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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?
From: Walker, Ed [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.
Analog Product Specialist
Texas Instruments Inc.
HC66 Box 203
Mountainair NM 87036