Fuses protect electrical devices and components from overcurrents and short circuits that occur in improperly operating circuits. When fault current reaches a predetermined magnitude for a fixed period of time, current flow is interrupted through the melting of an internal element.
Important performance ratings to consider when specifying fuses include voltage rating, current rating, rated braking capacity, and interrupt rating. The voltage rating of the fuse is the maximum voltage up to which a safe fault current interruption will occur. Consider the maximum effective operating voltage of the circuit. The rated current of the fuse should consider the maximum continuous operating current of the circuit and the influence of the ambient temperature. The rated breaking capacity of the fuse (interrupting rating) is the short circuit current at which the fuse can blow (at the rated voltage) without destruction or arcing being maintained. Consider the maximum possible fault current. Interrupt ratings can be high interrupt, medium interrupt, or low interrupt. Important performance characteristics to consider include fast acting and time lag. Quick-acting fuses are typically used in circuits with small inrush currents or where overcurrent or short circuit must be quickly interrupted, such as resistive loads. Time-delay fuses are used where high starting inrush currents will occur and decay gradually, such as inductive or capacitive loads. Important environmental parameters to consider when specifying fuse holders include operating temperature.
General specifications that apply to fuses include mountings, materials of construction, fuse types, and features. Mounting choices include solderable or surface mount, solderable with leads, and replaceable with holder or clips. Common materials of construction include glass, ceramic, and sand. Fuse types can be miniature, subminiature or micro, midget, automotive, blade type, PC board, and protective. Miniature cartridge fuses are commonly available in dimensions of 5x20mm and 6.3x32mm. Subminiature fuses have very small dimensions (no principal dimension greater than 10 mm) and are ideal for compact circuit board layouts. Subminiature fuses are also called microfuses. Midget refers to 13/32" diameter fuses. Automotive fuses are specifically for use in automobiles. Blade-type fuses are typically used in low voltage, high current applications. PC board fuses are specifically for use on PC boards. Protective fuses are used for protection of secondary circuits or low voltage ICs. Features include resettable and indicating. Resettable (polymeric or positive temperature coefficient - PTC) fuses are unlike traditional fuses that have to be replaced after a fault condition, this may not be necessary when using resettable fuses. The internal resistance of the component increases in response to an overcurrent or a short-circuit condition, limiting subsequent disturbances. After the fault condition is removed, the device cools and the internal resistance drops to nearly its initial value, permitting normal equipment operation to resume. Indicating fuses have visual indication and / or alarm activation. Used for telecommunication, computer and control circuit applications.