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Re: [802.3af] Late comment

All -

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? 
-44.7V? :-)


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
>         VportMax - VportMin
>         Zero
>         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].