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Choosing the right fuse

Choosing the right fuse can prevent damage to equipment, prevent costly maintenance, and protect the user

Santa Rosa, California

The role of circuit protection devices has traditionally been described as perhaps the least important aspect of a design: an afterthought and often irritating details. Today, the refinement of circuit design and the proper selection of a protective device require careful thinking in the early stages and everywhere.

It requires a competency, knowledge of different types of devices, an understanding of the different functions they provide, and the ability to determine the most appropriate device for an application. Choosing the right fuse ensures that the device continues to function properly and prevents expensive maintenance work due to failures. It protects the equipment as a whole and, more importantly, the safety of the user.

Choosing the right fuse requires careful consideration in the early stages of design as well as throughout the design.

Operating states of the circuit

To begin with choosing the appropriate fuse, one must understand the type of circuit that powers and protects it. Basic operating factors such as maximum steady-state voltage, current values ​​and ambient temperature must be defined. In addition, it is necessary to understand the peak value and duration / shape of surge currents that can exist.

Nominal current and ambient temperature

Like most electrical components, fuses must be reduced through temperature. For example, at 60C, a circuit that would use a 1A time delay fuse at room temperature would need a 1.25A fuse to support operation at the higher temperature (see Figure 1).

Fig. 1. The derating curve shown is a general curve for mean time delay (T) and fast action (F). Note the manufacturer's product-specific characteristics for each fuse type. It is important to note that the fuse has some resistance and therefore it will cause a voltage drop and degrade performance. Time-delayed fuses usually have lower voltage drops and power dissipation values ​​than equivalent fast-acting fuses.

For example, a 2 A 5 x 20 mm delayed fuse has a typical voltage drop of 60 mV, while the fast version has a voltage drop of 90 mV. The reason for this lies in the delayed fuses with a thicker fuse wire diameter, which results in a higher I²t value or energy required to melt the fuse wire. In addition, the safety wire is tinned. This means that fast-acting fuses in normal operation heat up to a higher temperature level before the interruption.

Backup location

The location of the fuse in a circuit is also important to prevent unnecessary heat build-up. The type and proximity of other components in the vicinity of the fuse will affect the ambient temperature.

Higher temperatures can affect the time and current properties specified by the manufacturer. Note that measures to dissipate heat directly from the fuses - enlarged solder pads, heat sinks, fans - can change the specified performance characteristics.

Breaking capacity

Breaking capacity is the maximum fault current at which the fuse can safely interrupt. Fuses that are used in situations in which fault currents exceed the breaking capacity can, in extreme cases, catch fire or even explode.

For example, a fuse with a breaking capacity of 35A should never be used if the power supply source exceeds 35A in the worst case, but should provide adequate protection when fault currents do not exceed 35A factors.

As a rule of thumb, it is recommended to use a highly break-proof fuse for circuits with inductive loads that are smaller than 0.9. A fuse with a low breaking capacity is usually sufficient for circuits with resistive / capacitive loads.

The actual power factor of the device can influence the values ​​given by the manufacturer. Some manufacturers offer additional ratings at different power factors to help customers determine the suitability of a product.

Time-current properties

Some applications have specific and easy to understand needs when it comes to the speed at which the fuse should blow. Sensitive semiconductor circuits often require fast-acting fuses that blow within a very short time, while devices that require large inrush currents may need a time-delayed fuse to prevent disruptive failures.

In cases where both types can be used, it is helpful to base your selection on the effects of ambient temperature and note that time-delayed fuses typically have a lower voltage drop than fast acting fuses and therefore consume less current. Pre-burning times with moderate overcurrents (1 <2, 5 * In) are almost the same (see Fig. 2). In the case of larger overcurrents (1 = 10, 0 * In), time-delayed fuses have higher switching times than fast-acting fuses.

Fig. 2. Pre-burn times for fast and time-delayed fuses with moderate overcurrents are almost the same.

Calculation of the melting energy

I 2 t is a measure of the energy required to melt the fuse wire in the fuse. The approximate time it takes for a fuse to blow can be determined by dividing the manufacturer's I 2 t value by the square of the expected fault current.

In the case of a fuse with I 2 t = 4.5 A 2 s, the expected fault current is z. B. 13 A:

t type = I 2 t / I 2 = 4.5 A 2 s / (1. 25 A * 10) 2 = 28.8 ms

Conversely, if the expected fault current value is known and taking into account a certain time in which a fuse is to penetrate, it can be determined which I 2 t value is required for a specific requirement. Once the required I 2 t has been determined, it is relatively easy to review the specifications of a potential fuse to find the fuse that meets your needs.

Available product types

The IEC standards cover miniature fuses for devices in a variety of different sizes and housing types, from 5x20mm cartridge types to 1206 SMD. Terminal variations include pig-tails, through-hole and SMD and are offered in bulk or tape and reel, typically for chip fuses.

There are some clear differences between IEC standards and UL standards, and when looking at domestic and export markets, compliance with one or the other or both must be considered. It is not necessarily possible to replace a UL fuse with an IEC, and vice versa, although one might think that a 1A fuse is just like another. Time-current characteristics vary, and if a UL fuse blows at 1A within hours, an IEC-designed fuse should not blow at all at 1A and could require a 2A fuse.

Make the final selection

As soon as the nominal voltage and currents, the breaking capacity and the time-current properties are determined together with the current I 2 t and the desired packet types are known, it is possible to examine candidates for an adaptation. If the fuse is to be installed in a fuse holder, it is particularly important to observe the power loss limits of the fuse holder and the fuse.

The right fuse holder

Fuses are often mounted in brackets for easy replacement. The fuse holder will have additional important electrical properties: contact resistance and maximum permissible fuse power loss.

You also have to consider that the holder and the fuse also have to be throttled at higher temperatures. Fuse holders are available in many styles for panel mounting in equipment that requires user replaceable fuses, and as clips or blocks that can be mounted to the housing or PCB in the equipment.

Both types can be found with quick connect terminals or PCB terminals, and PCB types are available for through hole or SMD connections. PCB brackets are available for vertical or horizontal mounting to allow flexibility in enclosure construction. Some types are even available with fuses already installed for faster assembly and lower installation costs. ■