RE: [802.3af] Late comment
In order to prevent voltages in excess of SELV, we do need to ensure that
multiple PSE ports are of the same voltage polarity when referenced to a
common return. Beyond this, I can't think of a reason for us to actually
specify this polarity (positive or negative) with respect to the common,
provided we maintain our existing mains earth port isolation requirement.
Yair Darshan <YairD@xxxxxxxxxxxxxx>@majordomo.ieee.org on 01/14/2003
Sent by: owner-stds-802-3-pwrviamdi@xxxxxxxxxxxxxxxxxx
To: "'Dave Dwelley'" <ddwelley@xxxxxxxxxx>, Geoff Thompson
Subject: RE: [802.3af] Late comment
I agree that if all ports are floating than you may have 2*Vport between
two ports however these ports would never touch each other under normal
operating conditions and installation.
In any case the preferred lead to be ground is the positive lead from
practical point of view (easy to design when the + is the common for a
single power supply) and from system point of view (positive lead to system
Currently in our spec we didn't defined if:
a. we stay floating in both leads or
b. ground the positive or
c. use the positive lead as a common point without grounding it to the
d. ground the negative or
e. using mixture of possibilities.
It is clear that option e is disaster to the standard.
option d is bad from practical design aspects and cost issues. (P channel
Option c : the best option by having a positive common point which is
Option b is proffered if we find good reasons why we should ground the
Option a is very good due to max completely floating outputs.
I suggest the following text to the standard:
1. The PSE port outputs shall be isolated from system ground, frame
ground and data circuits.
2. In multi port system all ports common point shall be the positive
From: Dave Dwelley [mailto:ddwelley@xxxxxxxxxx]
Sent: Tuesday, January 14, 2003 3:47 AM
To: Geoff Thompson
Subject: Re: [802.3af] Late comment
Geoff is right. There's nothing to stop someone from building a system with
a +/-48V supply, and wiring half the ports to -48 and half the ports to
+48. The fact that the supply is floating relative to earth ground doesn't
matter - there are at least two terminals that are 96V apart, and something
could short them together.
Technically, because we're isolated, it's completely arbitrary which end we
treat as "ground". Practically, there are arguments both ways:
- most non-telecom engineers (read: networking engineers) are most
comfortable with ground on the bottom, i.e., +48
- most telecom engineers are used to -48
- most engineers of both stripes like buying 100V NFets in preference to
PFets, suggesting switching the negative rail is best.
I'd vote we specify that the more positive rail (environment A only) is
"common to all ports" (don't call it "ground"), and the more negative rail
is switched. If we refer to that switched rail consistently as "isolated
-48V", it would be even more clear.
Now, what to call the rail that the logic on the isolated side runs from?
At 10:10 AM 1/13/2003 -0800, Geoff Thompson wrote:
>If you put a voltmeter between "hot" leads from 2 different PSEs,
>then is the max voltage between the leads (fault conditions included):
> VportMax - VportMin
> 2 X VportMax
>I contend that any answer above except [2 X VportMax] is OK
>We currently have no text to preclude [2 X VportMax].