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All current limit devices, including fuses and PTC can be characterized as having a defined current limit, as well as a current trip threshold (due to propagation delay).
You can instantaneously pull 30A through a .25A fuse or polyfuse due to their response time (seconds), or even through a current limited MOSFET due to is propagation delay (microseconds), the later being the shortest time period.
When defining your current limit you need to consider more than simply defining the value, you should consider how you wish to act on the fault (Shut down, poll for fault clearing, etc) as it impacts system operation. As well as consider what types of faults you wish to respond to (more than XmA for longer than Yms, etc).
Defining these things still allows for a variety of implementation choices, however they will give the overall system a bit more reliability.
From: R karam [mailto:rkaram@xxxxxxxxx]
Sent: Thursday, August 10, 2000 11:05 AM
To: Rick Brooks; stds-802-3-pwrviamdi@xxxxxxxx
Subject: More details...
I would have to revisit the PTC specs at this point, but on to the "active"
current limit, we will find that a hand full of analog houses have the process
and the expertise to deliver parts that can take the transients on and survive.
the good news is these parts do exist.
Since I am new to the standards, and the purpose I guess, is for us is to spec
a current limit in this case, we would have to guarantee ahead that an implementation
exist, yet not force any implementation. at this point it will be crowded around the
RJ45 to do the job properly. Until the volumes get higher and better alternatives are
Are we set on meeting the 2000v high potential spec for 802.3 when the connection is
live? (POWER IS ON) I heard that PBX's do not meet high-pot requirements- Arlan?
and may be someone from LUCENT can comment?
and if the active current limit is set to 350ma when the connection is live how does that factor into the high pot
breakdown?. I think we have been staying away from the isolation requirements, but if we
are talking current limits, then reality now sets in and we must face the details.
Come to think of it, how would the 2000v High pot be tested when the connection is live?
Currently we test high-pot with the board off NO POWER, and thank's to the magnetic the isolation
across it is NOT in question.