Fuses
The most commonly used protective device in a distribution circuit is the fuse. Fuse characteristics vary
considerably from one manufacturer to another and the specifics must be obtained from their appropriate
literature. Figure 3.3 shows the time-current characteristics which consist of the minimum melt
and total clearing curves.
Minimum melt is the time between initiation of a current large enough to cause the current
responsive element to melt and the instant when arcing occurs. Total Clearing Time (TCT) is the total
time elapsing from the beginning of an overcurrent to the final circuit interruption; i.e., TCT¼
minimum melt plus arcing time.
In addition to the different melting curves, fuses have different load-carrying capabilities. Manufacturer’s
application tables show three load-current values: continuous, hot-load pickup, and cold-load
pickup. Continuous load is the maximum current that is expected for three hours or more for which the
fuse will not be damaged. Hot-load is the amount that can be carried continuously, interrupted, and
immediately reenergized without melting. Cold-load follows a 30-min outage and is the high current
that is the result in the loss of diversity when service is restored. Since the fuse will also cool down during
this period, the cold-load pickup and the hot-load pickup may approach similar values.
The most commonly used protective device in a distribution circuit is the fuse. Fuse characteristics vary
considerably from one manufacturer to another and the specifics must be obtained from their appropriate
literature. Figure 3.3 shows the time-current characteristics which consist of the minimum melt
and total clearing curves.
Minimum melt is the time between initiation of a current large enough to cause the current
responsive element to melt and the instant when arcing occurs. Total Clearing Time (TCT) is the total
time elapsing from the beginning of an overcurrent to the final circuit interruption; i.e., TCT¼
minimum melt plus arcing time.
In addition to the different melting curves, fuses have different load-carrying capabilities. Manufacturer’s
application tables show three load-current values: continuous, hot-load pickup, and cold-load
pickup. Continuous load is the maximum current that is expected for three hours or more for which the
fuse will not be damaged. Hot-load is the amount that can be carried continuously, interrupted, and
immediately reenergized without melting. Cold-load follows a 30-min outage and is the high current
that is the result in the loss of diversity when service is restored. Since the fuse will also cool down during
this period, the cold-load pickup and the hot-load pickup may approach similar values.
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