Introduction to Selection Specifications for Fuses

Overview of fuse:

1. Structure: Small tubular fuse wires are often used for circuit overcurrent protection. The role of the tube body containing metal connection terminals on both sides of the element and the metal melt housing part in the pipe is to support and connect. The appearance of most fuses is cylindrical, called tubular; Key functions are determined by the internal solution.

2. Function: Fuses are connected in series in a circuit, generally requiring low resistance (low power consumption). Therefore, when the circuit operates normally, the fuse is only equivalent to a single wire, which can be used stably for a long time; When the current fluctuates due to power supply or external influences, the fuse can also withstand a fixed load range; "There is only a large overload current in the circuit - a fault or short circuit - that only the fuse can move, cutting off the current to protect the safety of the circuit.".

Principle: When the fuse wire is energized, due to the current converting heat, the temperature of the solution will rise, and when the load is due to the operating current or possible overload current, the heat generated by the current and the heat emitted by the radiation, convection, and transmission of the body and the surrounding environment based on the heat generated by the solution and the shell can gradually reach a flat balance; If the heat removal rate cannot keep up with the heating, these heat will be deposited on the solution one by one, increasing the temperature of the solution. Once the temperature reaches and exceeds the melting point of the solution material, it will melt and cut off the current, providing safety protection.

Type of fuse

1. Classification by application region: Due to the different starting points and experiences of industrial development around the world, there are still significant differences in the design and application of small conduit fuses, which currently have international recognition, mainly including: European specifications; North American specifications; Japanese specifications: There are other specifications that are only used within a limited range.

2. According to the fusing characteristics: According to different application regulations, various types of fuse breaking characteristics have been designed, and the fuse can also be divided into two types: fast fusing and slow fusing, and then subdivided into ultra-fast fusing; Medium speed fusing and extremely slow fusing, etc.

3. According to the division ability: From the perspective of the safe division current of the fuse, the fuse can be divided into two categories: high division and low division, as well as fuses that improve the division ability between the two.

4. Classification by size: There are many types of sizes for tubular fuses, and the commonly used ones are: Φ 6X30 （3AG）； Φ 5X20； Φ 4X15（2AG）； Φ 3X10； Φ 2X7, etc. 5. According to the type of structure, it is divided into two categories: pipe fuse end caps and solution welding connection methods: in pipe welding and in pipe welding.

According to the connection method, there are two methods for connecting a fuse to a circuit: directly welding it onto the circuit board (called PGT) and connecting it through other connectors.

7. Other classifications: According to the application field, pipeline fuses can be divided into industrial appliances and household appliances; According to the application industry, pipeline fuses can be divided into instrument, communication, power supply, lighting use, vehicle configuration, etc. Depending on where the fuse is connected in the circuit, as well as the primary and secondary fuses.

Parameters and terminology related to fuse links

1. Rated voltage (In)

Indicates the rated operating current on the fuse. This value is determined by the manufacturer as the current that the fuse can carry. The rated voltage is generally in standard gears such as 1, 1.25, 1.5, 1.6, and 2 (enterprise: A)

2. Rated current (Un)

Mark the rated current on the fuse to indicate the usable working voltage of the fuse. The rated current is generally 32, 63, 125, 250, and 600 V. Fuses are sensitive to changes in current rather than voltage. Fuses maintain their original appearance at any voltage from zero to the rated value, so they can be applied to all voltages below the rated current.

3. Voltage drop (Ud)

Voltage reduction at rated voltage at both ends of the fuse

4. Cold resistance (R)

The resistance value of the fuse itself when it is not operating. Most fuses are made with a positive temperature coefficient. Therefore, there are cold resistors and thermal resistors (voltage drop at rated voltage), in which the specific operating resistance is located. The cold resistance can be measured by measuring a current that does not exceed 10% of the nominal rated voltage of the fuse. Thermal resistance is generated based on a current flowing through a fuse that is equal to the nominal rated voltage.

5. Operating temperature

Refers to the temperature of the air directly surrounding the fuse and should not be confused with room temperature. In many specific places, the temperature of fuses is very high, such as fuses installed in enclosed spaces or around heating elements, such as resistors, transformers, inductive coils, etc.

6. Breaking Capacity

Also known as breaking capacity or short circuit short circuit capacity. The current at which the fuse is safely disconnected at a specified voltage. When the instantaneous overload current that may pass through the fuse exceeds the rated value, the fuse may shatter or explode, causing a risk. Therefore, it is specified that the fuse can remain intact (without bursting or breakage) after the protection action. The splitting ability of a fuse depends on the structure of the fuse. Most low division capacity fuses are glass enclosures. High division fuses typically have ceramic shells, many of which are also made of pure granular quartz material.

