since you are trying to validate that you are meeting a customer's specification, however unusual, you can use the calculation you propose. I would recommend, however, that you ask your customer to agree with your assessment methodology. Due to the unusual specification, they may have something else in mind.

  • \$\begingroup\$ Thank you for the reply. What is (if any) the equation that relates Air Gap Breakdown strength to applied voltage and distance? \$\endgroup\$
    – R.Joshi
    Jul 11, 2017 at 13:10
  • \$\begingroup\$ It is normally specified at 21.1 kV/cm RMS and 30 kV/cm peak although some safety certifications will require other metrics. \$\endgroup\$
    – Glenn W9IQ
    Jul 11, 2017 at 13:13
  • \$\begingroup\$ So according to those numbers. The breakdown strength of 0.00038m gap would roughly 1139V? (1cm/0.038cm = 26.32, 30kV/26.32 = 1139V)? \$\endgroup\$
    – R.Joshi
    Jul 11, 2017 at 13:20
  • \$\begingroup\$ That is correct. \$\endgroup\$
    – Glenn W9IQ
    Jul 11, 2017 at 14:47

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