Hi Peter:
I hope you have simplified the certification house response...
"However, opening of a trace during component abnormals
would be considered an unacceptable result."
Such a blanket judgement denies an engineering evaluation
of both the purpose of the test and the results of the test.
I said:
"Many, many fault-condition (abnormal) tests end up with an
open circuit. We don't treat all component open-circuits
as fuses. So, why should we treat an open trace as a fuse?"
When a semiconductor fails open as a result of an abnorml
test, the "open" is usually due to the fusing of an internal
bond wire. (Other semiconductor faults may precede the fusing
of a bond wire.) Such a result is commonly considered acceptable.
Yet, how does it differ from the opening of a board trace?
Let's consider first the purpose of the test. Most "abnormal"
tests are intended to ascertain whether a fire will result.
Other abnormal tests are intended to ascertain the failure of
insulation, usually be means of overheating.
Typical failure criteria for an abnormal test for fire is whether
or not ignition occurs. The opening of a trace is undoubtably
due to heating (due to excessive current, I*I*R). If no fire
results, then the constuction is accepatable.
Except, the termination of the overheating event was by opening
of the trace. We all know that there is a lot of variability
as to the time-current fusing characteristics of a trace. So,
we would need to conduct some further tests to ascertain that a
fire will not occur with the worst-case trace (i.e., larger
cross-sectional area).
Consider replacing the trace with a wire of relatively large
cross-section and repeating the test. Now, something else
fails and the test is terminated. There is either a fire, or
there is not. If there is no fire, then the fusing of the
trace SHOULD be acceptable. The two tests have proved that no
fire is possible.
So, one needs to know the purpose of the test, and one needs
to evaluate the test results in terms of the purpose of the
test.
It is irresponsible for a certification house to make such
a categorical statement as to whether or not a particular
failure (i.e., opening of a trace) is an unacceptable result.
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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> From: [email protected] (Peter Merguerian)
> To: [email protected], Rich Nute <[email protected]>
> Date: Thu, 7 Jan 1999 23:12:35 +0200
> MIME-Version: 1.0
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> Subject: Re: UL1950 Overvoltage Tests
> CC: [email protected], [email protected], <[email protected]>
> Priority: normal
> In-reply-to: <[email protected]>
> References: <[email protected]> from
> "Douglas McKean" at Jan 5, 99 02:19:49 pm
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> To Rich, Doug and All Members,
>
> I just got clarification from UL that opening of a trace during
> Overvoltage Tests is OK as long as unit passes either the Leakage
> and Dielectric Tests. However, opening of a trace during
> component abnormals would be considered an unacceptable result.
>
> Thank you everyone for all the input.
>
>
> Date sent: Tue, 5 Jan 1999 16:08:51 -0800 (PST)
> From: Rich Nute <[email protected]>
> To: [email protected]
> Subject: Re: UL1950 Overvoltage Tests
> Copies to: [email protected] (Product Safety Technical
> Committee)
> Send reply to: Rich Nute <[email protected]>
>
> >
> >
> > Hi Doug:
> >
> >
> > > Isn't this a variation of using the traces as a fusing element
> > > instead of using a real fuse?
> >
> > Not necessarily.
> >
> > In Peter's situation, the test was that of the integrity
> > of the insulation between the TNV circuit and the other
> > circuits.
> >
> > So, if the "fused" trace did not bridge the insulation,
> > then there should be no consideration that the trace was
> > being used as a fuse. It is simply a failure, and a
> > particular kind of failure -- open-circuit.
> >
> > Many, many fault-condition (abnormal) tests end up with an
> > open circuit. We don't treat all component open-circuits
> > as fuses. So, why should we treat an open trace as a fuse?
> >
> > It is unlikely that fusing of a trace will bridge a safety
> > insulation. On the other hand, fusing of a transformer
> > winding wire may be associated with enough heat to damage
> > the solid safety insulation within the transformer!
> >
> > On the other hand, if the fusing of the trace should
> > prevent an overheating situation (fire?), then the trace
> > probably should be evaluated for its fusing action.
> >
> > So, one needs to understand the nature of the test in
> > order to evaluate the results of the test to determine
> > whether the opening of a trace is a fusing action as
> > opposed to simply the end of a fault-condition test.
> >
> >
> > Best wishes for the New Year,
> > 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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>
>
> PETER S. MERGUERIAN
> MANAGING DIRECTOR
> PRODUCT TESTING DIVISION
> I.T.L. (PRODUCT TESTING) LTD.
> HACHAROSHET 26, P.O.B. 211
> OR YEHUDA 60251, ISRAEL
>
> TEL: 972-3-5339022
> FAX: 972-3-5339019
> E-MAIL: [email protected]
> Visit our Website: http://www.itl.co.il
>
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