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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Minimum Vertical Separation in PCB
As mentioned previously, flashover can occur not only through air, or over the surface of the PCB, but also through insulation. Our final concern is therefore avoiding flashovers through the PCB material inside the PCB the between copper layers. Note that typical FR-4 PCB material has a dielectric strength of 700 V/mil or 2756 V per 0.1 mm. So to hold off 2500 V, we basically need a minimum distance through PCB material of only 0.1 mm (4 mil) The rule is: Minimum distance through insulation (PCB) is 0.1 mm (4 mil) for 2.5 kV.
Note that standard PCB thickness is 63 mil or 1.6 mm. With a four-layer PCB, assuming the four copper layers are equidistant from each other, the distance from one layer to another will ...

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