>>>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!
