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Re: [8023-POEP] 2P v 4P and safety



Steve-

I finally got a few minutes to take a look at your note. My comments are mixed in below.


At 09:50 AM 5/23/2005 , Steve Robbins wrote:
PoE Plus Team,
 
Here are some thought I had over the weekend regarding 2P vs. 4P, and safety.  I checked with Mike, and he wrote that it's okay to post this on the reflector and get a converstation going.
 
1.  There are basically two roadblocks to higher power:
    a.  The limited amount of current that can be carried on a single wire (not a pair of wires).
         We must assume that sometimes a wire will break, or a connector pin will go bad,
         so all the current will be on one wire instead of a pair.  This fault condition must define
         the limit on how high I_CUT can go, for safety.
    b.  The affects of heating and current imbalance on the magnetics.

This is a very good point, especially for current standard mid-span PSEs and PDs that don't actually use Ethernet. We might be able to get some relief in other situations because the data function would tend to be compromised also. To depend on that though we would have to take a careful look. I have heard of 10BASE-T working at short reaches with one lead open.

 
2.  To make sure we get it right in terms of safety, I propose:
    a.  The objective regarding the goal of higher power should be amended to include
         a few words about safety.

I don't disagree but...
"safety" is a very touchy word with the IEEE and with its lawyers.
(I'm not an expert here but I know who to ask)
Also there is the difference between human safety and equipment safety.

    b.  Let's look for a worst-case scenario to keep in mind while we work on the standard.
         The best one I've thought of so far is the airline industry.  Assume PoE is used onboard
         a jet so that passengers can surf the net or watch DVDs in flight, without the batteries
         in their laptops dying.  How hot do you think the FAA will allow the wires to get?  How
         much current can a single conductor (in a bundle) carry before it reaches that temp?

Probably not actually the worst case. Airplanes are a pretty carefully controlled construction environment. We could get heavier gauge wire in planes. On the other hand, planes want to save weight. We might have a problem getting more than 2 pair out of them.

 
3.  I thought about Clays presentation a lot over the weekend.  It seems to me that reaching
     higher power (>30W) will require two things:
    a.  Current imbalance sensing to detect broken wires, so we can increase I_CUT beyond
         what a single conductor can handle, without compromising safety.
    b.  Active current balancing to keep the magnetics happy.  Although this won't solve the
         heating problem.
 
4.  Maybe it's time for an acronym here.  How about Active Current Balance (ACB) and Current Imbalance
     Sensing (CIS).  They seem to go together, so "ACB/CIS"?
 
5.  Compare the complexity of a 2P and a 4P system that carry equal power.  The 2P system
     would need about twice the current per conductor as the 4P system.  So, while the current on
     4P system might just be low enough to avoid the need for ACB/CIS, the 2P system would almost
     certainly need these features.  So the complexity difference between 2P and 4P may not be as
     big as previously thought, at least in some medium power range.  For much higher power, 4P
     would require ACB/CIS as well, but 2P becomes infeasable because of safety.
 
6.  Suppose PoE Plus allows both 2P and 4P.  How do we define this without getting into several
     different types of PSE and PD, with all the complexity of hooking them together?  I think the
     answer is simple:  We define a 2P system, and then say that it's legal to have one or two of these
     on the same cable.  So, two independant 2P systems make a 4P system.  I think this is really the
     only way to go, but of course I'm open to other ideas.
 
 
Steve Robbins