RE: floating power, be afraid...
Due to the inductive traits of the long wires my intuition tells me that the
NON ISOLATED DTE should designed with "NO USER ACCESSABLE METAL" on the
enclosure that the user could touch and get in contact with hazardous
levels.
Isolated power conversion would put a block in the path for directly passing
hazardous voltages to the user.
Bleeder resistors, on the non-isolated side to ground, would be good for
static buildup accumulation.
Bleeder resistors, on the isolated side to ground, would provide a path for
leakage current created by the switching action of the power supply coupling
through the transformer capacitance. A common problem in laptop computers.
EMP, solar flare and other voltage inducing phenomenon are impossible to
predict.
Sounds like a lot of guessing. The group still needs to get an agency(s)
participation. UL, CSA, TUV, and of course the Nordic countries are always
in their own field. This is imperative.
Ed Walker
Technical Staff
Analog Product Specialist
Texas Instruments Incorporated
HC66 Box 203
Mountainair, NM 87036
INTERNET: ed_walker@xxxxxx
WEB SITE: <http://www.ti.com>
Office = 505-847-0576
Fax = 413-280-0812
-----Original Message-----
From: Robert H Leonowich [mailto:leonowich@xxxxxxxxx]
Sent: Monday, March 05, 2001 2:38 PM
To: Rick Brooks
Cc: stds-802-3-pwrviamdi@xxxxxxxx
Subject: Re: floating power, be afraid...
Hi Rick,
I am in an awkward situation. Coming from the ex-phone company
I had access to "Propreitary" documents that I have seen
that discuss "anomolous" voltage generation in long
transmission wires. This info may be available in the public
domain, but I don't know where. I am not sure the voltages
are big enough to be concerned about, but they seem to originate
from solar activity. There was one incident reported where
a telegraph line ran for "several days" without the
application of power due to this mechanism.
I think longitudanal voltages are something to be considered.
Bob
> Rick Brooks wrote:
>
> 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