Re: Leakage current vs Hipot
For everyone's information, there is a IEEE standard for hipot. The
committee promulgating it is the Electronic Transformer Technical
Committee (ETTC) of the Power Electronics Society (formerly of the
"Walker, Ed" wrote:
> 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]
> 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 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]
> 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
> 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
> ed_walker@xxxxxx <mailto:ed_walker@xxxxxx>