I trying to hipot test a 12"x12" metal with kapton layer. Different sizes of metal (e.g. 1"x1", 4"x4", banana plug) is placed on top of kapton tape. The bigger metal produced higher leakage current as expected but it is also prone to hipot failure compared to the smaller sizes. What is the possible explanation for that.
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\$\begingroup\$ What happens when the smaller pieces are moved around on the larger sheet? \$\endgroup\$– Tim WilliamsCommented May 18, 2023 at 0:18
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\$\begingroup\$ Thank you for your reply. The smaller piece had smaller leakage current on different positions and no hipot failure. \$\endgroup\$– Ybx NopCommented May 18, 2023 at 16:50
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1 Answer
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When your dielectric fails, it doesn't fail all at once...the weakest area will break down first, protecting (most of) the rest. The weakness of the weakest area will be the tail of a probability distribution. When you have more area, you're likely to have more extreme outliers on that distribution.
The larger area doesn't really affect the breakdown, it just offers more opportunities for failure. It's not defective as long as the weakest point holds up to the specified voltage.
answered May 17, 2023 at 20:44
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\$\begingroup\$ thanks for the explanation. This is the reason why when I use banana plug the voltage can go up to 5KV without failure but when I use 4"x4' it usually failed at around 3.5KV. Would the current draw be a factor as well because in 4"x4" the current draw is much higher. \$\endgroup\$– Ybx NopCommented May 18, 2023 at 12:17
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\$\begingroup\$ I can't explain that offhand. One possible explanation could be that the 4x4 size requires you to use two pieces, and it's breaking down along the seam. I offer this as only one example of how the 4x4 case might differ from the others experimentally; without being there with you, there's far too many possibilities, but if the tape is rated at 5KV it shouldn't break down at 3.5. \$\endgroup\$ Commented May 18, 2023 at 12:56
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\$\begingroup\$ I really admire your knowledge and your explanations make sense. Would you be able to know why I get leakage current using AC but not on DC? Thank you. \$\endgroup\$– Ybx NopCommented May 18, 2023 at 16:46
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\$\begingroup\$ Because you're building a capacitor. Capacitors will conduct AC to an extent, with or without leakage, depending on their capacitance and the frequency of the test signal. \$\endgroup\$ Commented May 18, 2023 at 18:52
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\$\begingroup\$ The AC brings up another point...is the tape rated for 5KVAC or 5KVDC? 3.5KV RMS sine wave has a peak value of almost 5KV. \$\endgroup\$ Commented May 18, 2023 at 18:55