[8023-POEP] Comments on TR42.7 report regarding Current Carrying Capacity of Cabling

[Yair Darshan <YairD@xxxxxxxxxxxxxx>]

Title: Comments on TR42.7 report regarding Current Carrying Capacity of Cabling

Hi Val and Shadi, 

Thanks for the report.

Please see below my comments and advise for clarifying it.

Thanks

Yair

Darshan Yair
Chief  Engineer
PowerDsine Ltd.  -  The Power over Ethernet Pioneers

Tel:  +972-9-775-5100, Cell: +972-54-4893019
Fax: +972-9-775-5111

E-mail: <mailto:yaird@powerdsine.com>.  
<http://www.powerdsine.com>

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Slide 3, Temperature Rise Data: What is the "m" and "b" in the firs two rows in the table.

Slide 4, Temperature Rise Graph: Can you supply the value of "m" and "b" of the Fitting Function of the graph?

Slide 5, Recommendations for Max Current Carrying Capacity, 3^rd bullet: 720mA at PSE voltage of 50V minimum supplies 50V*0.72A – 12.5*0.72A^2=29.52W=~30W for 2P. Since the working assumption is that the max current value per wire is when all wires are conducting then the power over all 4 pairs is 60W and not 50W. Hence the total power over 100 cables in a bundle is 6000W and not 6000W. Please clarify the differences between this calculation to the 5000W result in this slide.

See also Slide 6 4^th bullet in which you say 30W/60W over 2P/4P which is the correct numbers for 0.72A and 50V at PSE.

Slide 6, Notes, 1^st bullet: Why now the max temperature rise is 12 degrees C and not 10 degree C as discussed earlier? Is the reason was to get 30W over 2P at 45 degree C?

Slide 6, Notes, 3rd bullet: "The 5000W (TBD) is an estimate for 10 degree C temperature rise in 100 cable bundle". This statement is not clear due to the following:

a) You already said in 1^st bullet that max temperature rise is 12 degree C.

    If the temperature rise is 10 degree C max then the max current is less then 720mA at 45degree C.

b)  It should be 6000Watts under the conditions you have specified. See also 4^th bullet in which you say 30W/60W over 2P/4P which is the correct numbers for 0.72A and 50V at PSE.

Slide 6, Notes, 4^th bullet: at 720mA which is the max current, the minimum voltage at the load is 50V – 0.72A*12.5 Ohm =41V and not 51V.

If you meant that this is the voltage at the PSE side then the voltage is 50V min and not 51V.