RP 156A - Electrical Circuit Protection Components: Fuses

Fuses are characterized by an internal metallic element (link) that is calibrated to melt at a specific current. This calibration enables different current ratings to be obtained. A fuse protects a wiring system when this element melts, opening the circuit. The fuse must then be replaced since the element is not repairable. A fuse is typically faster acting than a circuit breaker. A time to open versus current curve is used to help select the proper rating for a circuit based on the expected circuit load.

The Mini, ATC and Maxi fuse case color indicates the rating using a color convention chart illustrated in FUSE RATING AND COLOR.  The rating is also typically printed on the fuse case. The fuse element is also typically visible and provides a clear indication when the element has melted during an over current event. The Mega style bolt in fuse may or may not have color-coded cases, but should have a marking which is color coded to the amperage rating. Glass case fuses will have their rating printed on the fuse or stamped into the fuse ferrule and will not use color as an identifier.

The Mini, ATC and Maxi fuses refer to different physical size fuses. There are different current ratings available in each family with some duplication. These fuses are typically plastic cases containing the fuse element with metal terminals.

The MEGA/AMG and MIDI/AMI fuses are used in high current applications. The fuse usually is mounted to a threaded attachment fixture and secured with nuts. The MEGA fuse uses an internal metallic element calibrated to melt at specific heat induced levels.

The J-case fuse is similar in amperage capacity to a Maxi style fuse, however it is basically square in area. The engagement area of the fuse is comprised of female terminals and is intended to be plugged into an electrical center with matching male blades, spaced apart and tall enough to satisfactorily engage the fuse. The fuse element is positioned between the terminals internally near the top of the fuse, where a clear plastic window allows viewing for detection of an open element.

Glass fuses also utilize a calibrated fuse element in-side a glass case with a conducting cap at each end. This cap holds the fuse in the fuse holder or panel and conducts the circuit current through the element. Glass fuses vary by rating and physical size and are basically used with in-line fuse holders. Usage of this type of fuse on new design is very limited.