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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