A long long time ago.......(and I hope it never happens again!)
1. The construction details of a low voltage signal transformer
(telephony application) was described in our UL Report. For those who are
not familiar with such descriptions, it goes something like this: "Open
type. Stacked steel lamination, 0.19 by 0.19 by 0.19 in. thick. Bobbin,
phenolic. Crossover insulation by 0.02 in. gap with minimum 0.02 in. thick
polyester tape. Outer wrap is polyester tape, 0.02 in. thick. Insulation
between (TEL) and (SEC) provided by bobbin and one layer polyester tape,
0.02 in. thick." When the UL Inspector came across this, he asked me to
unwind the insulation and wrap so that he could inspect to the Report. The
transformer was varnished. I said that this was impossible. He then
suggested that I get a saw and saw it in half so that he could measure the
spacings and thickness of wraps, etc. I told him that for this procedure
to work, one needed a "laboratory" type saw so as not to deface spacing,
etc., in the process. A carpenter's saw would not do. I offered him a
sample transformer to take back with him and perform his inspections at his
office. He told me that it was my responsibility to provide proof that the
transformer complied. I placed the transformer in his hands and suggested
he call his supervisor for advise. After a lengthy conversation with his
supervisor, he returned the transformer to me and said he would not write
anything up. I was sympathetic to his plight and agreed that UL
engineering should not write descriptions of items that could not be
inspected in the field. He never asked to "saw" a transformer again.
My recommendation to UL was that they should either "bite
the bullet" and demand that all transformers be UL Recognized, or institute
a sampling collection program, much like they have with plastic enclosures,
where they take samples once a year and re-inspect them back in their
offices. Needless to say, nothing has been done about this yet.
2. The UL Inspector checked the flammability code on a printed wiring
board ('twas 94V-0) and then, per the UL Yellow Book,
asked me to provide proof that the solder temperature was not
exceeded during the solder-wave process. Good question! We do not
wave-solder our cards, but sub-contract them out! We hopped in the car and
went to the local PCB stuffer while horrendous thoughts drifted in my mind.
The manager there was very helpful;-- no, he was not running any cards for
us that day, but yes, he did have a log with all the pertinent data. His
records were impeccable: he had part numbers aligned with PCB fab vendors,
PCB flammability ratings, date codes, solder temperature in ºF, solder
dwell time in inches/minute, etc. The UL Book had solder temperature in ºC
and solder dwell time in cm per second (now it states "cumulative" dwell
time). After we figured out the math and converted everything properly we
had proof that the solder temperatures were not exceeded. I was very
pleased with the results of this effort. It was only as I was driving
home that night that it dawned on that the delta between the actual solder
temperature and the stated limit in the UL Book was so great that this UL
limit would never be exceeded, because if it were exceeded, the PCBs would
be a crisp dark toast and no sub-contractor in his right mind would deliver
such a result to his customers. The UL limit, after all, is for
flammability concerns and not for performance and reliability. Next time
the inspector came to visit, I informed him of my conclusions. He never
asked to see any wave solder logs again.
Tania Grant, Lucent Technologies, Octel Messaging Division
[email protected]
----------
From: Jon D. Curtis [SMTP:[email protected]]
Sent: Friday, July 02, 1999 5:54 AM
To: [email protected]
Subject: Follow Up Services/Factory Inspections
I am writing an article on preparing for factory inspections. Please
share with me your "gotchas" and any advice that a person should know
before the inspector arrives. What did the inspector look for? What
documents did you need to provide?
Any references on what to expect published by the NRTLs or others such
as NEMA?
Thanks in advance for your comments.
--
Jon D. Curtis, PE
Curtis-Straus LLC [email protected]
Laboratory for EMC, Safety, NEBS, SEMI-S2 and Telecom
527 Great Road voice (978) 486-8880
Littleton, MA 01460 fax (978) 486-8828
http://www.curtis-straus.com
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