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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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The Short-Circuit Enigma of the AF (and AT) Standard
It is often said that the maximum short-circuit as per the AF standard is when port voltage droops to 30 V. This is the number indicated in Figure 33C.6, page 97, and also in Figure 33C.4, page 95 in the AF standard.
Does that make complete sense? Consider this: The voltage at the PSE output during current limiting is ILIM × ROUT, where ROUT is the output (load) resistance and ILIM the actual set current limit of the PSE/ DUT (it can be anywhere between 0.4 to 0.45 A as per the AF standard). As a side note, observe that Figure 33C.7, page 98 of the AF standard seems to erroneously say that ROUT can be as low as 1 Ω. That would make the output voltage as low as 0.4 A × 1 Ω = 0.4 V during current ...

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