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Topic: Fuse rating question (15 msgs / 205 lines)
1) From: Mike McGinness
Would a 6A 250v fuse offer equivalent protection as a 12A 125v?
(was going to mark message OT but relates to roaster control unit design...)
MM;-) aka Kona Konnaisseur miKe mcKoffee
Dual Variable Transformer Rosto Roasting
Rocky grindin' - Miss Silvia brewin'
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2) From: Ben Treichel
I believe that is correct. Its a wattage/heating thing, and both are the 
same in those terms.
Ben
Mike McGinness wrote:
<Snip>
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3) From: Dan Bollinger
No.  You need a 12A 250V fuse.  It is common to use fuses rated as higher
voltages, but they must maintain the same amps.
dan
<Snip>
design...)
<Snip>
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4) From: Mike McGinness
Got two different answers so did what I could/should have done in the first
place, web search and research it! Dan is correct, Ben (& I)  were wrong.
Glad I checked. The fuse voltage rating is: "The ability of the fuse to
quickly extinguish the arc after the fuse element melts and the maximum
voltage that cannot jump across the gap of the fuse after the fuse opens."
The current rating is: "The amount of current the fuse will allow without
opening."  So yes higher voltage rating is fine and actually better, but
still using the same current (A) rating.
MM;-) aka Kona Konnaisseur miKe mcKoffee
Roaster control designer in training
From: "Dan Bollinger" 
<Snip>
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5) From: Rick Farris
Mike wrote:
<Snip>
Nope.  Just like the PIV rating on the diode, the voltage rating on a fuse
is like the speed rating on a tire.  As long as you have enough, you're ok.
Specifically, the voltage rating on a fuse is pretty much determined by its
length.  The longer the fuse, the higher the voltage rating.
-- Rick
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6) From: Rick Farris
Ben wrote:
<Snip>
BZZZZT!  Thank you for playing, but...  :-)
The voltage rating on a fuse is a safety factor -- if the fuse were very
short and the applied voltage very high, then the voltage could arc across
the contacts even after the fuse burned out.  So to get a higher voltage
rating, they make the fuses longer so that the (same) voltage can't arc
across.
-- Rick
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7) From: dewardh
Mike:
<Snip>
No . . . Dan got it right.  A fuse (fusible link) is a current driven device . 
.. . a resistor that melts (and opens the circuit) if it gets too hot.  It gets 
hot because of the power dissipated in it, and that power is determined P=I^2R, 
where R is the resistance of the fuse itself.  There is, ultimately, a voltage 
involved, but that's the voltage that developes across the fuse when current 
flows, and it's in millivolts . . . unrelated to the supply voltage in the 
protected circuit.  As to why one fuse would be "rated" 125V and another 250V, 
I have not a clue . . . a 6A fuse should fail if the current exceeds 6A, 
regardless.
Deward
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8) From: Ben Treichel
learn something new every day. And this time it evens makes sense.
Rick Farris wrote:
<Snip>

9) From: Dan Bollinger
I wasn't too worried about you Mike, the worst thing that would have
happened is that you'd have to take a trip to the hardware store to replace
the 6A fuse you blew!  :)  Dan
<Snip>
first
<Snip>
higher
<Snip>
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10) From: Dan Bollinger
<Snip>
dunno, perhaps because of the amount of insulation or to prevent arcing.
Dan
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11) From: Mike McGinness
From: "Dan Bollinger" 
<Snip>
Higher voltage rating higher arc protection after fuse blown from my
research...
MM;-) aka Kona Konnaisseur miKe mcKoffee
Dual Variable Transformer Rosto Roasting
Rocky grindin' - Miss Silvia brewin'
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12) From: Rick Farris
Mike wrote:
<Snip>
But don't forget that the fuses will be physically different sizes, so if
you already have the fuseholder, make sure the fuse you select will fit.
-- Rick
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13) From: Mike McGinness
From: "Rick Farris" 
<Snip>
You state the obvious. (which can often be over looked of course:-) But fuse
length and diameter aren't the only factors in determining it's rating. Many
same physical size fuses have different ratings, the internals be different
of course. But also make sure the fuse holder is current rated for the fuse!
(the first holder I picked up was rated 10A, took it back for a 30A...) You
can put a 15A fuse in a 10A rated panel mount fuse holder no problem.
Putting the fuse in no problem that is, potentially running the holder at
50% greater than rated could definitely cause a heat problem!
MM;-) aka Kona Konnaisseur miKe mcKoffee
Dual Variable Transformer Rosto Roasting
Rocky grindin' - Miss Silvia brewin'
homeroast mailing listhttp://lists.sweetmarias.com/mailman/listinfo/homeroast

14) From: dewardh
Dan:
<Snip>
Yep . . . as per Mike's search that seems to be the answer.  I'm so accustomed 
to the 3AG package (no arc suppression problem there, that I can see, at any 
"reasonable" voltage anyway) that I didn't even consider the possibility of a 
sustained arc (in a shorter package?).  Shoulda thought of it, though . . . arc 
suppression is a problem at power (distribution) line voltages both with 
switches and fault interrupters.  Just wasn't keeping the old noggen open 
enough . . . (must be getting old ).  Probably even knew the answer once 
upon a time . . . that's the worst part . . .
Deward
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15) From: Rich Adams
Yup, as stated previously by someone, the voltage rating is max voltage that
can be felt by a blown fuse before it starts to arc, thereby rendering the
fuse worthless as a fuse.


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