Fuseology Selection Guide Page 6 Fuseology Selection Guide

Fuseology Selection Guide www.littelfuse.com 4. OVERLOAD CURRENT CONDITION: The current level for which protection is required. Fault conditions may be specified, either in terms of current or, in terms of both current and maximum time the fault can be tolerated before damage occurs. Time-current curves should be consulted to try to match the fuse characteristic to the circuit needs, while keeping in mind that the curves are based on average data. 5. MAXIMUM FAULT CURRENT: The Interrupting Rating of a fuse must meet or exceed the Maximum Fault Current of the circuit. 6. PULSES: The general term "pulses" is used in this context to describe the broad category of wave shapes referred to as "surge currents" , "start-up currents" , "inrush currents" , and "transients" . Electrical pulse conditions can vary considerably from one application to another. Different fuse constructions may not react the same to a given pulse condition. Electrical pulses produce thermal cycling and possible mechanical fatigue that could affect the life of the fuse. Initial or start-up pulses are normal for some applications and require the characteristic of a Slo-Blo fuse. Slo-Blo fuses incorporate a thermal delay design to enable them to survive normal start-up pulses and still provide protection against prolonged overloads. The start- up pulse should be defined and then compared to the time- current curve and I 2 t rating for the fuse. Application testing is recommended to establish the ability of the fuse design to withstand the pulse conditions. Nominal melting I 2 t is a measure of the energy required to melt the fusing element and is expressed as "Ampere Squared Seconds" (A 2 Sec.). This nominal melting I 2 t, and the energy it represents (within a time duration of 8 milliseconds [0.008 second] or less and 1 millisecond [0.001 second]or less for thin film fuses), is a value that is constant for each different fusing element. Because every fuse type and rating, as well as its corresponding part number, has a different fusing element, it is necessary to determine the I 2 t for each. This I 2 t value is a parameter of the fuse itself and is controlled by the element material and the configuration of the fuse element. In addition to selecting fuses on the basis of "Normal Operating Currents" , "Rerating" , and "Ambient Temperature" as discussed earlier, it is also necessary to apply the I 2 t design approach. This nominal melting I 2 t is not only a constant value for each fuse element design, but it is also independent of temperature and voltage. Most often, the nominal melting I 2 t method of fuse selection is applied to those applications in which the fuse must sustain large current pulses of a short duration. These high-energy currents are common in many applications and are critical to the design analysis. The following example should assist in providing a better understanding of the application of I 2 t. EXAMPLE: Select a 125V, very fast-acting PICO II fuse that is capable of withstanding 100,000 pulses of current (I) of the pulse waveform shown in Figure 1. The normal operating current is 0.75 ampere at an ambient temperature of 25C. Step 1 - Refer to Chart 1 and select the appropriate pulsewaveform, which is waveform (E) in this example. Place the applicable value for peak pulse current (i p ) and time (t) into the corresponding formula for waveshape (E), and calculate the result, as shown: 1 1 I 2 t= - (i P ) = I 2 t = - (i P ) 2 t 5 5 1 -8 2 .004 = 0.0512 A 2 Sec. 5 This value is referred to as the "Pulse I 2 t" . Step 2 - Determine the required value of Nominal Melting I 2 t by referring to Chart 2. A figure of 22% is shown in Chart II for 100,000 occurrences of the Pulse I 2 t calculated in Step 1. This Pulse I 2 t is converted to its required value of Nominal Melting I 2 t as follows: Nom. Melt I 2 t = Pulse I 2 t/.22 0.0512/.22 = 0.2327 A 2 Sec. Step 3 - Examine the I 2 t rating data for the PICO II, 125V, very fast-acting fuse. The part number 251001, 1 ampere design is rated at 0.256 A 2 Sec., which is the minimum fuse rating that will accommodate the 0.2327 A 2 Sec. value calculated in Step 2. This 1 ampere fuse will also accommodate the specified 0.75 ampere normal operating current, when a 25% derating factor is applied to the 1 ampere rating, as previously described. 7. PHYSICAL SIZE LIMITATIONS: Please refer to the product dimensions presented in current Littelfuse product data sheets for specific information. 8. AGENCY APPROVALS: For background information about common standards, please consult the STANDARDS section of this guide or visit our Design Support web site at www.littelfuse.com/design-support.html. For specific agency approval information for each Littelfuse product, please refer to the data sheets within this catalog and information presented on www.littelfuse.com. As agency approvals and standards may change, please rely on the information presented on www.littelfuse.com as current information. 9. FUSE FEATURES: Please consult the specific product features presented within this catalog and on our web site at www.littelfuse.com. For additional information and support contact your Littelfuse product representative. Fuse Selection Checklist (continued) 6

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