RE: floating power, be afraid...

["Rick Brooks" <ribrooks@xxxxxxxxxxxxxxxxxx>]

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

thanks Peter.

1) as far as a possible grounding rule goes, it would be the same as the rule when installing
AC power distribution. At the source of the isolated power, one side of the power is tied to chassis ground.
In AC power distribution, this is done at the output of the isolation transformer. The conductor
that is hooked at this point to chassis ground is then called the "neutral". Therefore, the neutral
carries load current, but chassis ground does not handle load current. This method keeps the wires from
going to high volatges, except during transients.
So for DTE power, the single grounded point would be right at the output of the floating power supply
or battery.

2) As far as the 10 meg leakage path goes, I don't think that it presents a safety problem.
Right now we have high voltage caps whose leakage may look like 10 meg at DC anyway.
Also, the load current does not go through that 39K, since it is to ground.

The advantage of choice 1 is that there are proven large power distribution systems that use it already.
Also, it guarantees that the wires do not charge up to high voltages.

- Rick

-----Original Message-----
From:   Schwartz, Peter [SMTP:Peter.Schwartz@xxxxxxxxxx]
Sent:   Monday, March 05, 2001 1:54 PM
To:     stds-802-3-pwrviamdi@xxxxxxxx
Subject:        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 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