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



Hi Steve and all,

 

I basically agree with you regarding the situation you have described.

(One wire broken==>Current is doubled ==> temperature rise increased ==> cable and/or connector parameters may be impaired==>

 we need to sense this situation and protect the infrastructure).

 

Actually we are protecting the infrastructure in the IEEE802.3af by limiting the current, time and duty cycle. However, we are not sensing a broken wire in order to protect from doubling the current from the simple reason that it is not needed IF the time duration peak current and duty cycle of the event kept low enough so increase in temperature rise will not happen. The numbers of the IEEE802.3af were chosen in order to ensure this behavior (there are additional reasons for the numbers that we have decided in the IEEE802.3af and the end result was integration of all argument to get reliable operation while keeping the cost of silicon down)

 

I guess that we should aim to the same concept in our new project i.e. set the numbers that in case of failure keep the infrastructure parameters in their operation range without additional sensing and features in order to keep the system simple and low cost.

Probably we will need to revisit the protection mechanism by optionally adding a sense function that will alert of broken wire due higher current in 2P concept compared to the IEEE802.3; however we need test results in order to decide the way to go and some of us working on getting lab data.

 

 

Other comments:

 

1. PD has to have IEEE defined components (PHY or others) otherwise it is not a compliant PD. PD is a powered DTE. DTE is a device defined by IEEE802.3. Hence PD must meet IEEE802.3. We don’t care about other implementations that are using PoE technology and are not IEEE802.3 compatible.

 

2. You are right that a link is not required at the first place; actually according to the spec PSE operations are independent of data link status (33.2.4) however the spec allows wide flexibility for specify implementation specific errors under the variable "error_condition" which will results by power shut off.

In other words if A PSE vendor/ System designer decides that the PSE sees something that is not normal (he decides what is normal or not; he is the vendor and he should know.) and it is not specifically covered by the spec, (there are many scenarios that meets that definition) it can be considered "error_condition" as specified by the state diagram.

 

According to the above, if during operation system reports to PSE that "something bad has happen" (by getting information that data was good and now is dead and current is higher than normal for example) PSE is encouraged to do something about it.

 

3. I agree with the statement that true worst-case conditions in a 2P system must assume one wire is broken. In 4P, if we stay around the AF currents that we don’t have to take in consideration that we have problem if one wire was broken due to the fact that this condition was tested and we don’t have problems with it. If in 4P we will have higher current than 350mA than we have to test again.

 

Yair

 


From: owner-stds-802-3-poep@xxxxxxxx [mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of Steve Robbins
Sent: Wednesday, June 01, 2005 4:12 AM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] 2P v 4P and safety

 

 

Yair and all,

 

Perhaps I could make this a bit clearer.  Here is a hypothetical situation:  Suppose we determine that CAT5 cable can carry 800mA per twisted-pair (400mA per conductor), and under worst-case conditions (max amb temp, big bundle, etc.) the wire temp stays 10C below the point where the cheapest insulation starts to soften.  So we set I_CUT to 810mA, and it seems safe.

 

Now suppose that we have actual worst-case conditions somewhere, but then one wire breaks.  The whole 800mA is flowing through one wire, not two.  The wire resistance is doubled, so power dissipation in the twisted-pair doubles, and the temp rise increases.  (I'm not sure that the temp rise would double, since the power dissipation in the other pairs hasn't increased.  But obviously the wire carrying 800mA will get hotter, probably significantly hotter.)  If it goes up 11C, it exceeds the softening point.

 

True, the Ethernet data would be messed up, but who cares?  A PD doesn't even need to have a phy chip.  Loss of link is not a reason for the PSE to shut off power, at least under 802.3af, because link is not required in the first place.  In the above example the load current stays below I_CUT, so power stays on and the wire temp stays above the softening point until eventually something bad happens.

 

The point is, true worst-case conditions in a 2P system must assume one wire is broken.  In a 4P system, I think we'd have to assume one wire was broken in each of the pairs.  (Don't assume broken wires are statistically independant events.  If a wire breaks there is a good reason, and other adjacent wires could very well be broken too.  Once, when I was at Boeing I had a whole bundle of wires come loose in my hand when I gave it a small tug.  They all broke off right at the connector pins.  Yet every individual connector contact in that bundle had survived a much stronger pull test right after crimping.  A lengthy failure investigation determined someone had used an improperly calibrated crimp tool, which made the wire brittle and eventually led to fatigue.  After two years in the field, some were actually broken but still just barely touching, which led to intermittent equipment failures.)

