Lou,
It's my impression that the typical switching power supply, such as
that used for ITE type equipment, will apply basic insulation between
the primary circuits and earthed conductive parts on the primary side.
They do this in order to minimize the creepage/clearance requirements
and subsequently reduce the size of the supply. I'm referring specifically
to Tables 2G and 2F of 60950.
Thanks,
Carl
From: "Lou Aiken" <ai...@gulftel.com> on 02/03/2003 01:46 PM
To: Carl Newton/XYCOM@XYCOM, emc-p...@majordomo.ieee.org
cc:
Subject: Re: EN60950 protective conductor test (was Re: Circuit Breaker
Tripping Dring Fault Tests)
Carl, If the primary supply circuitry and components provide double or
reinforced insulation, nothing can become live in the event of a single
fault, the test becomes unnecessary, and I would argue that fact.
If the design does not provide double or reinforced insulation, the test
sounds applicable from points that could become live in case of a basic
insulation fault.
Regards,
Lou Aiken, LaMer LLC
27109 Palmetto Drive
Orange Beach, AL
36561 USA
tel ++ 1 251 981 6786
fax ++ 1 251 981 3054
Cell ++ 1 251 979 4648
From: <cnew...@xycom.com>
To: <emc-p...@majordomo.ieee.org>
Sent: Monday, February 03, 2003 11:19 AM
Subject: RE: EN60950 protective conductor test (was Re: Circuit Breaker
Tripping Dring Fault Tests)
>
>
>
> A slight divergence from the EN specifically, but I thought that the
> following would be helpful to this thread:
>
> I am presently working this issue with a UL engineer in accordance with
> UL 60950, 3rd Edition. I also have the UL 60950 3rd Ed. Test Data
Sheets.
> Their "Protective Earthing Trace Earth Fault Current Test", UL Doc.
190.eng,
> per Section 2.6.3.3 requires the following in my case:
>
> 1. Three samples are tested;
>
> 2. Trace resistance is measured before and after test. Resistance cannot
> exceed 0.1 ohms, and cannot change more than 10% after test;
>
> 3. AC source is 240 Vac, 200 amps (20A circuit breaker X 10), power
factor
> is 75 - 80% through shorted bus bars with a 20/30 A (20 in my case)
> service
> entrance type circuit breaker in series with the testing terminals.
The
> circuit
> breaker is connected to the bus bars by 1.22 m (4 ft.) of #12 AWG
wire.
>
> 4. The test circuit is connected to the DUT via the grounding lead of the
> 1.82 m (6 ft) power supply cord. If cord is not provided, then #16
AWG
> wire is used.
>
> 5. Test continues until ultimate results occur; e.g. CB trips, trace
opens,
> etc.
>
> My UL guy tells me that I should expect the typical service type CB to be
rated
> up to + 10%. So it appears that I need to concern myself with a burst
of
> current
> up to approximately 22 amps for the 20 amp AC circuit that my product is
being
> evaluated for.
>
> Carl
>
>
>
>
>
> From: "Chris Maxwell" <chris.maxw...@nettest.com> on 02/03/2003 09:29 AM
>
> Please respond to "Chris Maxwell" <chris.maxw...@nettest.com>
>
> To: emc-p...@majordomo.ieee.org
> cc: (bcc: Carl Newton/XYCOM)
>
> Subject: RE: EN60950 protective conductor test (was Re: Circuit Breaker
> Tripping Dring Fault Tests)
>
>
>
>
> This thread has been interesting. I am, at this moment, considering a
design
> where I am almost forced to use a PC (printed circuit) trace for Earth
ground.
>
> It seems funny to me that most equipment has been historically made with
18AWG
> protective ground pigtail wires; and 25A ground fault tests have been used
for
> years.
>
> Now that PC traces are being used for protective ground; we want to test
with
> 200A or greater impulse currents? I'm curious about what would happen to
your
> typical 18AWG line cord during this test. I'm wondering if the line cord
would
> fuse open?
>
> There are a couple of handy charts on the web.
>
> One is at www.kepcopower.com/nomovax2.htm this is a nomograph of maximum
> operating current, AWG and IR drop in the conductor. The point "A" is
generally
> considered the point of maximum IR drop. If you draw a line from point
"A",
> through a wire gauge size; you'll get a max current. Of course this is
steady
> state current; and the nomograph assumes a single wire. Wire bundles
would be a
> worse case. It's too bad that this chart doesn't contain the "fuse"
values for
> the wires as well (the I squared * T values).
>
> Another is at www.circuitboards.com/capacity.php3. This is a chart of
max
> current for PC traces. Remember that this is for TRACES and planes only;
it
> doesn't say anything about vias and other potential problems.
>
> At first pass, it seems that a trace size to handle twice the power cord's
max
> current, (from the nomograph) with a 10degC trace temperature rise (from
the PC
> trace chart), would be a good rule of thumb for the trace size. If I have
room,
> I'll just make it bigger. Once we pay for the PC board fabrication, the
copper
> is free!
>
> Even with an adequately sized trace; I can think of a few potential
problems
> with the trace to chassis connection:
>
> 1. Many layout people open up PC traces or planes around vias so that
only four
> little 20 mil wide bridges carry the current to the via. This is great
for
> soldering heat relief; but BAD for current carrying capacity. These
little
> bridges can fuse open in high current conditions. I am considering
solving this
> by not putting any thermal reliefs around your Earth ground vias and using
> multiple vias.
>
> 2. Another problem with these traces is using plated through vias with
screws
> through them. It has been found that plated through vias can crack when
they
> are put under pressure from screws. Some power supply manufacturers
solve
> this by bringing the Earth ground trace to the surface with vias near the
> chassis connection point; then route this to a solid plated pad on the
surface
> layer for chassis connection. I am considering this same solution as
well.
>
> 3. The third problem is mechanical. Once Earth ground brought to a pad
on the
> circuitboard; then there is still the issue of getting a good mechanical
mate to
> the chassis with a wide surface area. If the connection is made through a
> couple of teeth on a star washer; then there is a potential for localized
> heating. I'm just going to maximize surface contact area for this one.
I'm
> also considering using multiple board to chassis connection locations.
Every
> screw that connects the board to chassis is a potential Earth ground
connection.
>
> The last "fuse" in any power system is the cord connected to the product.
It
> seems to me, (just an opinion now) that a Earth ground system made to
handle the
> worst case current of your worst case power input cable (along with some
design
> margin) would stand a good chance of passing any regulatory test.
>
> Can any of the gurus see a problem with this?
>
> Chris Maxwell | Design Engineer - Optical Division
> email chris.maxw...@nettest.com | dir +1 315 266 5128 | fax +1 315 797
8024
>
> NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA
> web www.nettest.com | tel +1 315 797 4449 |
>
>
>
>
>
>
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