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January 20, 2010
Total Clearing Time


No matter if we’re talking about a fuse, circuit breaker, resettable PTC fuse, ground fault protector, MOV or any other type of power protection component, it will take a finite amount of time for the device to react and open to clear the circuit.

The total time to clear the circuit is called the clearing time and is made up of two components, the "melting time" (or the time it takes to melt the fuse element) and the "arcing time" (or the time it takes to extinguish the fault arc).

Fuse Clearing Time 

In the above diagram, the melting time is represented by "tm" and the arcing time is represented by "ta".  The total time it take to clear the circuit (clearing time) is represented by "tc" and is the sum of tm and ta.

The peak let-thru current is the maximum amount of current that the fuse allows through when it opens.  As you can see, It is significantly less than that of the total peak available current.  The lower the peak let-thru current, the more protection to downstream components and equippment.

The faster the fuse, the less time it takes the fuse element to melt and the less peak let-thru current.  The greater the voltage, the longer time it takes to extinguish the arc and eventually clear the fault. 

The total amount of energy is represented by the total of red and blue shaded areas above.  The energy is measured in i²t (amp-squared-seconds).

The I²t is a constant of the fuse and is dependant upon the fuse amperage, link material and overall construction.

I hope tha this brief lesson gives you a better understand about the physics behind the fuses’ operation.

Thanks to everyone for you continued support of OptiFuse where we hope that an informed customer makes a better customer.

Jim Kalb
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