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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Overview
If the Ethernet cable to the PD is suddenly disconnected and then immediately plugged into a “non-PoE-friendly” DTE (like a NIC), the terminations of the new device could get damaged. To avoid this situation two things are required: (a) the PSE must constantly sense the PD to ensure it is still present, and (b) the moment the PD is deemed disconnected, the PSE must remove power from the port. In fact, it is recommended that it actively “discharge” port capacitances to a safe level (2.8 V) within TOFF = 0.5 s. This maximum time was set based on how quickly a cable could be disconnected and then reconnected to another RJ-45.
The PSE is, in any case, constantly monitoring the port current, and within its accuracy of measurement can very ...

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