The fuse element in fuses and low-voltage load switches is connected in series with the circuit during operation, providing overload and short-circuit protection for wiring and electrical equipment. What principles should be followed when replacing the fuse element? The details are as follows:

1. Copper and aluminum wire cannot be used as the internal fuse element in high-voltage drop-out fuses.

The internal fuse of a high-voltage drop-out fuse is typically made of materials such as copper and silver. Because copper and silver have low resistivity and strong conductivity, the wire can be made thinner, which facilitates arc extinguishing. However, their high melting points can cause the fuse tube to overheat and become easily damaged. Therefore, fuses made of silver and copper are artificially lowered in temperature by soldering small tin or lead beads onto the fuse. When the fuse is heated to the melting point of tin (232°C) or lead (327°C), the small beads melt first, breaking the fuse. The arc formed at the break point causes the fuse to melt in both directions, thus protecting the circuit or electrical equipment from damage caused by excessive current and heat.

2. After the primary side fuse of a 10kV voltage transformer blows, it cannot be replaced with an ordinary fuse.

10kV voltage transformers are often protected by RN2 or RN4 fuses. The rated current is 0.5A, and the breaking current within 1 minute is 0.6-1.8A. Both fuse tubes are filled with quartz sand, resulting in excellent arc-extinguishing performance and a high interrupting capacity (minimum 1000MVA). Because the fuse element is made of nickel-chromium wire, the total resistance is approximately 90Ω, effectively limiting short-circuit current. If a conventional fuse is used instead, and the voltage transformer fails due to a fault or other reason, the fuse will neither limit the short-circuit current nor extinguish the arc, potentially damaging the equipment or even causing a system power outage. Therefore, when a voltage transformer fuse blows, it should be replaced with a fuse of the original specification rather than a conventional fuse.

3. High-voltage fuses filled with quartz sand can only be used on power grids with the same rated voltage.

In a fuse filled with quartz sand, when the melt melts, the arc burns in the narrow groove in the quartz sand. According to the principle of narrow slot arc extinguishing, the arc is in close contact with the surrounding filler and is cooled and extinguished. It has a strong arc extinguishing ability and can extinguish the arc before the current reaches its peak, thus having a current limiting effect.However, it will generate overvoltage, the extent of which depends on the voltage at the location of use. If used in a grid with a voltage lower than the rated voltage, the overvoltage could reach 3.5-4 times the phase voltage, causing corona in the grid and even damaging equipment on the grid. If used in a grid with a voltage higher than the rated voltage, the overvoltage generated by the fuse may cause the arc to reignite, making it impossible to extinguish again, causing the fuse housing to burn out. However, when used in a grid with the same rated voltage, the overvoltage at the time of melting is only 2-2.5 times the grid phase voltage, which is slightly higher than the line voltage of the equipment. Therefore, there is no danger.

4. When replacing a fuse, do not increase or decrease its rated current or voltage.

Only replace a fuse with one of the same capacity and voltage rating as the original. Otherwise, the fuse's protective function will be lost.

5. The fuse's rated current must match the fuse's rated current.

The same type of fuse can be equipped with fuse elements with different rated currents, but the fuse element's rated current must be smaller than the fuse's, or at most equal, and never exceed it. This is because the fuse's rated current is determined based on the heat generated by the contacts and terminals. Similarly, in load switches and fuse-type disconnectors, which combine a disconnector and a fuse element, the fuse element's rated current must be smaller than the switch's rated current, or at most equal, and never exceed it.

6. The rated currents of the fuses in the main switch and the sub-switch must never be reversed or the same.

Generally, power distribution lines use a combination of disconnectors and fuses as control and protection devices. The rated currents of the fuses in the main switch and sub-switch fuses are different, with the main switch having a higher rated current and the sub-switch fuse having a lower rated current.

