Five Criteria For Fuse Selection | Engineers' Insight | Avnet Abacus

Finding the right fuse for the task: Five criteria for fuse selection

By Jerry Moss | August 11, 2021

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One of the earliest and simplest electrical inventions, the fuse, has the job of being the weakest link in a circuit. When a fuse element is heated by the current to a certain temperature, it melts and interrupts the current. In this way, this inexpensive component prevents uncontrolled fault current or overcurrent from damaging a system.

Whatever the size, use or value of the electronic system, one or multiple fuses are needed for primary and secondary protection, on or off the circuit board. A good choice ensures the system is protected for the duration of its lifetime; a poor choice means an overwhelmed product support department at best.

Aside from packaging choice (SMD, thru hole, cartridge) and current and voltage ratings (AC and DC power), there are some important criteria to consider when choosing fuses:

1. Maximum operating current & operating temperature

Figure 1: De-rating curve for SCHURTER’s UMT 250 time-lag SMD fuse

Under normal operating conditions, a fuse is subjected to a maximum operating current and a maximum operating temperature. Fuse current ratings are measured at 23˚C but since a fuse is rarely operated at this temperature, ‘derating’ of the rated current is usually necessary. The higher the ambient temperature, the quicker the fuse will blow, so a higher amperage is necessary.

Taking the example of SCHURTER’s UMT 250 series of time-lag SMD fuses. When the operating temperature is 60˚C, a 1A fuse needs to be derated by 17%, so a rounded-up fuse value of 1.25A (1A / 0.83) would be required (figure 1, right).

2. Time-lag or quick-acting

Referred to as ‘trip characteristics’, a key decision is whether to use a time-lag or quick-acting fuse. With a thicker fuse element than their quick-acting counterparts, time-lag fuses trip at a slower rate at high current. A low resistance to current inrushes makes quick-acting fuses suitable for resistive loads, such as data or signal lines. Time-lag fuses demonstrate high resistance to inrush currents and suit capacitive loads, making them a better choice for motors, power supplies and any circuits with capacitors.

3. Melting integral value

Inrush current peaks occur when capacitors that are initially charged are switched on (figure 2, below). These take the form of short duration pulses and when they occur often, cause fuses to age prematurely. It is for this reason that manufacturers provide a melting integral value, or I2t value, and rated current for each fuse type. The I2t value represents the amount of thermal energy needed to melt the wire or fuse element. Displayed in graph form, the melting integral shows an exponential curve with a peak current value of Ip over a period of time, T. It can be calculated using the following formula: I2t Application = 0.5*Ip2*τ

SMPS-Switch-On-Curve-EN-Image

Figure 2: Typical switch-on curve of a SMPS when capacitors need to be charged

An overcurrent or inrush current peak in excess of the I2t value would melt the fusing element and interrupt the circuit, so the I2t value of the fuse must be higher than the calculated value for the application.

4. Number of pulses over lifetime

The number of pulses over the life of the equipment must be taken into account. A factor of 0.29 is used for 10,000 pulses with time-lag fuses.

Pulse resistant fuses are available, such as SCHURTER’s UMT-H, UMF 250 and UMT 250 series.

5. IEC 60127 or UL 248-14

The rated current of a fuse is designed according to either IEC 60127 or UL 248-14. The two standards mean that fuses are not directly interchangeable. Broadly, fuses designed in accordance with IEC 60127 may be operated continually at 100% of the rated current value, whereas fuses in accordance with UL 248-14 may be operated only at 75%. UL 248-14 specifies a minimum of 4h operating time at rated current.

IEC and UL standards also apply to fuse holders. IEC 60127-6 standard fuse holders need to specify rated power acceptance (e.g., 2.5W / 10A @ 23°C), rated current and rated voltage. For fuse holders approved to UL 512 and CSA C22.2 no. 39, only the rated current and rated voltage are specified.

Whichever standard your chosen fuse adheres to, bear in mind that fuses mounted in holders can influence each other with regard to temperature e.g., an operating current of 5A at 60 °C would result in a temperature increase in the fuse holder, leading to a need for derating. Fuse holder specifications have a recommended derating curve that should be taken into account.

Finally, a fuse might not solve the problem of overcurrents. When overcurrents are twice or three-times the rated current, a fuse becomes less accurate and is not suited to the task. Alternative overcurrent protection measures such as electronic protection, thermal overload elements or additional fuses are then necessary.

So with your fuse selection criteria in mind, it’s time to find the right fuse for your application. SCHURTER has a range of high performance SMD fuses and fuse holders designed for precise performance and circuit protection, with stock available for pilot samples from Farnell. Alternatively, if you have a question or would like to discuss your design in more detail, our team of technical specialists is on hand to help. Get in touch in your local language.