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)