Fuseology Selection Guide Page 2 Fuseology Selection Guide

Fuseology Selection Guide www.littelfuse.com The purpose of this introductory section is to promote a better understanding of both fuses and common application details within circuit design. The fuses to be considered are current sensitive devices designed to serve as the intentional weak link in the electrical circuit. Their function is to provide protection of discrete components, or of complete circuits, by reliably melting under current overload conditions. This section will cover some important facts about fuses, selection considerations and standards. The application guidelines and product data in this guide are intended to provide technical information that will help with application design. The fuse parameters and application concepts presented should be well understood in order to properly select a fuse for a given application. Since these are only a few of the contributing parameters, application testing is strongly recommended and should be used to verify performance in the circuit / application. Littelfuse reserves the right to make changes in product design, processes, manufacturing location and information without notice. For current Littelfuse product infomation, please visit our web site at www.littelfuse.com. AMBIENT TEMPERATURE: Refers to the temperature of the air immediately surrounding the fuse and is not to be confused with "room temperature." The fuse ambient temperature is appreciably higher in many cases, because it is enclosed (as in a panel mount fuseholder) or mounted near other heat producing components, such as resistors, transformers, etc. BREAKING CAPACITY: Also known as interrupting rating or short circuit rating, this is the maximum approved current which the fuse can safely break at rated voltage. Please refer to the INTERRUPTING RATING definition of this section for additional information. CURRENT RATING: The nominal amperage value of the fuse. It is established by the manufacturer as a value of current which the fuse can carry, based on a controlled set of test conditions (See RERATING section). Catalog Fuse part numbers include series identification and amperage ratings. Refer to the FUSE SELECTION CHECKLIST section for guidance on making the proper choice. RERATING: For 25C ambient temperatures, it is recommended that fuses be operated at no more than 75% of the nominal current rating established using the controlled test conditions. These test conditions are part of UL/CSA/ANCE (Mexico) 248-14 "Fuses for Supplementary Overcurrent Protection," whose primary objective is to specify common test standards necessary for the continued control of manufactured items intended for protection against fire, etc. Some common variations of these standards include: fully enclosed fuseholders, high contact resistances, air movement, transient spikes, and changes in connecting cablesize (diameter and length). Fuses are essentially temperature-sensitive devices. Even small variations from the controlled test conditions can greatly affect the predicted life of a fuse when it is loaded to its nominal value, usually expressed as 100% of rating. The circuit design engineer should clearly understand that the purpose of these controlled test conditions is to enable fuse manufacturers to maintain unified performance standards for their products, and he must account for the variable conditions of his application. To compensate for these variables, the circuit design engineer who is designing for trouble-free, long-life fuse protection in his equipment generally loads his fuse not more than 75% of the nominal rating listed by the manufacturer,keeping in mind that overload and short circuit protection must be adequately provided for. The fuses under discussion are temperature-sensitive devices whose ratings have been established in a 25C ambient. The fuse temperature generated by the current passing through the fuse increases or decreases with ambient temperature change. The ambient temperature chart in the FUSE SELECTION CHECKLIST section illustrates the effect that ambient temperature has on the nominal current rating of a fuse. Most traditional Slo-Blo Fuse designs use lower melting temperature materials and are, therefore, more sensitive to ambient temperature changes. DIMENSIONS: Unless otherwise specified, dimensions are in inches. The fuses in this catalog range in size from the approx. 0402 chip size (.041"L.020"W.012"H) up to the 5 AG, also commonly known as a"MIDGET" fuse (13/32" Dia.11/2" Length). As new products were developed throughout the years, fuse sizes evolved to fill the various electrical circuit protection needs. The first fuses were simple, open-wire devices, followed in the 1890's by Edison's enclosure of thin wire in a lamp base to make the first plug fuse. By 1904, Underwriters Laboratories had established size and rating specifications to meet safety standards. The renewable type fuses and automotive fuses appeared in 1914, and in 1927 Littelfuse started making very low amperage fuses for the budding electronics industry. The fuse sizes in following chart began with the early "Automobile Glass" fuses, thus the term "AG" . The numbers were applied chronologically as different manufacturers started making a new size: "3AG," for example, was the third size placed on the market. Other non-glass fuse sizes and constructions were determined by functional requirements, but they still retained the length or diameter dimensions of the glass fuses. Their Fuse Characteristics, Terms and Consideration Factors 2

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