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A fuse is a temperature-sensitive device that serves as an intentional weak link in an electrical circuit. It interrupts a circuit when an over-current condition, such as an overload or short-circuit, occurs. The over-current condition causes the fuse metallic element to melt, thereby opening the circuit and protecting downstream circuits. An overload is an over-current event which is confined to normal current paths. An overload occurs when the current exceeds the value for which the wires or equipment are rated. This can happen when too many devices are connected to the circuit, or when a device connected to the circuit malfunctions in a way that causes it to draw higher than normal current, usually in the range of one to six times normal current. Sustained overloads eventually overheat circuit components. Therefore, fuses must open circuits experiencing sustained overloads before damage occurs. A short-circuit event occurs when current flows out of its normal path. It occurs when an accident or malfunction creates an unintended path for the electricity to flow from the power source to ground. This shorter, more direct path to ground bypasses the resistance normally offered by the wiring and devices connected to the circuit. With virtually no resistance left to impede current flow, the voltage forces higher and higher current to flow through the wires to the point of the short-circuit. Under such a condition, the current will quickly build to such a high level that the heat generated can cause insulation to burn and equipment to be damaged unless the circuit is opened through the use of a fuse.
