RE: floating power, be afraid...

["Rick Brooks" <ribrooks@xxxxxxxxxxxxxxxxxx>]

Title: RE: floating power, be afraid...

What Roger says is true.
However, I don't think that is really a safety problem.
This setup would mean that you are testing 512 ports at the same time,
since all 512 RJ-45 ports would be getting the high voltage from the Hipot tester at the same time.

I think that the "greater than 2 meg at 500V", at the end of the Hipot test applies to a single MDI.

- Rick

-----Original Message-----
From:   R karam [SMTP:rkaram@xxxxxxxxx]
Sent:   Monday, March 05, 2001 2:34 PM
To:     Schwartz, Peter; stds-802-3-pwrviamdi@xxxxxxxx
Subject:        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 naïve) 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
>