It is NOT a waste of everyone's time. Thanks to everyone who has
contributed; you have caused some fresh wrinkles to be etched onto
our brains. With appreciation,
Tania Grant, Octel Communications Corporation
[email protected]
______________________________ Reply Separator _________________________________
Subject: RE: How long for continuous? -Reply
Author: Gordon Crawford <[email protected]> at P_Internet_mail
List-Post: [email protected]
Date: 11/15/96 3:44 PM
How long is this "continuous" going to go. Its a waste of everyones time!
>>> Tony Fredriksson <[email protected]> 11/15/96
10:32am >>>
>>>Every reasonable instance, that I can imagine, of injury or damage
resulting
>>>from an energy hazard, will happen very fast and be over and done
with in less
>>>than one second (human reaction time) or even faster (20 J @ 240
VA = 83 ms).
>>>So what possible significance can there be in relating this to a test
that
>>>runs for one minute or even longer?
All,
I believe that an energy hazard does not need to be an instantaneous
event as discussed above. I recall working with a computer once for
which, during assembly, it was possible to pinch one of the 5Vdc wires.
As this insulation broke down, a limited short circuit developed that was
not enough to trip a fuse or to open the circuit, but was enough to heat up
the enclosure plastic such that it ignited and smoldered, ultimately opening
a sizeable hole in the plastic. The thing sat there for hours and the
problem got worse and worse. Had the system been on a desk with
papers in contact with the area, a buliding fire could have easily been
triggered.
Limited short circuit conditions are often insidious and can go unnoticed
for long periods of time. Building fires from shorted electrical wiring are
another example of this phenomenon.
Egon is right that a PROPERLY chosen fuse can limit such hazards. In
such cases, limited shorts under the fuse hazard would not draw enough
current to cause and overheating problem with resultant fire hazard.
My 2 cents worth...
Regards, [email protected]
----------
From: Egon H. Varju
To: IEEE
Subject: RE: How long for continuous?
List-Post: [email protected]
Date: Thursday, November 14, 1996 6:58PM
This thread is getting very interesting. Trust Rich Nute to keep us on our
toes and help keep our minds from atrophying from disuse.
Many times when when I hear this question, the intent seems to be to
salvage a poor design and find out "how much can we get away with."
Wouldn't it make more sense to find out "how much is safe?"
Most responses so far seem to indicate that safety agencies consider
one minute or longer to be a reasonable number. Does this mean that we
are assuming that normal people are stupid enough to stand there for
one full minute, or longer, and watch their finger melt off? Seems to me
that in the unlikely event that any such people should exist, they probably
don't belong to the human race and, therefore, shouldn't be protected by
our safety standards or our laws.
Every reasonable instance, that I can imagine, of injury or damage
resulting from an energy hazard, will happen very fast and be over and
done with in less than one second (human reaction time) or even faster
(20 J @ 240 VA = 83 ms).
So what possible significance can there be in relating this to a test that
runs for one minute or even longer?
Well, I think we all know the answer to that: we're trying to use fuses to
limit enegry hazard. But fuses aren't intended to operate in fractions of a
second, except at extremely high overcurrent levels. In fact CSA/UL
fuses aren't even calibrated for operation in less than 2 minutes
(@200%).
Perhaps Rich's question should be "can a fuse be used to limit energy
hazard?"
Just some food for thought ...
Egon
Disclaimer: Definitely personal opinions!
