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In some refdesign I'm following (for PoE), they ask to a 1nF capacitor to earth/chassis (not GND).

One of the global design constraints is to maintain DC-resistance of at least 20M between all supplies/signals and earth/chassis (so I aim for at least 40M to have some margin). And isolation should be at least 2kV. NB : its OK if resistance goes bellow 40M at voltages above 500V.

My difficulty is how to find such capacitor? Filtering the capacitance and voltage is easy. But I haven't found any filters for DC leakage resistance (or leak current) on digikey/mouser/RS. For most capacitors, I can't even find this information in the datasheet.

Do you have any idea how to search for such a capacitor (excepted brute force, ie looking at all datasheets until I find one that fits)? Or at least what types of capacitors are good candidates?

NB : there is no indication about the capacitor, excepted this schematics : enter image description here

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  • \$\begingroup\$ Why do you want this? \$\endgroup\$
    – BeB00
    Apr 11, 2023 at 6:54
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    \$\begingroup\$ Also, I selected the first 1nF 2kV capacitor listed on digikey (digikey.com/en/products/detail/kemet/C1812C102KGRACAUTO/8109955) and the datasheet says that the resistance is 100Gohms, so I dont think you have much to worry about \$\endgroup\$
    – BeB00
    Apr 11, 2023 at 6:57
  • \$\begingroup\$ Thanks a lot for the part suggestion (nb : I'm still interested in a more general method). As for why I want this, I suppose you refer to the 2kV and >40M? Some of our clients monitor leakage currents real time (ie in use), and consider it a fault if it goes bellow 20M (in subsea systems, current leakage is a usefull warning for water infiltration). The 2kV is because we don't wan't ESD to damage the capacitor, and because we use a 500V insulation testor to measure insulation \$\endgroup\$
    – Sandro
    Apr 11, 2023 at 7:23
  • \$\begingroup\$ Sandro, there is no general method; it's a trawl through data sheets. \$\endgroup\$
    – Andy aka
    Apr 11, 2023 at 13:13

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No proper capacitor, especially of that voltage rating, will have a resistance that low, or conversely a leakage that high.

You need to find a quality supplier that you trust (so no sourcing them from auction sites), and the capacitor leakage will take care of itself.

You may need to be concerned about surface contamination, of the capacitor and the circuit it is used in. If the environment is dusty and/or humid, you could get board surface leakage. This can be mitigated by specifying the environment the board is to be used in, protecting the circuit from the environment with an enclosure or conformal coating, or designing the board with larger surface tracking distances between sensitive tracks.

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