TESLA INSTITUTE: explanation

The electrical system in every home has some form of circuit protection to shut off circuits in the event of an overload, short circuit or ground fault. In homes built after about 1965—or in older homes in which the electrical service has been updated - this protection is usually provided by a series of circuit breakers in the main service panel. Circuit breakers are mechanical devices that sense the amount of current flow and "trip" when the current flow exceeds the safe capacity of the circuit wires. However, if you have a home built before 1960 and the electrical service has not been updated, there is a good chance that you have a different of circuit protection—screw-in fuses found inside a main fuse panel.

How Fuses Work

Fuses are relatively simple devices. The fuses that protect individual 120-volt circuit are typically ceramic screw-in plugs that fit into threaded sockets in the fuse panel. A thin metal strip inside the fuse conducts all electrical flow through the circuit and if the current flow exceeds the current-carrying capacity of the metal strip, it overheats and melts, thereby interrupting the flow of current and shutting off the circuit. The fuse is a kind of early-warning system, which senses overloads and "blows" before the circuit wires themselves can overheat and possibly cause fire.

Larger 240-volt circuits, as well as the main fuse that controls the