 

 

Steve

-----Original Message-----
From: Yair Darshan [mailto:YairD@xxxxxxxxxxxxxx]
Sent: Tuesday, May 31, 2005 1:03 AM
To: Steve Robbins; STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: RE: [8023-POEP] 2P v 4P and safety

Hi Steve and all,

 

Please find attached below few comments on the thoughts presented below:

 

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.

----------------

 

1a. If a wire or a pin will break the data path will not work due to the fact that the magnetic will immediately saturate.(Lets assume that at the worst case all pairs are data pairs such in 1G)

There fore assuming the above condition may not be considered as a worst case that we have to handle since it is a case that the system is not working and we don't care what happens as long as we are not causing damage.

 

Regarding causing damage:  

If one conductor is break, we will have twice the current going through the other conductor and if we keep the TLIM/TOVLD and duty cycle than even for 2-4 times the current overload/Ilim numbers we probably will not have damage issue due to the averaging factor over time.

Connector pin according to its spec need to meet 750mA at 60C. With cables it is a temperature rise issue which is not an issue for the overload/ILIM timings and duty cycle.

 

Regarding safety issue: There is no safety issue here in terms of human safety. There is an issue of "Infrastructure safety or equipment safety" in terms of damage however as mentioned above, with TLIM/TOVLD and duty cycle limitation the equipment is kept un damaged and "safe".

 

1b. Yes, this is one of the major issues in 2P.

  

--------------

 

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.

    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?

 

-------

 

2a - We don't have a mandate to define safety (human safety) in our standard. We need to address

        the reader to current safety standard.

        We should address issues that have impact on keeping the equipment and infrastructure undamaged.

2b - I agree that we should look for the worst case HOWEVER the worst case scenarios should be real cases and not corner or theoretical cases in order not to increase the cost of solution or eliminating some implementation or support some applications.

Regarding the example presented: I guess that it is not the worst case since in aircraft designs there are a lot of margin in the design (Aircraft...) so in my opinion it is not representing the worst case.

--------

 

 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.

 

------

 

3a. The intent is not clear. Do you mean current balance in 4P?  If this is the case than Yes, we need to address this issue and your suggestions sounds reasonable.

3b- Active and passive current balancing to the magnetic was discussed in the AF and we didn't want to force this requirements in the standard due to the following reasons:

-          The vendor should keep his system working under his equipment spec. How he does it is implementation specific.

-          Forcing a requirement for active/passive current balancing will increase the port cost while in most cases it is not required (above some cabling length there is no issue)

Actually we address this issue in the informative part of the standard and let the designer to decide how to handle the worst case situation.

 

In our new project doing active current balancing will dramatically increase the port cost so the the 2P concept will cost much higher that the 4P... In addition as you have mentioned, the heat issue still exist.

 

Bottom line: In my opinion: Current balancing is not cost effective

--------

 

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 infeasible because of safety.

 

---

 

5. Agree. From complexity wise, the is no difference between 2P and 4P high power and in my opinion 4P from system point of view will cost less than 2P and will be more reliable by far.

   (Regarding safety: again there is no safety issue here (the system may not work... but no safety issue))

 

----

 

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 independent 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.

-----

 

6. Basically I agree with this concept which is what the 4P concept (part of it) represents: Let's take two improved AF power channels, combined them together to have twice or more the power by having the 4P..

The problem that I have with part of your suggestion is that it will allow at the PSE side, PSE that supports only 2P high power and PSE's that support only 4P which leads to the undesired environments were we have many different PSE's and PD's that will cause us complex system.

 

I suggest looking at the 4P as the only HP concept that will address all PD types.

By using this approach, the 4P design will be efficient, and simple.

And when it will be integrated, the difference in chip level between 4P and 2P will be small enough to stay cost effective.

From system point of view (cost, reliability, flexibility etc) 4P will represents better cost numbers than 2P.

 

 

Yair


From: owner-stds-802-3-poep@xxxxxxxx [mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of Steve Robbins
Sent: Monday, May 23, 2005 7:50 PM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: [8023-POEP] 2P v 4P and safety

 

 

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.

 

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.

    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?

 

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