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RE: [802.3af] Power isolation

Hi Wayne,


I'll summarize the the technical status of the isolation issue and than I'll refer to draft 3 section 33.4.1.


1.    According to the current IEEE802.3 standard, the MDI leads should be isolated from the port device circuits and frame ground. (in both sides of the MDI).

2.    In environment A there is no requirement for isolation between port to port.In Environment B there is a requirement for isolation between port to port.

3.    Now with the addition of the Power Over MDI technology (IEEE802.3af), the above applies too, however, it implies how to treat the PSE circuits and the PD circuits.

       In environment A, the PSE can use a single main power supply that delivers power to all ports. The power supply output should be isolated from

       frame ground and from port circuits (the circuits as defined by IEEE802.3) 

3.1    Due to the fact that a single power supply output is connected to all ports, there is no port to port isolation and it is OK according to (2).

3.2    From the above info, it is required to have isolation between PSE output port circuits and Port circuits.(they are not on the same ground!)


4.    At the PD side, if we read the wording of IEEE802.3, the MDI should be isolated from the DTE circuits and frame ground.

4.1  When adding a dc/dc converter that its input is connected to the MDI and its output is connected to the DTE circuits, the DC/DC converter

       should have isolation between its input and output in order to meet (1).


4.2    From safety standards point of view, if the PD is closed with plastic package (no frame ground or metal enclosure) and there is no metal  parts,

        accessible to the user, there is no need for using isolated DC/DC converter (which cost more than non-isolated one), however not using isolated DC/DC

        converter violates IEEE802.3 which requires isolation between MDI to DTE circuits.



According to the above, I believe that the wording used in draft 3, section 33.4.1 need to be modified accordingly.

Possible wording could be:


"The PSE output shall be isolated from port circuits, including frame ground (if any).

Port device circuits, including frame ground (if any) will be isolated from all MDI leads.

PD shall provide electrical isolation between port device circuits, including frame ground (if any) and all MDI leads."  or equivalent wording.


See the following link for more info: 









    Darshan Yair
    Chief  Engineer
    PowerDsine Ltd.  -  Powering Converged Networks
    1 Hanagar St., P.O. Box 7220
    Neve Ne'eman Industrial Zone
    Hod Hasharon 45421, Israel
    Tel:  +972-9-775-5100, Cell: +972-54-893019
    Fax: +972-9-775-5111

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