7. Overload and Time-Current Curves

It is one of the key parameters of a fuse. When the current flowing through the fuse exceeds the rated voltage, the fuse is blown, which is a load. The time current characteristic of a fuse is the relationship between the overload current and the melting time. The time current curve should be based on an average value.

Time Current Characteristics - Specifications

8. Melting heat energy I2t nominal

It is one of the key parameters for selecting a fuse, the energy value required to cut off the fuse, and the original parameter of the fuse itself, expressed in I2T. The I2T value is a parameter of the fuse itself, and its decisive factor is the material of the component and the shape of the fuse equipment, regardless of temperature and voltage.

9. Unless otherwise specified, the specifications are based on mm. Common tubular fuse outline specifications include Φ 6X30， Φ 5X20， Φ 3X10， Φ 2X7； Appearance specifications of common surface mounted fuse wires: 6.1X2.7X2.7, 10.1X3.1X3.1, etc

Selection factors and examples

1. Rated voltage

According to UL standards, note that the current derate for different standards is 0.75, which means that the specific stable operating current should not exceed 75% of In. The IEC standard is 1.0, which means that the specific stable operating current can be equal to In. According to UL specifications for fuses: When operating at 25 ℃, the operating current should not exceed 75% of the rated voltage of the fuse to prevent harmful fusing. For example, fuses with a rated voltage of 10A are generally not recommended to operate above 7.5A at a working temperature of 25 ℃. According to IEC standard fuse: the fuse can operate at rated voltage to complete maintenance. For example, a rated 10A fuse can be used for a specific working current of 10A.

For the working current of a single board, attention should be paid to the current at the allowable voltage. For example, the rated current is - 48-60V, allowing a 20% fluctuation. If the operating current of a board is 0.8A at - 48V, the operating current at - 38V is about 1A due to the stable power of the board. When selecting a fuse, it is necessary to use 1A as the working current of the single board. This should be noted in a wide range of input voltage applications.

During specific use, it is also necessary to consider whether the power chip has an undervoltage effect. For example, when the - 48V power chip is generally under voltage at 48V-35V, some power chips do not have an undervoltage effect, such as Huadian AV10 series power chips, which can work at - 12V, resulting in an input current that is about three times larger than usual.

Generally speaking, the optional current specifications provided by the supplier are lower than the gears described in the specification. It is recommended to select from existing current specifications, and the supplier is not recommended to conduct other designs.

Note: UL listing and UL approval

2. Rated current

The rating of the fuse should be equal to or exceed the high efficiency circuit voltage.

Precautions: Differences and judgments between AC and DC voltages

3. Operating temperature

The current carrying capacity test of fuses shall be conducted at an ambient temperature of 25 ℃, subject to changes in ambient temperature. The higher the ambient temperature, the higher the operating temperature of the fuse, and the shorter its service life. Conversely, operating at lower temperatures will increase the life of the fuse. Therefore, when selecting the rated voltage of a fuse, adjustments should be made based on the actual operating temperature of the fuse.

For example, if the normal working current of a single board is 1.5A, select a slow fusing fuse according to UL standards, and operate at room temperature, then:

Selection of fuse In=normal operating current/standard derating=1.5/0.75=2.0A (operating temperature 25 ℃)

If the fuse operates at a high temperature of 70 ℃, according to curve A (traditional slow fuse) in the figure below, it indicates that the temperature decreases by 80% at 70 ℃. under these circumstances,

Selection of fuse In=normal operating current/(normative derating * operating temperature derating)=1.5/(0.75 * 0.8)=2.5A (operating temperature 70 ℃)

Compare with the above calculation

Specific operating current

Working Practice Working Temperature

Required In1.5A, 25 ℃, 2.0A, 1.5A, 70 ℃, 2.5A, wherein: curve A: traditional slow fuse curve;

Curve B: Curve for fast fuse, fast fuse and spiral wound fuse

Table Common Temperature and Current Comparison Table

The information in the table is a common reduction in humidity (for reference only):

Operating temperature around the fuse*

40 ℃ 50 ℃ 60 ℃ 70 ℃ 80 ℃ 90 ℃ 100 ℃ 110 ℃ slow melting (curve A)

95% 90% 86% 80% 78% 70% 64% 58% rapid melting (curve B)

