1.Fuse Related Parameters and Terminology

(1). Rated Current (In)

The rated working current marked on the fuse. This value is determined by the manufacturer and represents the current the fuse can carry. Rated currents are typically standard recommended ratings, such as 1, 1.25, 1.5, 1.6, 2, etc. (Unit: A)

(2). Rated Voltage (Un)

The rated voltage marked on the fuse indicates the maximum working voltage at which the fuse can be used. Common standard rated voltages include 32, 63, 125, 250, 600V. Fuses are sensitive to changes in current rather than voltage. A fuse remains unaffected from zero up to its maximum rated value, allowing its use under any voltage lower than its rated voltage.

(3). Voltage Drop (Ud)

The voltage drop across the fuse at rated current.

(4). Cold Resistance (R)

The resistance of the fuse when it is not in operation. Most fuses are made from materials with a positive temperature coefficient, thus there are cold resistance and hot resistance (voltage drop at rated current), with the actual working resistance in between. Cold resistance can be measured using a current not exceeding 10% of the fuse's nominal rated current. Hot resistance is based on the voltage drop when the current flowing through the fuse equals its nominal rated current.

(5). Ambient Temperature

Refers to the air temperature directly surrounding the fuse, not to be confused with room temperature. In many practical situations, the temperature of the fuse is significantly high, for example, when the fuse is installed in a confined space or near heat-generating components such as resistors, transformers, inductors, etc.

(6). Breaking Capacity (Breaking Capacitor)

Also known as interrupting capacity or short-circuit rated capacity, it refers to the maximum current that the fuse can safely interrupt at a specified voltage. When the transient overload current that may pass through the fuse exceeds the rated value, the fuse may break or explode, causing danger. Therefore, the fuse is required to remain intact (without bursting or breaking) after the protective action. The breaking capacity of a fuse depends on its construction; low breaking capacity fuses are mostly glass-bodied, while high breaking capacity fuses generally have ceramic bodies, many of which are also filled with pure granular quartz material.

(7). Time-Current Curves (Overload and Time-Current Curves)

One of the most important parameters of a fuse. When the current passing through the fuse exceeds the rated current, the fuse blows, indicating an overload condition. The time-current characteristic of a fuse is the relationship between overload current and blowing time. The time-current curve is based on average values.

(8). Nominal Melting I^2t

One of the most important parameters for selecting a fuse, it is the energy required to open the fuse, an inherent parameter of the fuse itself, represented by I^2t. The I^2t value is a parameter of the fuse itself, determined by the material of the component and the shape of the fuse element, independent of temperature and voltage.

(9). Size

Unless otherwise specified, dimensions are in millimeters. Common cylindrical fuse sizes include Φ6X30, Φ5X20, Φ3X10, Φ2X7; common surface mount fuse sizes include 6.1X2.7X2.7, 10.1X3.1X3.1, etc.

2.Fuse Selection Factors and Examples

1).Rated Current

Pay attention to the current derating under different certification standards. For fuses certified according to UL standards, the derating is 0.75, meaning the actual steady-state operating current should not exceed 75% of In. According to IEC standards, the derating is 1.0, meaning the actual steady-state operating current can equal In. For fuses certified under UL standards: operating at 25°C, the working current should not exceed 75% of the fuse's rated current to avoid harmful blowing. For example, a fuse rated at 10A is generally not recommended to operate at currents greater than 7.5A at an ambient temperature of 25°C. For fuses certified under IEC standards: the fuse can operate at its rated current for protection. For example, a 10A rated fuse can be used for a 10A actual working current.

For the working current of a single board, the current at the lowest allowed voltage should be considered. For example, if the rated voltage is -48—60V, allowing for 20% fluctuation, and the working current of the single board at -48V is 0.8A, then due to the constant power of the single board, the working current at -38V working voltage is approximately 1A. When selecting a fuse, 1A should be considered as the working current of the single board. This is particularly important in applications with a wide input voltage range.

Also consider whether the power supply module has an under-voltage protection feature. For instance, a -48V power supply module generally activates under-voltage protection at -35V, but some modules lack this feature. For example, the Huadian AV10 series power modules can actually operate at -12V, which could cause the input current to be more than three times the normal condition.

In general, suppliers offer fewer current specifications than the standard recommended positions. It is advised to choose from the existing current specifications provided by suppliers rather than requesting custom designs.

Note: The main difference between UL Listed certification and UL Recognized certification lies in the scope and standard of certification. UL Listed certification (UL Listing) indicates that a product has undergone comprehensive testing and evaluation, meeting specific safety standards, whereas UL Recognized certification (UL Recognition) is typically for component products, which are evaluated as part of a larger system. Understanding this is crucial when selecting fuses, as it determines whether the fuse is suitable for a particular application.

