Hi Peter:
I'm not an expert in UL 508, and have been only
peripherally involved in some product certifications.
So, my comments may not be highly accurate.
As a general rule, UL almost always allows testing
in lieu of constructional requirements. This is
specifically stated in the Foreword to every UL
standard:
"D. A product employing materials or having forms
of construction differing from those detailed with
the requirements of this Standard may be examined
and tested according to the intent of the requirements
and, if found to be substantially equivalent, may be
judged to comply with the Standard."
However, under some UL engineers, such testing may be
resisted. Usually, this means he doesn't understand
the intent of the requirements and cannot come up with
a suitable test. Some proposals on your part should
soften this attitude. Personally, while I've encountered
resistance, I've never been held to construction, and
I've always been able to test. But, it usually takes a
test proposal from me before it will work.
The issue of the PWB being capable of carrying fault
currents is not easy. Its a function of cross-sectional
area. Often, PWB construction does not adequately control
the copper thickness. So, UL is justifiably reluctant to
rely on the PWB to carry the fault current.
If the product is cord-connected, insist on testing with
the power cord and fuse in place. 18 AWG power cords will
limit the current to the range of 200 amps due to the wire
resistance and contact resistance. This makes it easier
for a PWB to pass the test.
The UL test is a justifiably tough test. You should be
looking at very wide traces to carry the fault current.
Before you submit for testing, you should be confident
that the PWB will indeed carry the fault current without
damage!
Best regards,
Rich
-------------------------------------------------------------
Richard Nute Product Safety Engineer
Hewlett-Packard Company Product Regulations Group
AiO Division Tel : +1 619 655 3329
16399 West Bernardo Drive FAX : +1 619 655 4979
San Diego, California 92127 e-mail: [email protected]
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