Current limiting and non fuse

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A current limiting fuse must begin to melt in 1/4 of one cycle and be completely open in less than 1/2 cycle. In doing this, it limits the amount of let-through current through the fuse.
 
mark32 said:
Perhaps a dumb question but what is the difference between a current limiting and non current limiting fuse?
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perhaps life and death
 
ryan_618 said:
A current limiting fuse must begin to melt in 1/4 of one cycle and be completely open in less than 1/2 cycle. In doing this, it limits the amount of let-through current through the fuse.
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Oh boy, some magic equipment we have here. So if I apply, say, twice the nameplate current to the fuse it will open in 1/2 cycle?
 
weressl said:
Oh boy, some magic equipment we have here. So if I apply, say, twice the nameplate current to the fuse it will open in 1/2 cycle?
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Sure, if that current is in its current limiting range. If it is a 2000 HP motor and you put a 20A current limiting fuse on it, I'm sure it would. :)
 
Fuses do two basic tasks, both based upon current.

Task one: open under long-time over current conditions, commonly referred to as an "overload" (but not to be confused with a Motor Thermal Overload). The task we are speaking of is a "circuit" overload. So if you have a 200A fuse, there is a specific time-current curve which will tell you how you can predict the time it will take to clear at, say, 250A. This curve is also called an "I^2t" (I squared t or current squared x time) curve, meaning that the higher the current is over the rating, the faster it clears, and the rate is based upon the square of the current, so it is fairly steep.

Task two; interrupt a Short Circuit as quickly as possible. This means that at some level, the fuse will NOT wait any longer than it has to to clear a rapid rise in current well beyond the fuse rating, such as what you would see in a short circuit.

A Current Limiting fuse adds another task on top of that: clear no matter what in such a short time (that 1/2 cycle) that up to the limits of the fuse class, only so much current will get through the fuse. Here is an example to help illustrate this (not taken from a fuse book, so don't accept these numbers as gospel).

200A std or CL fuse; circuit overload of 250A, it takes 2 minutes for the fuse to clear that overload based on the I^2t curve.

200A std fuse; short circuit situation with 100,000A available in the circuit, takes 2 cycles to begin clearing, then one more before current actually stops flowing. In that time, the current in the circuit rose to 50,000A. If the equipment downstream of that fuse was only rated to withstand 10,000A, then that equipment becomes shrapnel. The magnetic forces created by that much current flowing through it blows it to pieces.

200A CL fuse; same scenario but the fuse clears in 1/2 cycle, so the fault current downstream only reaches 5,000A. Equipment downstream has a withstand rating of 10,000A, so no problem.
 
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