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RE: Leakage Currents; Special Equipment Environments


Thank you for the clarification as to which end causes the concern.

Your comment "currents could be passed thru a person if metal contact on the
DTE allowed it..." leaves me with only one question further: could this not
be made the responsibility of the equipment manufacturer?  Or are you taking
the worst-case scenario, where for instance a conventional type VoIP phone
ends up in a hospital environment, without having been specifically selected
as appropriate for that environment?  I wonder if there are legal issues
involved here too, unless the manufacturers and the users of the DTE must
take the responsibility for putting the right equipment into the right

No contentiousness is intended whatsoever.  I find this an interesting
issue, but my company has no vested interest, so I merely curious to hear
your opinions, and those of others.


Peter Schwartz
Applications Engineer
Micrel Semiconductor
Phone:	408.435.2460
FAX:	408.474.1210

	-----Original Message-----
	From:	Walker, Ed [SMTP:ed_walker@xxxxxx]
	Sent:	Thursday, September 21, 2000 13:45
	To:	'Schwartz, Peter'
	Subject:	RE: Leakage Currents; Special Equipment Environments

	It is not the load end that is the concern. It is the source. If you
suppose that a DTE is powered on the LAN and is floating (not grounded) then
currents could be passed thru a person if metal contact on the DTE allowed

	All we need to do is find out from the safety agencies what is
allowed and put it in the 802 standard. 

	Medical grade supplies are expensive. We don't have to go that far. 


	-----Original Message----- 
	From:   Schwartz, Peter [ mailto:Peter.Schwartz@xxxxxxxxxx
<mailto:Peter.Schwartz@xxxxxxxxxx> ] 
	Sent:   Thursday, September 21, 2000 11:19 AM 
	To:     'IEEE 802.3 'DTE Power'' 
	Subject:        Leakage Currents; Special Equipment Environments 


	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
	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
	simple and relatively inexpensive.  Again, this can be done on a
	basis, and addresses the concerns of many different "critical"

	Or maybe there are just some environments for which the Powered
	concept just is not suitable without extensive equipment
modifications at 
	one or both ends of the link: I am thinking now of explosive
	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
	have done for years - enumerate the special environments, and
	special equipment for those environments only. 


	Peter Schwartz 
	Applications Engineer 
	Micrel Semiconductor 
	Phone:  408.435.2460 
	FAX:    408.474.1210 
	peter.schwartz@xxxxxxxxxx < mailto:peter.schwartz@xxxxxxxxxx
<mailto:peter.schwartz@xxxxxxxxxx> > 

	        -----Original Message----- 
	        From:   Walker, Ed [SMTP:ed_walker@xxxxxx] 
	        Sent:   Thursday, September 21, 2000 8:11 
	        To:     'Rick Brooks'; 'IEEE 802.3 'DTE Power'' 
	        Subject:        Leakage current vs Hipot 

	        The hipot test is for basic high voltage breakdown within
	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
	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
	secondary.  Xc=1/2piFC in the transformer couples over the high
	switching waveform of the primary front end and the current needs a
	path. Operating leakage current is tested only once during the
	safety testing for the agencies. 
	        They are 2 different things. 


	        -----Original Message----- 
	        From: Rick Brooks [ mailto:ribrooks@xxxxxxxxxxxxxxxxxx
<mailto:ribrooks@xxxxxxxxxxxxxxxxxx> ] 
	<mailto:[ mailto:ribrooks@xxxxxxxxxxxxxxxxxx
<mailto:ribrooks@xxxxxxxxxxxxxxxxxx> ]> 
	        Sent: Wednesday, September 20, 2000 7:11 PM 
	        To: Walker, Ed 
	        Cc: IEEE 802.3 'DTE Power' 
	        Subject: RE: Leakage current 

	        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
	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
	medical, it would be safer, but could add a lot of cost. 

	        I believe that for non medical applications the SELV levels
	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
	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] 
	        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
	something like 5 or 10uA from live wire to earth ground.  Typical
	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
	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
	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

	        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
	to the AC input via the ground return or it will present a shock

	        The 802.3 must get a UL/TUV/EN safety member on the team to
	these issues. 

	                        Ed Walker 
	                Technical Staff 
	                Analog Product Specialist 
	                Texas Instruments Inc. 

	                        HC66 Box 203 
	                Mountainair NM 87036 

	ed_walker@xxxxxx < mailto:ed_walker@xxxxxx <mailto:ed_walker@xxxxxx>
>  < mailto:ed_walker@xxxxxx <mailto:ed_walker@xxxxxx>  
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