RE: (ET) fuses and breakers

[Larry Elie <lelie ford com>]

Thanks for setting us straight Dave.  I was the confused individual who 
proceeded 
to spawn confusion.  I have done fuse testing (20+ years ago) but never 
done a 
breaker test in my life.  I should have guessed an I square R for the 
power in the 
'passive' device.  Duh.

Now the problem.  Even today, someone posted shunt values for the meter of 
an 
E-20.  Note the E-20 has a 50 Amp breaker on the armature.  The values for 
the 
green range extended to 75 Amps.  This compares rather well to the values 
I 
saw with a Hall-Effect Clip-on (to my Fluke digital) on my E12.  The 
yellow range 
extended to 121 Amps, and the red ended at 156 amps.  I was able (by 
pushing 
on a dirt pile with the blade to nearly pin the meter (~200 Amps at Hall) 
for tens of seconds.  
I have pushed gravel for over an hour between the upper green and yellow.  
These are 
on the order of 100 Amps, or more than twice the value of the 40 Amp 
breaker in my 
E12.  I have never tripped it.  Someone else posted a clip-on value of 600 
Amps, which 
I think should take out the fusible link, but it didn't.  Even for a 
'slow-blow' breaker, I have 
a hard time understanding how this could work.  Secondly, even at 100 
Amps, we 
are only talking about something like 5 HP; and GE's 'rating' would be as 
bad as 
the Sears ratings on their saws.  At 40 amps, my GE would be about 2 HP, 
making 
Sears look honest.  Sure, part of the 'rating' might include the deck 
motors, but they 
are only about 1 HP (by my calculations... now suspect) each.  I do design 
of high-field 
devices (clutches) for part of my job, and I am having some real problems 
here.  

The bottom line is how can an E-20 win in a tractor pull?  And how does a 
40 Amp breaker 
handle 100 Amps for over an hour?

Larry Elie


----------
From:   David Howard[SMTP:howie sssnet com]
Sent:   Sunday, October 31, 1999 7:39 AM
To:     elec-trak cosmos5 phy tufts edu
Subject:        (ET) fuses and breakers

    There has been alot of confusion and misinformation on the fuses and
breakers.  The equation stated before for heating in an electrical circuit
is not right.  The temperature in a circuit is related to the current (I)
squared times the resistance of the circuit or device in the circuit or I^2
* R. Someone wrote about the different fuse voltage ratings and thinking
that the higher voltage rating ment that you had to have that voltage and
current to cause the fuse to blow.  Not true!!!  the voltage ratings show
what type of fuse is needed to ensure that when the fuse operates and  that
the circuit opens.  Whenever a fuse opens, an arc is created across the
opening element.  If the gap is not large enough then the circuit can
actually stay energized instead of opening.  This gap that is required is
determined by the system's voltage.  So whether you have a 30 amp 
automotive
fuse ( rated at 12 V ) or a household fuse at 30 amp ( rated at 120 or 220 
V
) they both blow at 30 amp.  It is just that the automotive fuse is smaller
than the household fuse because it only has to ensure an open circuit at 12
V instead of 120 V or 220 V.

    A fuse is a relatively  instantaneous device. Up to about 102 % of its
rating a fuse will work.  Above that, the element inside will rapidly melt
and separate, opening the circuit.  

    A breaker, on the other hand does not instantly open when its rating is
exceeded.  The most common breaker that we see is the Bimetalic element.  
It
works by having two different metals that heat diffently.  As they heat 
they
move and cause the circuit to open circuit.  As the metals cool they move
back and the circuit can be made again.  When a breakers rating is 
exceeded,
the breakers opertate on what is known a an I^2 T curve.  That is current
vs. Time curve.  The higher the current the faster the breaker trips until
it is actully an instantaneous device also.  

    The same thing applies the the breaker elements.  Because the 
temerature
of the element is I^2*R, it does not matter what the voltage rating of the
breaker. It will still go at the same current. 

    There are different types of breakers and fuses.  Some are "quick blow"
and some are "slow blow".  depending on your application you might need to
experiment with differnt types to find on that provides for turning the
component on and still provide running protection.

    If you have any questions please respond.  I work in the electrical
testing and maintenance field and do this for a living.  We have a high
current machine that we use to test 480 V breakers.  This machine only puts
out .5 to 2.0 V while providing up to 50,000 amps. So we trip breakers 
every
day with low voltage but the proper amount of current.

Dave Howard
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