White Paper Fuse Selection www.schurter.com/downloads 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 ex- plained 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 ex- ceeded by many multiples. Important facts with regard to fuseholders are given under point 3, where the correct selec- tion of fuse and fuseholder combinations is explained. 40% 30% 20% 10% 0 -10% -20% -30% -40% -40C -20C 0C 20C 40C 60C 80C 100C Ambient Temperature Ta C Fig. 1: shows the de-rating curve of the time-lag SMD fuse UMT 250. Point 1: Normal Operation After Switching ON Under normal operating conditions, a fuse is subjected to a maximum operating cur- rent and a maximum operating temperature. A derating of the rated current is therefore usually necessary since a fuse is rarely oper- The function of a fuse is to interrupt an uncon- With overcurrents up to 2x or 3x the rated cur- ated at the set ambient temperature of 23 C. trolled fault current or overcurrent before seri- rent, a fuse becomes less accurate and, as As an example, lets look at a scenario using a ous damage can occur, such as the overheat- such, not so well suited for these conditions. time-lag SMD fuse such as the UMT 250 from ing of equipment. Because a fuse is designed Other overcurrent protection measures such SCHURTER. With an operating temperature of using a fusing element, it is particularly suited as electronic protection, thermal overload ele- 60 C, in accordance with fig. 1 the fuse needs for reliable interruption of short-circuits. ments or additional fuses are then necessary. to be derated by 17%, i.e., when the operat- Fuses 01 Derating of rated current in %White Paper Fuse Selection www.schurter.com/downloads Pre-Arcing Time ing current is 1 A 60 C, a rounded-up fuse ON. These pulses can be many multiples of the value of 1.25A (1A / 0.83) is necessary. rated current of the fuse, but are mostly of a very Rated Current In 1.0 x In min. 2.0 x In max. Fuses can be in accordance with IEC 60127 or short duration. UL 248-14. Because of the different definitions The area beneath the curve is called the melt- 2 2 0.375 A - 5 A 4 h 60 s between the two standards, fuses are not ing integral or I t value. The I t value is defined directly interchangeable as follows: fuses in as that amount of energy necessary to heat accordance with IEC 60127 may be operated Table 1: Pre-arcing time of a fuse continually at 100% of the rated current value, according to UL 248-14. whereas fuses in accordance with UL 248-14 Inrush Current only at 75%. UL 248-14 specifies a minimum 13A of 4h operating time at rated current (table 1). a certain temperature has been reached when the fuse element melts (opens) and interrupts The self-heating effect of time-lag fuses is less the circuit. I RMS than that of quick-acting. This can be seen All measures for cooling such as ventilation, from the typical values of voltage drop. For heat sinks, large solder surfaces or heat ac- example, a 2 A 5x20 mm glass time-lag fuse cumulators (fig. 2) change the time/current has a typical voltage drop of 60 mV, whereas a characteristic of the fuse and thus should be 5.4 A typical quick-acting version is 90 mV. This dif- avoided. 2 I t ference is evident by the thicker fuse element 2.6 A 2 (higher melting value I t, see point 2) that is Point 2: In-Rush Current Peaks 1.6 A Operating Current 1 A 1 A necessary for time-lag fuses. It should also be In-rush current peaks (fig. 3) arise through ca- Time 0.006 s 0.012 s 0.018 s 0.024 s 0.03 s noted that fuses are heated by the current until pacitors that are initially charged when switched 1 2 3 4 5 Fig. 3: Typical switch-on curve of a SMPS when capacitors need to be charged. up and melt a wire or fuse element. Generally this is an exponential curve having a peak cur- rent value of Ip over a period of time called , at which point the current has reached 37% of the peak current value. As an example, lets take the time-lag SMD fuse UMT 250, 1 A. The 2 I t-value can be calculated with the following formula using a peak current of Ip = 13 A and a = 6 ms: 2 2 I tApplication =0.5*Ip * 2 2 2 I tApplication =0.5*(13 A) *6 ms=0.507 A s In addition to the start-up current, the number of pulses over the life of the equipment must be taken into account since premature ageing of the fuese needs to be considered. A factor of 0.29 is used for 10 000 pulses with time-lag fuses (see table 2). 2 2 I tmin Fuse T =I tApplication/F 2 2 2 I tmin Fuse T =0.507 A s/0.29=1.748 A s Manufacturers provide the melting integral val- ue in the catalog for each fuse type and rated current. For example, (table 3) with the IEC time-lag SMD fuse UMT 250. The 1 A rated Fig. 2: Fuses mounted in holders can influence 2 2 current fuse has an I t value of 2.8 A s, that is, each other with regard to temperature. with an over-current (short-circuit) or an inrush Fuses 02