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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
----- Original Message -----
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?
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