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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Causes of Isolation Breakdown and Recommended Minimum Clearance
When we apply 2500 V DC, the first and most obvious, though not likeliest cause of breakdown is a flashover through air. Air is an insulator, with a certain dielectric constant and also a certain breakdown voltage. Its breakdown voltage is often stated in literature to be 33 kV/ cm, or 3.3 kV/mm, and since there are 394 mil in 1 cm that is equivalent to 33,000/394 = 84 V/mil. Therefore, to hold off 2500 V DC, we need a minimum of 2500/3300 = 0.75 mm or 30 mil. To account for variations in the breakdown voltage based on atmospheric variations, we should leave a little more margin here. We thus fix this minimum distance as 40 mil or 1 mm for meeting 2.5 kV.
Note that the distance ...

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