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