Fuseology Selection Guide Page 3 Fuseology Selection Guide

www.littelfuse.com Fuseology Selection Guide designation was modified to AB in place of AG, indicating that the outer tube was constructed from Bakelite, fibre, ceramic, or a similar material other than glass. The largest size fuse shown in the chart is the 5AG, or "MIDGET," a name adopted from its use by the electrical industry and the National Electrical Code range which normally recognizes fuses of 9/16" 2" as the smallest standard fuse in use. FUSE SIZES SIZE DIAMETER (Inches) LENGTH (Inches) 1AG 1/4 .250 5/8 .625 2AG - .177 - .588 3AG 1/4 .250 1 1.25 4AG 9/32 .281 1 1.25 5AG 13/32 .406 1 1.50 7AG 1/4 .250 7/8 .875 8AG 1/4 .250 1 TOLERANCES: The dimensions shown in this catalog are nominal. Unless otherwise specified, tolerances are applied as follows. Tolerances do not apply to lead lengths: .010" for dimensions to 2 decimal places. .005" for dimensions to 3 decimal places. Contact Littelfuse should you have questions regarding metric system and fractional tolerances. FUSE CHARACTERISTICS: This characteristic of a fuse design refers to how rapidly it responds to various current overloads. Fuse characteristics can be classified into three general categories: very fast-acting, fast-acting, or Slo-Blo Fuse. The distinguishing feature of Slo-Blo fuses is that these fuses have additional thermal inertia designed to tolerate normal initial or start-up overload pulses. FUSE CONSTRUCTION: Internal construction may vary depending on ampere rating. Fuse photos in this catalog show typical construction of a particular ampere rating within the fuse series. FUSEHOLDERS: In many applications, fuses are installed in fuseholders. These fuses and their associated fuseholders are not intended for operation as a "switch" for turning power "on" and "off " . INTERRUPTING RATING: Also known as breaking capacity or short circuit rating, the interrupting rating is the maximum approved current which the fuse can safely interrupt at rated voltage. During a fault or short circuit condition, a fuse may receive an instantaneous overload current many times greater than its normal operating current. Safe operation requires that the fuse remain intact (no explosion or body rupture) and clear the circuit. Interrupting ratings may vary with fuse design and range from 35 amperes for some 250VAC metric size (520mm) fuses up to 200,000 amperes for the 600VAC KLK series. Information on other fuse series can be obtained from the Littelfuse. Fuses listed in accordance with UL/CSA/ANCE 248 are required to have an interrupting rating of 10,000 amperes at 125V, with some exceptions (See STANDARDS section) which, in many applications, provides a safety factor far in excess of the short circuit currents available. NUISANCE OPENING: Nuisance opening is most often caused by an incomplete analysis of the circuit under consideration. Of all the "Selection Factors" listed in the FUSE SELECTION CHECKLIST, special attention must be given to items 1, 3, and 6, namely, normal operating current, ambient temperature, and pulses. For example, one prevalent cause of nuisance opening in conventional power supplies is the failure to adequately consider the fuse's nominal melting I 2 t rating. The fuse cannot be selected solely on the basis of normal operating current and ambient temperature. In this application, the fuse's nominal melting I 2 t rating must also meet the inrush current requirements created by the input capacitor of the power supply's smoothing filter. The procedure for converting various waveforms into I 2 t circuit demand is given in the FUSE SELECTION CHECKLIST. For trouble-free, long-life fuse protection, it is good design practice to select a fuse such that the I 2 t of the waveform is no more than 20% of the nominal melting I 2 t rating of the fuse. Refer to the section on PULSES in the FUSE SELECTION CHECKLIST. RESISTANCE: The resistance of a fuse is usually an insignificant part of the total circuit resistance. Since the resistance of fractional amperage fuses can be several ohms, this fact should be considered when using them in low-voltage circuits. Actual values can be obtained by contacting Littelfuse. Most fuses are manufactured from materials which have positive temperature coefficients, and, therefore, it is common to refer to cold resistance and hot resistance (voltage drop at rated current), with actual operation being somewhere in between. Cold resistance is the resistance obtained using a measuring current of no more than 10% of the fuse's nominal rated current. Values shown in this publication for cold resistance are nominal and representative. The factory should be consulted if this parameter is critical to the design analysis. Hot resistance is the resistance calculated from the Fuse Characteristics, Terms and Consideration Factors (continued) 3

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