Standard definition of current-limiting fuse (original text):

GB/T13539 (IEC60269) current-limiting fuse: "A fuse that, within a specified current range, limits the current significantly below the expected peak current due to the fuse element melting."

UL248 Current-limiting fuse:
"A fuse that, within a specified overcurrent range, limits the clearing time at rated voltage to an interval equal to or less than the first major or symmetrical current loop duration; and limits the peak current to a value less than the available peak current."

 

The impacts of not limiting fault current on circuits and equipment are as follows:

Mechanical stress (Ip): The huge electromagnetic force generated by short-circuit current can cause severe distortion of conductors (such as busbar bending, cable displacement), or even break connection components.

Thermal stress (I²t): The enormous heat generated by short-circuit current can burn conductors and insulation materials.

Arc risk: Continuous high current produces high-temperature arcs at the breaking point, scorching surrounding objects.

Current-limiting capability is an important advantage of fuses. Current-limiting fuses can quickly interrupt fault circuits before the fault current reaches its peak.

 

Current-limiting capability of overcurrent protection devices:

The degree of current limitation depends on the type of selected overcurrent protection device.

 

 

Mechanical stress is proportional to the square of the "peak current" (F ∝ I PEAK 2 )

Thermal stress is proportional to the square of the "RMS current" (Q ∝ I RMS 2 )

Limiting permissible current (Ip) and (I²t) are indicators to measure the current-limiting capability of fuses.

 

 

Comparison between fuses and circuit breakers.

 

How should we use the permissible current curve?

(1) The horizontal axis is labeled as "Expected fault current" with units in A (RMS symmetrical value).

(2) The vertical axis is labeled as "Peak permissible current" with units in A.

(3) In circuits with a typical power factor of 0.15, the maximum peak current generated by the circuit is 2.3 times the RMS symmetrical value, represented by a curve with a 2.3:1 slope.

(4) The fuse current-limiting characteristic curve shows the peak current that a given fuse can pass under various expected fault currents.

The above figure demonstrates how to use the permissible current curve. Suppose a 400A Class T fuse (VTF-400A) is to be used, and the expected fault current is 50kA RMS. Without fuse current limiting, the circuit can generate a maximum peak current of 115kA (50kA multiplied by 2.3). Locate the 50kA expected fault current on the bottom of the curve (1), then move upward to the intersection with the VTF-400A fuse curve (2). From this point, move horizontally left to intersect the maximum peak current curve (3). Finally, read the permissible current value of about 6.5kA at the bottom of the curve (4).
Thus, using the permissible current curve, the VTF-400A fuse can limit the peak current of an available 50kA fault current to 15kA, with a permissible current limit of 6.5kA.

Vicfuse UL general protection fuses are UL listed under JDDZ category certification, with maximum Ip and I²t values specified in the UL248 standard.

 

Vicfuse semiconductor protection series fuses also possess current-limiting capability.

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