On 10/14/2015 4:40 PM, Dan Roman wrote:
The other Brian wrote:
Does anyone have first-hand experience dealing with EMC failures in the field?
If you fail by 1db, are you dragged
through the mud, fined, banned, prosecuted, black helicopters circle your
house, masked men drag you out of bed in
the middle of the night? Or is action taken only on severe non-compliances?
What's the likely scenario?
Three short stories. Customer complains they are having problems with
our very large format flat bed laser scanner. Trip to Buffalo, NY in
January with analyzer, antennas, etc. Found significant "noise" in the
very quiet RF amplifier circuits. Much chin scratching, head shaking,
hand wringing later, I found I could listen to a local radio station in
the video path of our product. Tracking it down was a bear but finally
found it when I let a scope probe drop and it contacted the metal
chassis of the product. Zero span was a wonderful tool to let me listen
to 5 different radio stations on the product and all at frequencies
below 100 MHz. On the chassis. Then I found it on the power lines. And
then I found it on the steel column supporting the roof of the metal
building. The final culprit was a radio station 3 blocks north and 7
stories up in the air. With a very large antenna on the roof. Made the
entire building hot and resonant at various frequencies. Fixes to the
product were difficult, but we eventually succeeded.
Second story was when some government inspector stopped in at a
customer's store in The Netherlands. Took a sample of our product back
to the government lab for testing. 9 dB over the limit at 48 MHz.
Product was a high end DVD player. Just not possible according to my
test lab reports. Pulled two units from our EU warehouse and had them
tested in Germany. Made sure the serial numbers bracketed the one the
government lab said was bad. Not a peep. 4+ dB margin on both units. I
called a contact at our EU office and she was able to work her way into
the lab and ask lots of questions for me. She had no electronics
knowledge, but worked the customs side of things. So she knew a bit
about handling bureaucrats. Through her, I was able to show the lab had
a bad test set up. The lab manager re-tested according to my
configuration "suggestions" and found the unit was compliant after all.
The original test engineer did not work there shortly thereafter.
Last story just recently. Israeli customer bought some products (AV
gear) and as we normally do, we allowed them to submit for local country
approvals. Conducted EMI failed by 12 dB at 4 MHz. Very helpful customer
took lots of photographs for me. Contacted the factory (in Asia) and
they suggested an "improvement" that might fix the problem. There was a
large ferrite core on the AC Mains just in from the appliance inlet that
fed up to the front of the box where the switching power supply was
located. The factory "improvement" was to move the ferrite an inch to
the left and mount it to the side of the chassis and to add several more
twists to the discrete line/neutral conductor pair. Customer reworked
the product and it passed. Then the Israeli test lab required them to
rework the remaining 9 units same way, selected two more for testing
and all passed. Case closed. Almost. Looked back at the photos in the
original EMC test reports and the ferrite was in different location from
the production models Much closer to the inlet and much closer to the
side of the product keeping it away from the digital audio amplifier
modules. It looks like the production line was getting sloppy in the
placement of the ferrite and wire and caused the failure. We added more
inspection criteria to our factory inspection checklist after that one.
So one customer reported problem and two government gotcha's.
Fortunately I was able to work the three problems and find solutions
that did not involve recalls, prohibitions, or fines. And I never had to
leave the comforts of home. Except for the month I spent in Buffalo one
winter week in January.
Scott
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