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RE: floating power, be afraid...

See my comments bellow.

> -----Original Message-----
> From:	Rick Brooks [SMTP:ribrooks@xxxxxxxxxxxxxxxxxx]
> Sent:	ב, מרץ 05, 2001 10:54 PM
> To:	stds-802-3-pwrviamdi@xxxxxxxx
> Subject:	floating power, be afraid...
> This reflector has been very quiet, so maybe I can stir up some
> controversy? 
> A) We are allowing "Env A" PSE's in 802.3af 
> B) These "Env A" PSE's could have many ports which share the same large DC
> power source 
> therefore there could be many meters of cable that are all hooked together
> at DC. 
> Example: 200 ports, 100 meters each is 20 km, 656,000 ft, or 12.4 miles. 
> So, 20 km of cable that is all hooked together at DC, and each PSE port is
> delivering DTE power 
> Now, I plug in one more UTP cable and PD into this PSE system. 
> I'm thinking that there could be a very large transient as the new port
> turns on due to all the static 
> charge that could be on all those 20 km of cables. 
> Now, in reality, there is probably enough leakage to ground to bleed off
> the charge, but 
> can we be sure of this? 
> Proposals: 
> 1) make DTE power be tied to ground in the same way as telecom (48V) power
> is, and as most power 
> distribution systems in the world are. The connection to ground could only
> be at one point, of course. 
> This would require changing or at least interpreting the existing 802.3
> spec. 
	[Yair Darshan]  It could be good idea if the +48V will be considered
as system ground. 
	It also can help to achieve PSE power supply isolation requirements
and safety standards.
	It is done in Telecom world, However how the requirement of having
isolation between MDI to chassis will be met? 

> 2) Have each PSE port contain some leakage path, maybe around 10 Meg ohm, 
> it must be greater that 2 Meg which is the 802.3 spec. 
	[Yair Darshan]  It should. we are using 10MEG to chassis in similar

> 3) Make this concern that of the large PSE producer only and not of the
> 802.3af 
	[Yair Darshan]   PSE is a term that is defined in the 802.3af, thus
it should be specified under the same paragraph dealing with safety etc.  

> Does anyone out there share these concerns? Any other ideas? 
> - Rick