Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

RE: [802.3af] Late comment

10 meg resistor.  Very good point.

Ed Walker   
Analog Product Specialist 

Texas Instruments Incorporated 
HC66 Box 203 
Mountainair, NM 87036 

INTERNET: ed_walker@xxxxxx 
Office = 505-847-0576
Fax = 413-280-0812  

 -----Original Message-----
From: 	bachand@xxxxxxx [mailto:bachand@xxxxxxx] 
Sent:	Wednesday, January 15, 2003 3:16 PM
To:	Steven Murray
Cc:	Jack Andresen; cmjones@xxxxxxxxx; 'Geoff Thompson';
Mike_S_McCormack@xxxxxxxx; stds-802-3-pwrviamdi@xxxxxxxx
Subject:	Re: [802.3af] Late comment


Although it may make sense to tie the common to earth ground, the existing 
IEEE-802.3 standard has a nasty little requirement that it must be floating.

In fact it's further specified that the system should be able to have all 
conductors of all ports tied together, then hi-pot tested at 2250Vdc,
the wires and the chassis.

Therefore it was decided that this group will not make any changes to the 
existing 802.3 standard.

I would however suggest a high resistance (possibly 10M) or so, between the 
common and chassis, in order to drain static charge.


Gerard E. Bachand
Power Systems Engineer
7 Atwood Terrace
Cherry Valley, MA 01611
> "Jack Andresen" <jandresen@xxxxxxxxxx> wrote:
> > Any common point needs an impedance to ground.
> I would agree with this comment, and add:
> It is unusual practice to distribute a commoned
> conductor to a number of physically seperate 
> locations without providing an earth return, somewhere.
> If a fault should happen that causes a hazardous
> voltage to be applied to the commoned conductor,
> that hazardous voltage will then be present in a 
> number of physically separate locations.  This could
> create a hazard for a person handling a connector,
> or a hazard for the electronics in certain circumstances.
> If the common point were grounded, the hazardous voltage 
> would normlly only affect a smaller number of devices:
> Particularly, just the devices on the cable to
> which the hazardous voltage was applied.
> In the situation where a fault causes a hazardous voltage 
> to be applied to the commoned (but ungrounded) conductor
> for an extended period of time, and the electronics is 

> not affected, the fault is likely to remain undetected
> until such time as it causes a failure or a person comes
> into contact with a conductor somewhere in the system.
> A danger of electrocution might exist in this circumstance,
> the issue being that the fault may remain undiscovered until
> such time as it harms a person.  In point-to-point data wiring
> (eg a LAN Hub and a PC) such a fault condition would really
> only affect the two endpoints.  However with commoned
> (but ungrounded) POE wiring, the hazardous voltage could 
> well be present at many physically seperated locations.