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Currently I am designing a circuit that is required to be PoE powered. We are debating whether to use an isolated or a non-isolated supply.

The device is wall mounted and in a plastic enclosure. No metal parts are exposed to the end user.

This answer said that this is the only requirement to be able to use a non-isolated DC-DC converter, but I would like to know if someone can reference an actual document were this is stated. It will be sold in Germany, so there may be additional regulations that do not apply to the whole world.

EDIT: To be clear: we are designing a powered device not a PSE.

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When is it allowed to use non-isolated PoE?

Never.

IEEE 802.3at specifies that isolated power supplies be used. If it doesn't use isolation then it's not called PoE.

We are debating whether to use an isolated or a non-isolated supply.

It's not PoE unless you use an isolated supply. It becomes something else.

Think about what ethernet does - it provides massive insulation against indirect lightning surges by using magnetically coupled data interfaces. This isolation would be ruined by a non-isolated DC power supply. Not only would it become much more susceptible to surges but the impedance balance of signals would be significantly compromised and speed performance would be very poor.

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  • \$\begingroup\$ I agree that isolation is much safer and it may be required to be fully IEEE 802.3af/at compliant (although I could not find any reliable source stating that PDs must be isolated. I only found sources stating that PSEs must be isolated). But a lot devices like video cameras use non-isolated PoE designs and work well with standard compliant PoE switches. \$\endgroup\$ – Karsten Apr 27 at 20:57
  • \$\begingroup\$ The devil is in the detail and not in the generalisms. Isolation isn't really about safety - it's about protection against surges (EN61000-4-5) and best data performance by ensuring proper impedance balance. \$\endgroup\$ – Andy aka Apr 28 at 7:41
  • \$\begingroup\$ We are just talking about the PoE part. The data lines will be isolated with proper transformers. My question was more about whether it is allowed or not. Not if it is recommendable. In our case the PSE is located in the same housing a few cm away from our device so most of the standard concerns don't really apply in this case. Speed is also no concern, because even a 10BaseT connection would be plenty enough. \$\endgroup\$ – Karsten Apr 30 at 11:06

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