I'm currently designing a PCB that will be powered by PoE (or external supply).

I have some doubts about what isolation the PoE supply (PSE, typically a PoE switch) is guaranteed to provide :

  • are the Ethernet wires (and their common mode voltages) isolated from the supply voltage of the PSE (that can be DC)?
  • on a switch with multiple PoE ports, are the ports isolated compared to each other?

For now, I just have a power ORing with the external power supply, and all signals are sharing the same ground, which is also the ground of the external power supply.

Until now, I was assuming PoE supplies where guaranteed to be isolated, but I'm starting to have some doubts, so I's rather check (if isolation isn't guaranteed at PoE side, I will have to do quite some modifications on my side)

PS : in case it matters, this PCB will be used as part of a robot, with short (<3m) ethernet cables. All external signals are referenced to the same ground as the external power supply (if present)

  • \$\begingroup\$ Isn't it required that both the PSE and PD are fully isolated, as even standard Ethernet requires that the wires can have 1500 Vrms potential difference from the device potential? Which means that your PoE powered board must also have an isolated converter from PoE wires to your system ground? If you do your device by the rules, you don't need to worry how the PSE is implemented. And if you did not do your device properly, then you should be worried about meeting another device which did not do it properly. \$\endgroup\$
    – Justme
    Apr 27 at 10:49
  • \$\begingroup\$ So if I understand correctly, if PSE is conform to standard, it grantees 1500V insulation between the ethernet cable and its supply/frame. Correct? So I can assume there is insulation (provided I buy a decent PSE) and not add insulation on my, but If I do so, I'm not allowed to claim to be a PoE device. Correct? \$\endgroup\$
    – Sandro
    Apr 27 at 11:35
  • \$\begingroup\$ And do you know if there is also 1500V isolation between the different ports of the PSE? \$\endgroup\$
    – Sandro
    Apr 27 at 11:36
  • \$\begingroup\$ You might want to double check that the specific equipment you get comes from a reputable manufacturer and is standards-compliant, because I wouldn't be surprised if some cheap ones skimp on the isolation. \$\endgroup\$
    – Hearth
    Apr 27 at 15:13


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