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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.

Karsten
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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.

It has been brought to my attention of an amendment to the question: -

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

My response is that it's still beneficial to use a decent isolated DC-to-DC converter in the powered device (PD) because if there are any external interfaces in the PD to the outside world (aka user ports) then, to avoid upsetting the data integrity of the ethernet comms, isolation is required "somewhere" and, the likeliest place to put it (to obtain best cost and performance) is by using a decent isolated DC-to-DC converter.

winny
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Andy aka
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  • 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. – Karsten Apr 27 '20 at 20:57
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    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. – Andy aka Apr 28 '20 at 07:41
  • 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. – Karsten Apr 30 '20 at 11:06
  • Hi @Andyaka I'm trying to figure out if it is safe for me to attach my oscilloscope ground to the isolated PoE power supply. Is it safe to do so? (connect scope ground to all 4 pins 3,6,7,8 on PSE side of a 803.2bt connection) Or is it only safe to connect the scope ground to one or one pair of these? Or none? – Steven Lu May 20 '22 at 04:26
  • @andyaka Are you talking about the PD or the PSE? I think the user is talking about the device end. You can still run the PD DC DC converter non isolated. Usually the DC DC converter is in between the rectifiers and the isolation transformer anyway if you are running a PS scheme. The PSE always needs to be isolated – Voltage Spike May 03 '23 at 18:00
  • @VoltageSpike it looks like the OP edited the question after I made an answer. Now we are in uncharted waters because, to use a decent isolated supply in the PD is fine but, to use a non-isolated supply in the PD requires isolation on any interfaces to the outside world to maintain ethernet integrity. So, I'm thinking I might add that statement. – Andy aka May 03 '23 at 18:09
  • @Andyaka True that was added later but the part "I am designing a circuit that is required to be PoE powered" implies PD not PSE, I'm working on the same problem as the user and the answer looked like it needed clarification – Voltage Spike May 03 '23 at 18:11
  • I've amended my answer @VoltageSpike Also note that the sentence following was this: *We are debating whether to use an isolated or a non-isolated supply* and that implies to me the power source (aka supply) i.e. the PSE end. – Andy aka May 03 '23 at 18:14
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Practical Guidelines to Designing an EMI Compliant PoE Powered Device with Non-Isolated DC/DC

https://www.ti.com/lit/an/slua454/slua454.pdf

The circuit uses the TPS23750 in a non-isolated buck converter topology to produce a 5-V at 2-A output. The IEEE 802.3-2005 standard requires that circuits connected to the 10/100Base-T Ethernet port be isolated from all external electrical connections that can be touched by users or connected to other equipment. PDs without these connections may use non-isolated converters for their simplicity

Neil
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