7. Do not rashly replace the fuse of a load switch after it has blown.

Whether the fuse is caused by a short circuit, overload, or other reason, do not rashly replace the fuse. The cause must be identified and corrected before replacement. Also, do not replace each phase individually; replace all fuses in the same load switch. Because the fuse that did not blow has also been through the heat, continued use may cause it to blow during normal operation. Therefore, while the new and old fuses may have some similar characteristics, their fusing characteristics and cross-sectional areas will differ.

8. For the same load switch or fuse-type disconnector, only replace fuses of the same type. Do not mix fuses of the same capacity but different types.

Fusing characteristics vary greatly between different fuse types. For example, the breaking current of a lead-tin alloy fuse is 1.5 times the rated current; a copper wire fuse is 2 times; and a zinc fuse is approximately 1.3 to 2.1 times.

9. Always disconnect the power supply when replacing the fuse element.

Disconnect the isolating switch and never operate with power on. This prevents contact between your body or tools and live parts, which could cause electric shock. This also prevents the possibility of pulling out a blown fuse element while it is under load.

10. Do not allow the fuse to come into contact with the housing.

When replacing lead-tin alloy fuses, for fuses approximately 60-85mm in length, do not allow the fuse to come into direct contact with the protective housing during assembly.

When assembling copper wire fuses, do not allow the housing to come into direct contact with the fuse. The length must be sufficient. Wires with a diameter of less than 0.46mm should be 60-85mm long, and those with larger diameters should be no less than 100mm. When replacing fuses in small to medium-sized spare asynchronous motors, the rated current of the fuse must not exceed 1.5 times the rated current of the motor being protected; it should generally be around 2.5 times.

11. When replacing the fuse element of a plug-in fuse, do not remove or discard the asbestos padding inside the porcelain base.

Fuses with a current rating of 30A and above have asbestos padding between the arc-extinguishing and arc-extinguishing sections to reduce vibration, provide heat insulation, and aid in arc extinguishing.

12. Do not arbitrarily change the operating mode of a screw-in fuse.

Spiral fuses (RL1 type) offer strong interrupting capacity, compact size, ease of use, and safety and reliability. They are widely used for short-circuit protection in the main circuits, control circuits, and lighting circuits of electrical equipment.

When the fuse in the fuse tube blows, it should be replaced with a new one according to the rated current of the load.

13. In incandescent lighting circuits, fast-acting fuses with low thermal capacity should not be used for protection.

The instantaneous current drawn by an incandescent lamp when cold is very high, reaching 12 to 16 times the rated current of the bulb. The duration of the starting surge current (the time it takes to return to normal) is 0.05 to 0.23 seconds (the higher the bulb power, the longer this time). However, the protective characteristic of a fast-acting fuse is that it will melt within 0.2 to 0.02 seconds when interrupting 4 to 6 times the rated current. This means that the fuse of a fast-acting fuse will melt within the duration of the incandescent lamp's starting surge current.

14. Correct and reasonable fuse selection is crucial to ensuring the safe operation of circuits and electrical equipment. The selection principles are: 1. The fuse should blow after a certain period of time when the current exceeds the equipment's normal value. 2. The fuse should not blow during normal short-term overcurrent events (such as motor startup).

The selection method (in environments not strictly required above) is as follows:

1. In lighting and electric heating equipment circuits

(1) The rated current of the fuse in the main line is equal to 0.9 to 1 times the rated current of the watt-hour meter;

(2) The rated current of the fuse in the branch line is equal to 1 to 1.1 times the sum of the rated currents of all loads in the branch line.

2. In AC motor circuits

(1) The rated current of the fuse in a single AC motor circuit is equal to 1.5 to 2.5 times the rated current of the motor;

(2) The rated current of the fuse in a circuit with multiple AC motors is equal to 1.5 to 2.5 times the rated current of the largest motor in the circuit plus the sum of the rated currents of the other motors.

3. AC welding machine AC The fuse in a single AC welding machine circuit can be estimated using the following simple method: 

(1) When the power supply voltage is 220V, the rated current of the fuse is equal to 6 times the power (kW) of the welding machine. 

(2) When the power supply voltage is 380V, the rated current of the fuse is equal to 4 times the power (kW) of the welding machine.