RE: current limit on 48 VDC

["Rick Brooks" <ribrooks@xxxxxxxxxxxxxxxxxx>]

Title: RE: current limit on 48 VDC

Bob,
I have actually looked at those words, I guess I read than differently than you do.
Then again, what are you actually saying?

Unless we plan on measuring the temperature of the cable plant, 350 ma is the maximum that we must design to.
We must assume that there is a large bundle somewhere in the cable plant, not separate and happily cooled cables.

Is anyone suggesting that we measure the cable plant temperature?
If we could do that somehow, then we could be well above 350 ma at 25C, and derate above that temp.
I don't think this would be cost effective.

- Rick

-----Original Message-----
From:   RDLove [SMTP:rdlove@xxxxxxxxx]
Sent:   Thursday, August 10, 2000 4:21 AM
To:     stds-802-3-pwrviamdi
Subject:        Re: current limit on 48 VDC

Rick, I believe you are using the 350 mA as a worst case number when that was not the intention.  Let's look at the words from the SC25/WG3 Liaison report to us.

Referencing Document  ISO/IEC JTC 1/SC 25/WG 3 N 602, page 2, carefully read both the response and the rationale.
Question 2: Allowable loop dissipation (temp rise in worst case
bundles), maximum temperature of cabling behind
the wall?
Response:
The maximum current for categories 5 and higher and classes D and higher
should not exceed 0,175 per conductor (0,35 A per pair) for ambient temperatures
up to 60 °C for solid cables (45 °C for 0,4 mm equivalent stranded cables).
Analysis assumes that conductor diameters of less than 0,4 mm (or equivalent)
are not used.
Note - No change required to 11801.

Rational

Assuming an allowed 40 °C rise for PVC insulating materials and an ambient
temperature of 25 °C, the current capacity of 0,5 mm (24 AWG) and 0,4 mm (26 AWG)
conductors is 2,1 A and 1,3 A respectively. Using the assumption that the total current in
all conductors of multiple conductor cables may not exceed 20% of the sum of the
individual ratings of all conductors, the current capacity for is:
Solid cable: 2,1 A * 0,2 = 420 mA per conductor
Stranded cable: 1,3 A * 0,2 = 260 mA per conductor
A maximum current of 420 mA per solid conductor allows for 840 mA per pair based on
an ambient temperature of 25 °C. Derating by a factor of 53 % (using the chart shown in
IEC 60603-7 from 25 °C to 60 °C) allows for a steady state current of 395 mA per pair.
This current assumes that stranded cables are exposed to ambient temperatures of
less than 45 °C. Connector current capacity of 0,75 A @ 60 °C per conductor as
specified in IEC 60603-7 is not the limiting factor.

Best regards,
 
Robert D. Love
President, LAN Connect Consultants
7105 Leveret Circle
Raleigh, NC 27615
Phone: 919 848-6773
Fax: 720 222-0900
email: rdlove@xxxxxxxx <mailto:rdlove@xxxxxxxx>

----- Original Message -----
From: Rick Brooks <mailto:ribrooks@xxxxxxxxxxxxxxxxxx>
To: stds-802-3-pwrviamdi@xxxxxxxx <mailto:stds-802-3-pwrviamdi@xxxxxxxx>
Sent: Wednesday, August 09, 2000 9:01 PM
Subject: current limit on 48 VDC




Welcome back Roger.

Since we are putting in our share of opinions, here is mine.
If we assume that 350 ma per pair, 175 ma per pin, is the maximum continuous current that we are allowed to have.
Then for PTC protection alone, we would have select a PTC with a lower current value than 350 ma,
since these devices are not particularly sharp, just as they are not fast.
So, we would have to choose a PTC with (I'm guessing), say 250 ma limit at low ambient temperature.
This choice would drastically lower our guaranteed output power, especially for higher ambient temperatures.
We may only be able to get 5 watts to the PD.

Has anyone worked out an example with real numbers?

So, I think that we will need some form of electronic current limit on each port, so we can run close to the 350 ma limit.

If we use PTC's at all, it will be to prevent fires, in the case of when the electronic current limit fails.

thanks,
- Rick





we need more dialog...
anyone for discovery?   tennis?