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I read that active POE is safe, and that when performing the handshaking it can increase the voltage up to 10.1V. So I infer that generally Ethernet devices can handle 10V no problem. But what about 24V?

If one connects 24V passive PoE (pin 1&4 to +24V, pin 7&8 to gnd) to a non-PoE device, like a laptop, I have read that it could be damaged. As far as I understand ethernet is an isolated system, that use transformer couplings. If you put a DC voltage on the transformer nothing should happen.

How could a non POE device be damaged by a passive 24V POE device?

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    \$\begingroup\$ "If you put a DC voltage on the transformer nothing should happen" - except the voltage spike when it turns on/off, and the transformer burning up \$\endgroup\$
    – FrancoVS
    Apr 29, 2021 at 12:32
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    \$\begingroup\$ I have witnessed burned magnetics because of this - be careful. Not all Ethernet jacks can accept PoE. \$\endgroup\$
    – rdtsc
    Apr 29, 2021 at 13:27

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If PoE is passively injected without detection, it usually means that the supply voltage is applied to pins 4&5 and the supply return to pins 7&8.

If the device has DC coupled common mode termination on the cabling side of the Ethernet transformer, it basically means the transformer center taps are connected with 150 ohms of resistance, and yes, it will be damaged from applying PoE passively.

Devices that will tolerate passively fed PoE have AC coupled common mode termination where capacitors isolate the DC path between pairs.

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  • \$\begingroup\$ That makes sense, do you know how common R termination is compared to C termination? \$\endgroup\$
    – run_the_race
    Apr 29, 2021 at 14:38
  • \$\begingroup\$ That means the device's ability to withstand passive PoE could be measured, right? If there's a fixed resistance between pairs (no capacitor), it's likely to burn out. \$\endgroup\$
    – akwky
    Sep 15, 2021 at 8:37

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