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Re: [8023-POEP] Cabling and wire current capacities



Hi All,

 

 

I have red the last correspondence regarding safety aspects and would like clear few issues.

 

  1. IEEE802.3af as part of IEEE802.3 address two environment conditions that affects the isolation requirements of a PoE port and address the case of which part of the system segments (e.g. cables) are located outside the building or crosses buildings.

 

1.1   The requirements of these environments deal with the isolation voltage and the test requirements.

1.2    Regarding other aspects of the safety issue such as power limit, voltage limit and current limit it addressed in the IEEE802.3af spec:

      1.3.1  SELV – Voltage is limited to 60V max at operating voltage is limited to 57V.

      1.3.2  Limited power source: Limited to <100VA as per EN60950 definitions. We have indicated at the Vancouver meeting that the max value

                will be less or much less than this value.

1.3.3          The PoE port is equipped with current limit protection at 450mA in addition to secondary protection as per the limited power source requirements. The PoEp current at its worst case will be still less that 100V/44V, or 100V/51V etc or less that 1.3A per conductor per UL spec which is even lower.

 

  1. All other protection circuits needed to meet surge voltage or other immunity test voltages are implementation specific and it is practiced per vendor product definition which is out of scope for us.

 

 

  1. The bottom line: IEEE802.3af is design to meet safety requirements. PoEp will meet too these requirements as long as the topics above are met and maintained.

 

 

Yair

 

From: owner-stds-802-3-poep@xxxxxxxx [mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of Michael McCormack
Sent: Wednesday, February 02, 2005 12:26 AM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] Cabling and wire current capacities

 

 


I am on the hook for supplying test data on the cable temperature rise of cables in bundles caused by adding power.  I will not be testing what the cable rating should be.

Mike

Please respond to "IEEE 802.3 Power over Ethernet plus"<stds-802-3-poep@xxxxxxxx>

Sent by:        owner-stds-802-3-poep@xxxxxxxx  

To:        STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
cc:        
Subject:        Re: [8023-POEP] Cabling and wire current capacities


I think that there is a more rational approach here.  Rampant
conservatism is not the road to progress.  The point is to determine,
with reasonable judgement and consideration, the actual ampacity based
on factual information whose underlying assumptions are clearly
documented.



I know that the NEC (National Electrical Code) considers this wiring to
be Class 2 (Inherently limited < 100VA)  in article 725.  I cannot find
another NFPA (www.nfpa.org <http://www.nfpa.org/> ) standard that gives
more detail about a case like this.  Does anyone know of one?



I remember that someone was going to see if they could get actual test
data - and I am not sure if it was Alan Flatman or not.



Regards,

Martin



Martin Patoka

Systems Engineer

Texas Instruments

214-567-5487

mpatoka@xxxxxx



-----Original Message-----
From: owner-stds-802-3-poep@xxxxxxxx
[mailto:owner-stds-802-3-poep@xxxxxxxx] On Behalf Of Steve Robbins
Sent: Monday, January 31, 2005 9:08 PM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: Re: [8023-POEP] Cabling and wire current capacities



Guys,



This is just a suggestion, but instead of consulting NASA specs, I would
ask UL (Underwriters Lab) or some other group whose opinion will count
when it comes to fire safety.  (I spent many years working on the
electric power system for the international space station, and don't
have a lot of trust in specs written by those "rocket scientists" at
NASA.)



Here's what makes me a bit nervous about the ampacity discussions I've
been reading in these emails:  One of the great things about PoE is that
it's the first international power standard, but one of the scary things
is that it's the first power standard where most hook-ups will be done
by some Joe Blow IT guy instead of a licensed electrician.  There will
be people using cables that are not in the best of condition, hanging
them over sharp metal edges, hanging them near heat sources, tying them
into large bundles, doing bad crimp jobs, hooking them into patch panels
with loose screws, etc.  There will be plenty of instances where
something is getting a lot hotter than you expect.  All it might take is
a few small fires scattered around the planet for big insurance
companies to start putting clauses in their policies for corporate
customers prohibiting the use of PoE in office buildings.  That would be
the end of this technology.



I'm sure that SAE or MIL standards will be useful as guidelines.  But
when it comes to safefy in electronics, people look for the UL or CE
marks on products, not NASA, SAE or even the IEEE.  I don't know if UL
or CE have specs for wire ampacity, but I'd feel better if I knew they
were part of the discussion.



Anyway, that's my two cents.  Sorry if it seems alarmist.  I just think
we should be very conservative about ampacity.  More conservative than
organizations who might have effective veto power, such as big insurance
companies.



Steve Robbins

-----Original Message-----
From: owner-stds-802-3-poep@xxxxxxxx

[mailto:owner-stds-802-3-poep@xxxxxxxx]On Behalf Of Derek S. Koonce
Sent: Monday, January 31, 2005 5:20 PM
To: STDS-802-3-POEP@xxxxxxxxxxxxxxxxx
Subject: [8023-POEP] Cabling and wire current capacities


As I stated I would do, here is information I have on the
cabling and wire current capacities. I scanned in some documents and,
fortunately, found the complete NASA related document. I cannot attach
the files to this email because the server limits to 100K of
attachments. However, those that would like the files I will be glad to
forward upon request.



The key document is the MIL-W-5088K. This document has been
transferred to SAE control under standard AS50881. The SAE web site has
a cost of this document, non-member, of $59. Maybe someone on this list
has this copy or can obtain it to share with the list.



The NASA TM102179 document discusses the cabling design for
space payloads. I feel this would be the best starting point.


http://snebulos.mit.edu/projects/reference/International-Space-Station/T
M102179.pdf



Surprenant is a cabling company that has some data on cabling
derating. But their ampacity chart starts at 18 AWG and goes up from
there.



A fusing current document was passed to me, years ago, from a
line of engineers and is more of an eye-opener. The fusing current for
26 AWG wire is 20.5 A. Quite a bit to look at, but clearly unreasonable.



Looking at the NASA document (referenced above) and running the
numbers for 100% wire usage, 50 F temp rise we would get a maximum
current loading of 3.588 A per wire. (A 4.46V drop based on 2-pair
conduction for 100 m)



This is very large and would put a maximum limit on the power.
This does not take into account for voltage drop along the cable.



A bit more searching brought up the following website

http://www.powerstream.com/Wire_Size.htm

This states that 26 AWG wire can handle up to 2.2 A for chassis
wiring and 0.361 A for power wiring. These currents relate to a voltage
drop of 2.74 V and 0.45 V for 2-pair conduction for 100 m.



If we look at any baseline for current in the wire. I would say
the 0.361 would be a good start. Then add on upping the voltage to 55 V,
we can see about 39 W on the PD side.



Derek Koonce

Architect, Standard Product Group

JSI Microelectronics

4235 Forcum Ave., Ste. 500

McClellan, CA 95652

916-648-2089 x114



 

P