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

RE: floating power, be afraid...




Hi Peter

the 39k causes a problem for the high pot test,
when 2000v low freq is applied for 1 sec...
roger


At 01:53 PM 3/5/01 -0800, Schwartz, Peter wrote:
>
>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 
>