2). Rated Voltage

The rated voltage of the fuse should be equal to or higher than the operating voltage of the system. Using a fuse with a voltage rating lower than the system's operating voltage may lead to unsafe interruption under overvoltage conditions, thus posing safety risks. For example, if the system's operating voltage is 220V, then the fuse's rated voltage should be at least 220V or higher.   

3).Ambient Temperature Effect

The current carrying capacity of a fuse is tested at an ambient temperature of 25°C. This test is influenced by changes in ambient temperature. The higher the ambient temperature, the higher the operating temperature of the fuse, and thus, the shorter its lifespan. Conversely, operating at lower temperatures will extend the life of the fuse. Therefore, when selecting the rated current of a fuse, adjustments should be made based on the actual operating ambient temperature of the fuse. For example, if a certain PCB operates normally at 1.5A and uses a slow-blow fuse certified according to UL standards at room temperature, then:

Select fuse In = Normal operating current / Certification standard derating = 1.5 / 0.75 = 2.0A (Operating ambient temperature 25°C)

If the fuse operates at a high ambient temperature of 70°C, according to curve A in the figure below (traditional slow-blow fuse), it indicates a temperature derating of 80% at 70°C. In this case,

Select fuse In = Normal operating current / (Certification standard derating * Operating temperature derating) = 1.5 / (0.75*0.8) = 2.5A (Operating ambient temperature 70°C)

Through the above calculations, we can compare the different requirements.

Figure 2 shows the characteristic curve of the impact of ambient temperature on current carrying capacity.

Where:

Curve A: Represents the characteristic curve of traditional slow-blow fuses;

Curve B: Represents the characteristic curve of very fast-acting, fast-acting, and spiral-wound fuses.

The table below provides a common temperature-current reference chart

The data in the table are common temperature deratings (for reference only):

*Ambient temperature around the fuse

*Refers directly to the air temperature surrounding the fuse, which should not be confused with room temperature. In many practical situations, the temperature of the fuse can be significantly high, such as when the fuse is installed in enclosed spaces or near heat-generating components like resistors, transformers, inductors, etc.

4). Voltage Drop and Cold Resistance

Generally, the resistance of a fuse is inversely proportional to its rated current. It is preferable to select a fuse with as low a resistance as possible to minimize power loss. The voltage drop across a fuse is measured at its rated DC current. Since fuses with smaller rated currents have higher resistance, they have a more significant impact on low-voltage power systems. Care should be taken regarding the resistance when selecting small-sized fuses.

5). Time-Current Characteristic Curve

This is one of the most important criteria for selecting a fuse. It determines whether the fuse can effectively protect the circuit by blowing correctly under fault current conditions. Each type of fuse has its own time-current characteristic curve. The curve's horizontal axis represents current, while the vertical axis represents blowing time. Typically, this curve is referenced during selection, and key points on the curve are used as criteria. The choice of key points varies with the fuse certification type; for UL-certified fuses, typical key points include 110%In, 135%In, 200%In, while for IEC-certified fuses, typical key points include 135%In, 210%In, 275%In. The relationship between blowing time and key points is further discussed in section 3.7. When selecting a fuse, it is necessary to determine the allowable duration for the fault current to safely exist in the circuit before causing damage.

For example, a certain fast-acting fuse certified according to IEC standards with a rated current of 5A was tested. It was found that during a particular fault condition on the PCB, the fault current passing through the fuse was 10A, i.e., 200%In. According to the time-current characteristic curve of the fuse, it could operate for up to 30 minutes before blowing at 200%In. When the fuse was bypassed, allowing the PCB to operate under this fault current for 30 minutes, a fire occurred. This indicates that the selection of this fuse was inappropriate. Before the beginning of the fuse's blowing action, the protected device had already encountered an unsafe condition, failing to achieve the protective purpose.

Attention: Differences and selection criteria between fast-blow and slow-blow fuses

6). Interrupting Rating

The rated interrupting capacity of a fuse must meet or exceed the maximum fault current in the circuit. When the protected system is directly connected to the power supply input circuit and the fuse is placed at the power supply input, a high interrupting capacity fuse must be used. In most secondary circuits, especially when the voltage is lower than the supply voltage, a fuse with a lower interrupting capacity is sufficient.

7). Nominal Melting I^2t

For situations where the fuse must withstand high-energy currents, i.e., large current pulses over short durations, such as surge currents, start-up currents, inrush currents, and other similar "pulse" type transient conditions, the fuse should be able to withstand the energy of such high-energy currents without failing. The rated nominal melting I^2t of a fuse is determined through laboratory testing, and each fuse specification has a single rated nominal melting I^2t value.