BlueLight Fuse Selection Guide

Release Time: 12 Nov,2021

To facilitate the selection of appropriate fuse links for the components, circuits, or equipment requiring protection, this guide has been prepared. The selection of fuse links can be carried out according to the following procedure:

Factors Taken into Consideration

※The safety certification of the fuse holder shall be determined based on the safety certification requirements of the complete equipment; accordingly, the fuse holder can be preliminarily classified as either IEC-compliant or UL-compliant.

※Space constraints in the circuit during design.

※Installation method.

※The rated voltage shall be greater than or equal to the effective circuit voltage.

※The breaking capacity shall be greater than the maximum fault current in the circuit.

※When the entire device is switched on, does inrush current occur in the circuit? In some circuits, inrush current is normal; in such cases, time-delay or medium-time-delay fuses should be used.

※Current Selection

a) The current and duration for which the fuse tube must interrupt the circuit (this requirement is determined by the design engineer based on the specific protection needs of the circuit); refer to the I–t curve for the corresponding model and select the maximum rated current that meets the requirements as the upper limit, A1.

b) The steady-state current through the fuse tube (depending on the specific circuit).

c) For details on the differences in rated current between IEC-standard and UL-standard fuse links, see “Steady-State Current” below.

d) The impact of ambient temperature on the current-carrying capacity of fuses is detailed in the following section, “Ambient Temperature.”

e) The impact of pulses (impulse current, surge current, inrush current, and current transients) on fuse life is described in detail in the “Pulses” section below.

f) Compare the inrush current and its duration with the I-T curve for the corresponding model.

After comprehensively considering the above factors, select the most suitable model and rated current.

When A1 > A2, select the corresponding fuse model with a rated current of A2.

When A1 ≤ A2, select the corresponding fuse model with a rated current of A1.

Steady State Current

There are differences in conditions between practical applications and laboratory settings, such as:

※A fuse box is sometimes used.

※Cross-sectional area of the wires in the circuit.

※Contact resistance of the fuse holder, etc.

Taking the above factors into account, the fuse tube selected for operation at 25°C must meet the following conditions to ensure its continuous and reliable performance:

IEC specification: The rated current In of the fuse tube = steady-state current / 0.9

UL specification: The rated current In of the fuse tube = steady-state current / 0.75

Environment Temperature/THE ENVIRONMENT TEMPERATURE

The current-carrying capacity test for fuse links is conducted under ambient conditions of 25°C; however, the current-carrying capacity of a fuse link is influenced by ambient temperature. As ambient temperature increases, the temperature rise of the fuse decreases, resulting in a lower current-carrying capacity. Therefore, when selecting a fuse, the ambient temperature surrounding the fuse must be taken into account. The effect of ambient temperature on the current-carrying capacity of various types of fuses is illustrated in the figure below:

Pulse/PULSE

Pulses generate thermal cycling, which in turn induces mechanical fatigue and affects the fuse’s service life. During design, the pulse I should be ² T is much smaller than the fuse’s rated melting thermal energy I. ² T. Fuse life (number of pulse cycles it can withstand) versus U (U = pulse I ² T-value and fuse I ² Refer to the table below for the relationship of the T-value ratio.

Number of withstandable pulses	U (ratio)
100,000 times	22%
10,000 times	29%
1,000 times	38%
100 times	48%

The melting thermal energy I of fuses of each specification provided in this catalog ² For reference, the following table provides the I for various typical pulse waveforms. ² Approximate formula for the T-value:

Test/TEST

The selected sample must be tested in the actual protected circuit to verify the chosen fuse. This verification shall encompass tests under both normal and fault conditions to ensure that the selected fuse operates properly within the protection circuit.

Test Method for Thermal Fuses

Explanation of Temperature Fuse Terminology

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