Thread: What else do we want?
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Unread 23-06-2002, 23:19
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#0047 (ChiefDelphi)
  
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Timing is everything...
Posted by Joe Johnson.   [PICTURE: SAME | NEW | HELP]


Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems.

Posted on 7/13/2000 5:26 PM MST


In Reply to: Dumb Question posted by Dodd Stacy on 7/13/2000 3:04 PM MST:



Dodd,

The thing about fuses is that the amp rating does not tell all. Time is a very important factor.

A fuse that is rated for a particular amperage will be able to carry that amount of current forever at the rated temp (e.g. 20 deg C). They can carry twice that current for a shorter period of time, 3 times the current for an even short time, etc. As I recall, the 60 amp fuse can carry 120 amps for something on the order of 100 seconds.

Circuit breakers have a similar method of rating EXCEPT they typically have a much lower margin of safety (if you want to call it that -- it could be thought of as a higher margin of safety if you thing about 'safe' as being 'sure not to start a fire' rather that as 'sure not to trip' but I digress...) For example a 30 amp breaker while never tripping at room temp at 30 amps may trip in 30 seconds at 40 amps and 10 seconds at 60 amps.

Using the example numbers above, 3 thirty amp curcuit breakesr carrying 120 amps would trip in 30 seconds (each breaker carrying 40 amps). A fuse would require 100 seconds to trip. SO... 3 thirty amp circuit breakers would protect 1 sixty amp fuse. Yes, 2 thirty amp breakers would protect the fuse as well but it would be TOO well in a sense limiting current to much less than would normally go through the fuse without trouble -- note that 120 amps would trip them in just 10 seconds . Also note that 4 thirty amp breakers would do no good what ever as they could carry 30 amps each (120 amps) forever without tripping while the fuse would blow in 100 seconds. The numbers are approx. but the idea is the same.

As to mfg. variation, this is not a problem. Yes, one of the breakers blows first but as soon as it does, the other two blow almost instantly after because they are carrying 50% more current all of a sudden!

I hope this clears things up a bit.

Joe J.


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