Fuse Selection White Paper Find the Right Fuse! Criteria for Correct Fuse Selection Circuitry overcurrent protection rarely receives the attention it deserves. An inadequately thought out selection of fuses can lead to the breakdown of equipment and installation, resulting in high replacement costs and dissatisfied customers. This technical article focuses on the correct selection criteria for fuses and fuseholders, and should help you take the more important aspects into consideration. Normal operation after switching ON is explained under Point 1. This Point should always be taken into consideration. Point 2 is only necessary with capacitive loads, present when the charging of capacitors after switching ON, leads to high in-rush current peaks and the rated current of the fuse is exceeded by many multiples. Important facts with regard to fuseholders are given under Point 3, where the correct selection of fuse and fuseholder combinations is explained. Derating of rated current in % Fig. 1: shows the de-rating curve of the time-lag SMD fuse UMT 250. The function of a fuse is to interrupt an uncontrolled fault current or overcurrent before serious damage can occur, such as the overheating of equipment. Because a fuse is designed using a fusing element, it is particularly suited for reliable interruption of short-circuits. With overcurrents up to 2x or 3x the rated current, a fuse becomes less accurate and, as such, not so well suited for these conditions. Other overcurrent protection measures such as electronic protection, thermal overload elements or additional fuses are then necessary. Point 1: Normal Operation After Switching ON Under normal operating conditions, a fuse is subjected to a maximum operating current and a maximum operating temperature. A «derating» of the rated current is therefore usually necessary since a fuse is rarely operated at the set ambient temperature of 23 ˚C. As an example, let’s look at a scenario using a time-lag SMD fuse such as the UMT 250 from SCHURTER. With an operating temperature of 60 ˚C, in accordance with Fig. 1 the fuse needs to be derated by 17%, i.e., when the

Fuse Selection White Paper operating current is 1 A @ 60 ˚C, a rounded-up fuse value of 1.25A (1A / 0.83) is necessary. Fuses can be in accordance with IEC 60127 or UL 248-14. Because of the different definitions between the two standards, fuses are not directly interchangeable as follows: fuses in accordance with IEC 60127 may be operated continually at 100% of the rated current value, whereas fuses in accordance with UL 248-14 only at 75%. UL 248-14 specifies a minimum of 4h operating time at rated current (Table 1). The self-heating effect of time-lag fuses is less than that of quick-acting....

Fuse Selection White Paper Variants Order Number Rated Cur- Rated Voltage Rated Voltage rent [A] [VAC] [VDC] Breaking Capacity Voltage Drop Voltage Drop Power Dis- Melting l2t 10.0 1.0 ln max. 1.0 ln typ. sipation 1.25 ln typ. [A 2s] [mV] [mV] ln typ. [mW] Table 3: Versions of UMT 250 with details of the melting integrals, voltage drops and power losses for every current rating. current peak in excess of this value, the fusing element melts and interrupts the circuit. In our example the I2t value of the fuse (2.8 A2s) is higher than the calculated value (1.748 A2s); that is, the selection would...

Fuse Selection White Paper The fuse power dissipation is calculated with the rated current and typical voltage drop value (voltage level over the fuse at rated current) according to the catalog. Ceramic fuses such as, SPT 5x20, 6.3 A have a typical voltage drop of 70 mV at rated current. Correct dimensioning can be ascertained through the use of the following equation: PFuse= IN*UVoltageDrop_typ Combination is correct layed out, when PFuseholder > PFuse + PContact According to the IEC standard, the contact resistance Rc between fuse and clip is a maximum of 5 mΩ. Power dissipation can be calculated...