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