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To my knowledge, the PoE standard requires a Switch to be capable of detecting whether a connected device is PoE capable before delivering the full power via PoE. Why is there still often a warning, non-PoE Equipment might suffer damage if connected? This seems kind of contradictory to me?

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There are some poorly designed or constructed devices. We experienced this with some devices that had pins shorted in a way that requested PoE from the switch, but the devices could not handle PoE, and they blew up when connected to a PoE switch interface. This was a design defect in the devices, and we needed to disable PoE on the switch interfaces where those devices were connected.

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In addition to Ron's anwer, some PoE switches support proprietary, pre-standard PoE modes that weren't as thoroughly designed as 802.3af/at. When active, sending power to a device not supporting it gets more likely.

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    More to the point, some PoE sources provide a passive form of PoE, where instead of negotiating for power, power is always applied. – YLearn Feb 25 '18 at 21:50

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