It's very easy for our electrician to handle fuses. Small power applications use thin copper wires, while larger ones use thicker copper wires. If a fuse blows, it can be replaced with a higher-rated one. In recent years, our textile mill has significantly reduced the number of fuses used. Let me take our factory as an example. For high-voltage power distribution rooms, we use two types of fuses: RN2 0.5A for high-voltage voltage transformers and 3.15A RN2 for 30KVA transformers. In the low-voltage distribution room, we only have a 1A RT14 fuse for the secondary control loop and a 50/63A capacitor protection fuse for the capacitor cabinet. In the workshop, we mainly use an RT0 200A fuse in the distribution cabinet and RT14 fuses ranging from 2-10A for the secondary control. We also use quick-break fuses for transistor protection in HY492 inverters. Other fuses are now replaced by high-voltage vacuum circuit breakers, low-voltage circuit breakers ranging from 100-3200A, and miniature circuit breakers from 1-32A.
For reference, here is some historical data about our company:
Figure 4 shows the inverse time characteristic curve of a fuse. Each fuse element has a minimum melting current, which varies depending on temperature. Although this current is influenced by environmental factors, it is usually negligible in practical applications. The ratio between the minimum melting current and the rated current of the fuse is called the minimum melting coefficient. For standard fuses, this coefficient typically ranges from 1.1 to 1.5, and is generally considered acceptable if it exceeds 1.25. This means that a 10A fuse will not blow if the current is below 12.5A.
6.2 Selection Method
The rated current of a fuse element can be selected as follows:
(1) When protecting smooth loads without a starting process, such as lighting circuits, resistors, or electric furnaces, the rated current of the fuse should be slightly greater than or equal to the load’s rated current.
(2) For single motors operating continuously, the fuse rating can be selected based on the maximum starting current, or using the formula:
Ir ≥ (1.5~2.5)ie
Where Ir is the fuse’s rated current, and ie is the motor’s rated current. If the motor starts frequently, the coefficient can be increased to 3–3.5, depending on the actual conditions.
(3) For multiple long-term running motors (such as main power lines), the formula becomes:
Ir ≥ (1.5~2.5)ie max + Σie
Here, ie max is the rated current of the largest individual motor, and Σie is the sum of the rated currents of the other motors. To prevent over-level fusing and reduce accident scope, the upper-level (main supply) and lower-level (branch line) fuses should be properly coordinated. Ideally, the upper-level fuse should have a rated current about 1.6 times that of the lower-level one.
7. Conclusion
I have thoroughly understood the characteristics of fuses, and even though I was initially unfamiliar with these small components, I now have a better grasp of how they function. This knowledge allows me to apply them more effectively and ensure reliable performance in our electrical systems.
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