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When a current is interrupted, an arc is generated. This arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium the arc forms in.

Figure 3: Circuit breaker II.10.1.Protection with fuse In electronic and electrical engineering, a fuse is a type of resistance resistor that acts as a sacrificial device to provide over current protection, of either the load or source circuit. It essential component is a metal wire or strip that melts when too much current flows, which interrupts the circuit in which it is connected. Short circuit, overloading, incompatible loads or device failure are the prime reasons for excessive current. A fuse interrupts excessive current (blows) so that further damage by overheating or fire is prevented. Wiring regulations often define a maximum fuse current rating for particular circuits. Over current protection devices are essential in electrical systems to limit threats to human life and property damage. Fuses are selected to allow passage of normal current plus a marginal percentage and to allow excessive current only for short periods. Slow blow fuses are designed to allow higher currents for a modest amount of time longer, and such considerations are and were commonly necessary when electronic devices or systems had electronic tube tech or a large number of incandescent lights were being powered such as in a large hall, theater or stadium. Tubes and incandescent lights each have reduced current needs as they heat up to operating temperatures for their internal resistance grows as they are heated. The same physics principle causes the fuse material to melt, disconnection the circuit from power.