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Understanding Current Limitation
Current limitation is one of the most important features of today's fuses. By isolating a faulted circuit before the fault current has sufficient time to reach its maximum value, a current-limiting fuse can:
- Limit the total energy delivered to arcing faults.
- Limit thermal and mechanical stresses created by the fault currents.
- Reduce the magnitude and duration of the system voltage drop caused by fault currents.
- Minimize downtime, since current-limiting fuses can be precisely and easily coordinated even under short circuit conditions.
The degree of current limitation provided by a fuse is measured two ways — peak let-thru current (lp) and I²t. Maximum allowable lp and I²t values are specified in UL standards for all UL Listed current-limiting fuses, and are available on all semiconductor fuses.
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Short circuit currents usually have a decaying asymmetrical wave shape in the first several cycles of the fault, as illustrated in Figure 1. The first peak is the highest; in theory, the peak available fault current can be anywhere from 1.414 to 2.828 times the RMS available in a circuit where the impedance is all reactance with no resistance. In reality, all circuits include some resistance, and the 2.3 multiplier has been chosen as a practical limit.
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Figure 1: Short circuit currents usually have a decaying asymmetrical wave shape in the first several cycles of the fault.
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Let-Thru Current
Let-thru current is the current passed by a fuse while the fuse is interrupting a fault that is within its current-limiting range.
As the fault current begins to rise in the first half cycle, the element in the current-limiting fuse melts. The added impedance created by the resultant arc causes the current to max out and begin to decrease.
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The highest instantaneous value reached is referred to as peak let-thru current, and is expressed as a peak instantaneous value (lp). Figure 2 shows the Ip for three different fuses subjected to the same available fault current. Although all three fuses have the same ampere rating, the current-limiting performance in the short circuit region is dramatically different.
Ip is useful in predicting peak electromagnetic forces created throughout the faulted circuit. The peak forces will be proportional to the square of the instantaneous value of current of the first half-cycle peak, or Ip2, for current-limiting fuses.
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Ip versus I²t
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When it comes to predicting the energy delivered to a fault, Ip has rather limited usefulness. Two fuses can have the same Ip, but different total clearing times, as shown in Figure 3.
The fuse that clears in time "A" will provide better component protection than the fuse that clears in time "B." Fuse clearing I²t takes into account Ip and total clearing time. Fuse clearing I²t values are derived from oscillograms of fuses tested within their current-limiting range and are calculated as follows:
I²t = ∫tI²dt
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Figure 3: Two fuses can have the same Ip, but different total clearing times.
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The "t" in the equation is the total clearing time for the fuse. Although I²t is actually (IRMS)²t, it is generally understood that the "I" in I²t is really IRMS, and the RMS is dropped for the sake of brevity.
In Figure 3, note that since clearing time "B" is approximately twice clearing time "A," the resultant I²t for that fuse is greater than the I²t for the fuse with clearing time "A," and its level of protection is correspondingly lower.
The I²t passed by a fuse depends on the fuse characteristics and applied voltage. As application voltage decreases, I²t will decrease. Unless stated otherwise, published I²t values are based on AC testing. The I²t passed by a fuse in a DC application may be higher or lower based on the voltage, available fault current and time constant of the DC circuit.
Fuse I²t value can be used to determine the level of protection provided to circuit components under fault current conditions. I²t is proportional to the energy delivered to the faulted circuit (I²Rt). I²t for a specific fuse design will be lower for higher fault currents until the fuse reaches adiabatic melting.
Manufacturers of diodes, thyristors, triacs and cable publish I²t withstand ratings for their products. To protect one of these devices, a fuse should have a clearing I²t that is lower than the withstand I²t of the device.
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Using peak let-thru charts
