IEEE Std C37.48.1-2011 pdf download – IEEE Guide for the Application, Operation, and Coordination of High- Voltage (>1000 V) Current-Limiting Fuses.
4.3 Interrupting characteristics After the fuse element melts, the fuse must interrupt the current (which continues to flow through an arc). After interruption, the fuse must withstand any immediate transient recovery voltage (TRV) condition and the subsequent steady-state recovery voltage. When a fuse melts, there will always be some period of arcing before the current is interrupted. The melting time (adjusted by an allowance for the manufacturer’s plus variations for manufacturing tolerances, etc.) is added to this arcing period to obtain the total-clearing time. Total-clearing TCC curves are drawn to present this information. Normally, at long melting times, arcing as a portion of total clearing time is negligible, while at short melting times it can be significant. Thus, for a given fuse, the minimum-melting TCC curve and the total-clearing TCC curve tend to diverge at shorter times (higher currents), as illustrated in Figure 1B, which shows the minimum-melting and total- clearing time-current-characteristic curves plotted for the same fuse. The two TCC characteristics described are used to coordinate with comparable characteristics of other fuses or protective devices to selectively isolate faulted circuits and to protect equipment. This is a very important consideration in the operation of electric systems.
Current-limiting fuses, by contrast, can limit faults in magnitude and duration when they melt before the first major peak of the fault current. A current-limiting or “current zero forcing” fuse begins limiting the rising fault current as soon as its element melts and thus prevents the current from reaching its prospective peak value. The current-limiting action results from melting of a rather long fuse element that interacts with the constraining and cooling medium (typically sand), to introduce a quickly rising equivalent resistance into the fault circuit. Figure 2b shows a current-limiting fuse carrying a fault current that causes it to melt before the first peak. As soon as the fuse arc voltage equals the system instantaneous voltage, the current peak occurs. After this the current falls and the fuse arc voltage exceeds the system voltage because circuit inductive voltage opposes the falling current and adds to the system voltage. The fuse changes a high- current, low power factor fault circuit into a lower current, higher power factor circuit. As a result, the current is forced to near zero well before the normal current zero of the circuit and close to the voltage zero. This typically occurs within less than 1/2 cycle of the fault initiation. Because the current zero is moved close to the voltage zero, low TRVs result. The operation of current-limiting fuses will be expanded upon in 4.6.2. The various types of current-limiting fuses include commutating, as well as non-commutating, electronically actuated fuses.