99% 98% 97% 96% 95% 94% 93% 92% * refers to the temperature of the air directly surrounding the fuse and should not be confused with room temperature. In many specific places, the temperature of fuses is very high, such as those installed in enclosed spaces or around heating elements, such as resistors, transformers, inductive coils, etc

4. Voltage drop and cold resistance

In general, the resistance of a fuse is inversely proportional to the rated voltage. The lower the resistance of a fuse, the lower the power consumption of the fuse. The voltage drop of the fuse shall be tested at the DC rated voltage. Due to the high resistance of fuses with low rated voltage, the impact on low-voltage power is also significant. When selecting small size fuses, attention should be paid to the effect of resistance.

5. Time-current characteristic curve

Is one of the key criteria for selecting a fuse. When a fault current occurs, the correct fusing determines whether the fuse can effectively protect the circuit. The fusing characteristics of each type of fuse have their own time current curve of [y1]. The abscissa of the curve is the current, and the longitudinal axis is the time of rupture. In general, during the selection process, this curve is used as a reference and based on the key points in the curve. The selection of key points varies depending on the type of fuse. UL fuses generally choose 110% In, 135% In, and IEC fuses generally choose key points such as 200% In, 135% In, 210% In, and 275% In. For the relationship between melting and breaking time and key points, refer to the introduction in 3.7. When selecting a fuse, it is necessary to determine how long the protective fault current can safely exist in the circuit.

For example, according to IEC standards, the rated voltage of the fast fuse is 5A. The measurement board has a fault, and the fault current flowing through the fuse is 10A, that is, 200% In. According to the time current characteristic curve of the fuse, at 200% In, it may take 30 minutes for the fuse to fuse. At this time, the fuse is short-circuited, causing the board to be tested to operate under this fault current for 30 minutes. As a result, a fire occurs, indicating that the use of the fuse is inappropriate. Before the fusing starts, the protective device is unsafe and cannot achieve maintenance results.

Precautions: Differences and judgments between fast melting and slow melting fuses

6. Breaking capacity level

Different specifications of fuses have different segmentation capabilities. Please refer to 3.6 for actual data. The rated breaking capacity of the fuse must meet or exceed the fault current in the circuit. When the protection system is directly connected to the power input circuit and the fuse is placed in the power input section, a high division capacity fuse must be used. In most secondary circuits, especially when the voltage is less than the power supply current, it is sufficient to use a fuse with a low division capability.

7. Melting heat energy I2t nominal

Fuses must withstand high energy currents, i.e., large current pulses with short duration, such as impulse currents, starting currents, inrush currents, and other similar "pulse" type circuit transients. The fuse should be able to withstand the kinetic energy of this high energy current without abnormal short circuits. The nominal melting heat energy I2t rating of a fuse is measured by the laboratory, and each type of fuse has only one rated nominal melting heat energy I2t.

For example, in practical applications, the rated nominal melting heat energy I2t of the fuse wire is derated by 38%, i.e., the rated I2t * 38% should be greater than the instantaneous kinetic energy (pulse) that may occur during specific use. If the cycle pulse frequency exceeds 1000 times, further derating calculation shall be performed according to Figure 4.8-3. At the same time, because different suppliers have different I2ts in the same quantity, it is necessary to consider fuses with a smaller rated I2t, which can also withstand corresponding pulse energy.

For the slow start circuit currently used by the company, it should be determined whether the fuse can withstand the impact of the starting current based on the specific detection situation. Please refer to the instructions in this article.

Note: Impulse current and pulse

For example, the UL standard fuse has a rated current of 125V/1.0A, a normal load working current of 0.75A, and a working temperature of 25 ℃. It can quickly fuse and withstand 10000 pulse currents of the pulse waveform shown in Figure 4.8-1.

The first step is to calculate the pulse I2t.

Select a typical waveform E according to the typical pulse waveform kinetic energy calculation method in Figure 4.8-2. The waveform E formula is brought in with the following values:

I2t=(1/5)ip2t =（1/5） × eighty-two × 0.004=0.0512A2Secece

Flow 2: Calculate the required fuse I2t

Required fuse I2T=pulse I2T/0.22=0.0512/0.22=0.227

Process 3: Check that the rated I2t of the fuse is 0.281A2Sec>0.2327A2Sec, that is, it can withstand pulse cycle kinetic energy. At the same time, due to the UL specification, the rated voltage is reduced by 25%, and the fuse can also adapt to a stable operating current of 0.75 amperes.