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RE: floating power, be afraid...




Rick:

The concern seems reasonable.  My 2c worth:

I'd like to see a single-point ground [as you outline in (1)], but I doubt
that will happen.  Even if it did/could happen, there is no clear way of
saying where in a building all the various switches, hubs, routers, etc.
might be - so it might be difficult (pardon me if I am being na´ve) to
specify just where that single point is.

Therefore, I am given to favor your second plan, wherein each PSE port gas a
small bleeder path to its local ground.

Some quick math:  (10 Meg / 256 ports) = 39k from PSE ground to earth
ground.  Does this introduce any safety hazard(s)?

Peter Schwartz
Applications Engineer
Micrel Semiconductor
Phone:	408.435.2460
FAX:	408.456.0490
peter.schwartz@xxxxxxxxxx

	-----Original Message-----
	From:	Rick Brooks [SMTP:ribrooks@xxxxxxxxxxxxxxxxxx]
	Sent:	Monday, March 05, 2001 12:54
	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. 

	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. 

	3) Make this concern that of the large PSE producer only and not of
the 802.3af 


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