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When a conductor from one potential comes in contact with the system ground or a conductor from another potential, a short circuit can occur. The short circuit provides little or no impedance to the power source, so that the current can rise to 100 times the full-load current levels in one or two cycles. That is, currents of 50,000 A or larger are possible if the current is allowed to build.
Figure 1 shows a graph of the current developed by a short circuit. Notice that the current continues to increase with each cycle of the ac voltage. As this current increases, it will build up powerful magnetic forces and tremendous amounts of heat energy that will cause the metal conductors and terminals to melt and explode.
Above: Fig. 1 (a) Example graph of short-circuit current. (b) Example of graph of current when a circuit breaker is used to protect against short-circuit current. (c) Example graph of current when a fuse is used to protect against short-circuit current.
A circuit breaker is an electromechanical device that requires approximately one-half of an AC cycle (positive or negative half of a sine wave) to sense the short circuit and another half-cycle to trip its mechanical contacts. The short-circuit current may still reach 40,000 - 45,000 A during this period of time before the circuit breaker can open its contacts fully. In some cases, the heat from this short-circuit current is so intense that it will actually weld the contacts of the circuit breaker together so that they can't open even though the circuit breaker’s trip mechanism has activated. When this occurs, the short-circuit current continues to increase and eventually causes severe damage.
In comparison a fuse can sense the overcurrent as it begins to build and its fusable link will melt before the current increases to a dangerous level. A graph of the protection that a fuse can provide during a short circuit is also shown in Figure 1. From this graph, notice that the fuse element will open as soon as the current reaches the overcurrent level. That is, the short-circuit fault will be sensed and opened in less than a half-cycle. The only problem with the single- element fuse is that it must be sized up to six times the full-load current level to allow the motor to start. This leaves two alternatives when providing short-circuit and overload current: fuses in combination with motor starters or circuit breakers, or the use of a dual-element fuse.
|Protecting Against Slow Overcurrent