Leakage Currents; Special Equipment Environments
Ed:
The point which you raise is valid, and deserves addressing. But, as far as
the PSE goes, does this necessarily need to affect the cost and design of
every powered port? I think the matter might be better addressed by having
the PD use a medical-grade DC/DC converter at the load end, if leakage will
be of such vital concern.
If that still does not satisfy, then perhaps a separate class of
"Medical-Grade" PSE equipment could be designated at a later time, for those
markets which need it.
One could I suppose also start looking at GFI-type equipment, which is
simple and relatively inexpensive. Again, this can be done on a per-port
basis, and addresses the concerns of many different "critical" environments.
Or maybe there are just some environments for which the Powered Device
concept just is not suitable without extensive equipment modifications at
one or both ends of the link: I am thinking now of explosive environments.
Imagine the possible arc which occurs by pulling a live 48V, 300mA
connection apart at the end of 100M of cable. In this case the RJ-45 may
simply be an inadequate connector.
Summary: Special environments certainly do deserve consideration. But part
of addressing the needs of those environments while allowing the
"mainstream" environments to proceed in an economically viable manner may
well be to do what the power supply industry, and the regulatory agencies,
have done for years - enumerate the special environments, and require
special equipment for those environments only.
Respectfully,
Peter Schwartz
Applications Engineer
Micrel Semiconductor
Phone: 408.435.2460
FAX: 408.474.1210
peter.schwartz@xxxxxxxxxx <mailto:peter.schwartz@xxxxxxxxxx>
-----Original Message-----
From: Walker, Ed [SMTP:ed_walker@xxxxxx]
<mailto:[SMTP:ed_walker@xxxxxx]>
Sent: Thursday, September 21, 2000 8:11
To: 'Rick Brooks'; 'IEEE 802.3 'DTE Power''
Subject: Leakage current vs Hipot
Rick,
The hipot test is for basic high voltage breakdown within the
isolation transfomer and other isolation paths. This test is supposed to be
done with the equipment OFF. This is how we did it for 10 years at Compaq.
HiPot is performed with a high DC voltage. HiPot is done on every piece of
product in the production phase.
The leakage current I speak of is tested with the equipment ON and
is caused by capacitance between the AC primary front end and the isolated
secondary. Xc=1/2piFC in the transformer couples over the high voltage
switching waveform of the primary front end and the current needs a return
path. Operating leakage current is tested only once during the product
safety testing for the agencies.
They are 2 different things.
THIS IS WHY THE COMMITTEE NEEDS TO HAVE A SAFETY MEMBER ON BOARD.
Ed
-----Original Message-----
From: Rick Brooks [mailto:ribrooks@xxxxxxxxxxxxxxxxxx]
<mailto:[mailto:ribrooks@xxxxxxxxxxxxxxxxxx]>
Sent: Wednesday, September 20, 2000 7:11 PM
To: Walker, Ed
Cc: IEEE 802.3 'DTE Power'
Subject: RE: Leakage current
Ed,
I think that the issue you bring up is covered to some extent by the
Hipot test that every 802.3 compliant port must pass.
The maximum leakage current that is acceptable in this test is
higher than the numbers that you quote for medical.
Still it does guarantee that the DTE load current will return on the
data cable.
The main power supply driving all the powered ports, if it is say an
AC to DC type, must meet a different Hipot test
at the AC input.
I think that we should not form this DTE power spec to accommodate
medical, it would be safer, but could add a lot of cost.
I believe that for non medical applications the SELV levels are
sufficient. Isn't our situation exactly where SELV applies?
One worry that I have is that I've seen a few brand x,y,z 10/100
boxes that do not pass Hipot, but still work fine
from the users point of view.
Will DTE powered boxes that do not pass Hipot exist on the market in
the future, and if so, will there be new safety concerns,
and user complaints?
Could be.
- Rick
-----Original Message-----
From: Walker, Ed [SMTP:ed_walker@xxxxxx]
<mailto:[SMTP:ed_walker@xxxxxx]>
Sent: Monday, September 18, 2000 5:18 PM
To: IEEE 802.3 'DTE Power'
Subject: Leakage current
I just thought of something else to consider. Leakage current specs
are very tight in many environments such as medical. I believe it is
something like 5 or 10uA from live wire to earth ground. Typical computer
type specs are 5 mA. It does not take much current through the human body
to stop a heart. That is what this is all about. You can feel a couple of mA
of 60Hz current like a "tingle".
The current is caused by capacitance in the winding of the isolation
transformer in the power supply. There is also some current associated with
the bleeder resistor on the front end of an AC input supply-we wont worry
about that right now. If the power source for 802.3 is coming from the AC
line voltage you must put something in the 802.3 spec to address the leakage
current.
All current from the source should go down the wire/pair to the DTE
and be returned to the source down the other wire/pair. It must not return
to the AC input via the ground return or it will present a shock hazard.
The 802.3 must get a UL/TUV/EN safety member on the team to evaluate
these issues.
Ed Walker
Technical Staff
Analog Product Specialist
Texas Instruments Inc.
HC66 Box 203
Mountainair NM 87036
505-847-0576
ed_walker@xxxxxx <mailto:ed_walker@xxxxxx> < mailto:ed_walker@xxxxxx
<mailto:ed_walker@xxxxxx> >