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Reply To: Protection Riddle No.97 – Short circuit current limiter


    A type of fuse sometimes used in power applications is the current-limiting fuse. This fuse is defined as “a fuse that, when it is melted by a current within its specified current-limiting range, abruptly introduces a high arc voltage to reduce the current magnitude and duration.” This type of fuse is basically different from the current zero waiting types. Here the principle is called current limiting or energy limiting. It does this by introducing a high resistance into the circuit. This limits the current, but it also improves the power factor, making the current more in phase with the voltage. The current-limiting fuse, however, causes the current to fall to zero as the voltage goes to zero, thereby limiting the maximum current to an amount called the “let-through current.” There are three basic types of current limiting fuses. The first type is the backup current-limiting/ use. This fuse is very effective at high fault currents, but is not able to interrupt low currents, hence it must always be used with a conventional over current device to react to low fault currents. The second type is the general purpose current-limiting fuse that, according to ANSI standard, is a current-limiting fuse that can interrupt a current that causes the fuse to operate in one hour or less. The third type is a full range fuse, which is designed to interrupt any current that causes its fusible element to melt under normal fusing conditions. The best way to show the effectiveness of the current-limiting fuse is by computing the so called I2t factor, which is the current time integral. This factor represents the heating that can occur per increment of resistance. Therefore, it is proportion al to the allowable energy in the circuit. There are two parts of the I2t factor. The first part is the melting 12t , which can be determined by calculation. The second part of the 12t factor is that which occurs after arcing begins and continues until full current interruption occurs. This part must be determined by test. The first requirement of any fuse is to extinguish the arc. The second requirement is that the cutout must be able to withstand the normal full rated voltage across the open fuse link. This usually involves a high frequency transient that rapidly damp s out, followed by the normal power frequency voltage, which will be applied across the open cutout. The third requirement of the fuse design is that it must be capable of being coordinated with other fuses or other devices, so that the outage caused by the fault is restricted to the minimum protective zone. An ordinary circuit-breaker does not provide protection against impermissibly high peak short circuit currents, because it is too slow. Only the Is-limiter is capable of detecting and limiting a short-circuit current in the initial rise, i.e. in less than one millisecond. The maximum instantaneous current value that occurs remains well below the peak value of the short-circuit current of the system. The Is-limiter is a current-limiting switching device, which detects and limits the short circuit current in the initial rise. The short-circuit current through the Is-limiter is limited so quickly that it does not contribute in any way to the peak value of the short-circuit current at the fault site. In principle, the Is-limiter consists of an extremely fast switching device that can conduct a high rated current, but has a low switching capacity and a parallel configured fuse with high breaking capacity. To achieve the desired short switching delay, a small charge is used as energy storage for opening the switching device (main current path). Once the main current path has been opened, the current still flows through the parallel fuse, where it is limited within 0.5 ms and then is finally interrupted in the next voltage zero. references: – ABB SWITCHGEAR MANUAL – POWER SYSTEM PROTECTION (IEEE Press Power Engineering Series P. M. Anderson, Series Editor)