Is temperature a key factor to consider when choosing a
fuse?
The answer is absolutely affirmative. The ambient temperature is not only a factor to consider, but also one of the core basic parameters when selecting fuses. Neglecting the influence of temperature and selecting fuses solely based on the rated current of the circuit is a common and potentially serious mistake in engineering practice. The essence of a fuse is a "thermal element", and its working principle is entirely based on thermal effects. Therefore, the balance between ambient temperature and its own heat generation and dissipation directly determines its protective performance, lifespan, and reliability.
1、 Root cause exploration: The intrinsic relationship between the working principle of fuses and temperature
To understand the importance of temperature, one must first understand how fuses work. The core of a fuse is a melt, usually made of low melting point metals such as tin, lead, zinc, and their alloys. When current flows through a melt, heat is generated according to Joule's law (P=I ² R). Under normal operating current, the heat generated will be dissipated to the surrounding environment through the conduction, convection, and radiation of the melt itself, the arc extinguishing quartz sand filled inside, and the outer shell and terminals of the fuse, thereby achieving a thermal equilibrium state where the melt temperature remains below the melting point and the fuse is safe and sound.
When the current exceeds the rated value and reaches overload or short circuit conditions, the heat generation rate increases sharply, far exceeding the heat dissipation rate. The temperature of the melt rises rapidly until it melts and disconnects the circuit, thus providing protection.
From this working principle, it can be seen that every link in the entire process is closely related to "heat". And environmental temperature is a crucial external variable in this heat balance equation.
2、 How does temperature specifically affect the performance of fuses
The impact of environmental temperature on fuses is comprehensive and profound, mainly reflected in the following aspects:
1. Impact on the melting characteristics (the most critical impact):
Early melting or false melting in high-temperature environments: This is the most direct risk. Assuming a fuse with a rated current of 1A melts after a long time at a current of 1.1A at room temperature of 25 ° C (following its time current characteristic curve). If it is placed inside a high-temperature chassis at 70 ° C, even if the circuit current is still normal at 1A, the initial temperature of the fuse melt has changed from 25 ° C to 70 ° C. This means that the additional heat generated by the current required to reach its melting point is greatly reduced. The result is that this 1A fuse may not be able to work stably at 1A current in high temperature environments, and may even melt under normal current, causing the equipment to shut down unexpectedly. This type of fault is difficult to troubleshoot and affects equipment availability.
Late melting or inertness in low-temperature environments: In extremely low-temperature environments, the opposite is true. The initial temperature of the melt is extremely low, requiring more heat to raise it to the melting point. This will result in a longer fuse blowing time and a slower response. In the case of overload, this means that the protected sensitive components will withstand longer periods of abnormal current impact, which may result in overheating and damage; In the case of a short circuit, a longer melting time means that the short-circuit current will last longer, which may cause greater arc damage and thermal shock to the circuit, contactor, and other components.
2. Impact on aging rate and lifespan:
Long term exposure to high temperature environment, even if the current is normal, the fuse melt will continue to be under a high temperature stress. This will accelerate the oxidation, lattice structure changes, and mechanical fatigue of the molten material, leading to its gradual aging. An aging fuse may experience drift in its actual melting characteristics, with a slight increase or decrease in melting point, making it more unpredictable. Ultimately, it may melt prematurely below the rated current (becoming fragile due to aging), or fail to disconnect in a timely manner under true fault current due to changes in its characteristics. This slow performance degradation is implicit, but the harm is enormous.
3. Redefine the rated current value - Derating:
Due to the aforementioned impact, all professional fuse manufacturers will provide clear "temperature reduction curve" or "derating curve" in their product specifications. This curve tells us that as the ambient temperature increases, the maximum sustained current that the fuse can safely carry must correspondingly decrease.
For example, a fuse with a rated current of 10A at 25 ° C may have a specification stating that it must be downgraded to only be used in circuits up to 7A or even lower at an ambient temperature of 70 ° C. Therefore, the "rated current" of a fuse is never an absolute constant value, but a value bound to a specific reference temperature (usually 25 ° C or 20 ° C). The responsibility of an engineer is to find the correct and reduced current value based on the maximum operating temperature of the equipment, using this curve as the basis for selection.
3、 How to consider temperature factors in actual selection
1. Determine the worst-case ambient temperature: First evaluate where the fuse will be installed on the equipment. Is it on the PCB board near the high-power heating components? Is it deep inside a sealed fanless computer case? Or is it in an outdoor cabinet? Measure or estimate the maximum ambient temperature (Ta) of the location during long-term operation at full load of the equipment. We must consider the worst-case scenario and leave a certain margin.
2. Refer to the manufacturer's derating curve: Obtain the technical manual of the intended fuse model and find its temperature derating curve. Based on the worst-case environmental temperature determined in the first step, find the corresponding maximum allowable operating current percentage on the curve.
3. Calculate and select: Divide the normal operating current (Ir) of the circuit by the derating percentage to obtain the nominal rated current value (In) of the required fuse at 25 ° C. The formula can be simplified as: In ≥ Ir/(derating percentage). For example, if the working current of the circuit is 1A and the rated current drops to 70% at an ambient temperature of 70 ° C, a fuse with a nominal rated current of at least 1A/0.7 ≈ 1.43A needs to be selected. Usually, the specifications of 1.5A or 2A are chosen upwards, but at the same time, it is necessary to verify whether its time current characteristics meet the protection requirements.
4. Consider pulse and cold shock: In circuits with frequent switching and surge currents (such as motors and capacitive loads), high temperatures can reduce the fuse's ability to withstand pulse shock, which may cause it to melt at the moment of startup. Sometimes it is necessary to choose a slow break fuse with stronger impact resistance and also perform temperature derating calculations.
In summary, the environmental temperature and the choice of fuse are not unrelated, but rather deeply coupled and determine the success or failure of their work. It directly distorts the core time current protection characteristic of the fuse by changing the thermal equilibrium point of the melt, and affects its long-term reliability. Incorporating temperature factors into selection considerations is not an optional 'optimization', but an engineering design criterion that must be strictly followed. Ignoring this point is like building a bridge on unknown ice or lava, and the effectiveness of the entire circuit protection system will become extremely unreliable and dangerous. Therefore, the first task for every responsible engineer when selecting a fuse is to ask themselves, "How hot will it work in
