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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: http://grouper.ieee.org/groups/802/3/af/public/may00/stewart_1_0500.pdf Regards Yair.
Darshan Yair
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