About Fuses
Aug 04, 2023
Introduce
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A fuse refers to an electrical device that uses the heat generated by itself to fuse the melt and disconnect the circuit when the current exceeds a specified value. A fuse is a circuit breaker that melts the melt with the heat generated by itself after the current exceeds a specified value for a period of time, thereby causing the circuit to break; A current protector is made using this principle. Fuses are widely used in high and low-voltage distribution systems, control systems, and electrical equipment. Short circuit and overcurrent protectors are one of the most commonly used protective devices.
Working principle
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The basic working principle of a fuse involves the use of a fusible element, typically made of a metal or an alloy with a low melting point. The most common material used for fuses is a mix of tin and lead, known as solder. When the current flowing through the fuse exceeds its rated value, the temperature of the fusible element increases due to the Joule heating effect. As the temperature reaches the melting point of the metal, the element melts and breaks the electrical circuit, interrupting the flow of current.
Here's a step-by-step explanation of the working principle:
- Normal operating conditions: Under normal conditions, when the current flowing through the circuit is within the fuse's rated current, the fusible element remains in a solid state. It presents negligible resistance to the current flow, allowing electricity to pass through the fuse unimpeded.
- Overcurrent condition: If the current in the circuit exceeds the fuse's rated current, such as during a short circuit or excessive load, the current passing through the fusible element increases substantially.
- Joule heating effect: The increased current causes the fusible element to heat up due to the Joule heating effect. This phenomenon occurs when current flows through a resistive material, generating heat proportional to the square of the current.
- Melting of the fusible element: As the temperature of the fusible element rises, it eventually reaches its melting point. At this critical temperature, the metal changes from a solid to a liquid state.
- Circuit interruption: Once the fusible element melts, it breaks the electrical circuit, creating an air gap in the fuse. This gap prevents any further current flow through the circuit, effectively disconnecting the faulty or overloaded section from the rest of the system.
- Protection: By interrupting the circuit, the fuse protects electrical equipment and wiring from the harmful effects of excessive current. It prevents electrical fires, damage to appliances, and other potential hazards.
Common types
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- Cartridge Fuses: These fuses consist of a cylindrical body made of ceramic, glass, or other non-conductive materials. Inside the cartridge, there is a fusible link made of metal. Cartridge fuses can be found in two main categories: D-type and NH-type fuses. D-type fuses are general-purpose fuses used in industrial and residential applications, while NH-type fuses are larger and used in high-current industrial applications.
- Plug Fuses: These are screw-in fuses designed to fit into standard Edison-base fuse holders. Plug fuses are commonly used in residential electrical systems to protect branch circuits and appliances. They are available in various current ratings and are often color-coded for easy identification.
- Blade Type Fuses: Also known as automotive fuses, blade fuses are commonly used in vehicles to protect various electrical circuits. They have a plastic body with two or more metal prongs on the back. Blade fuses are available in different sizes, such as mini, standard, and maxi, each with varying current ratings.
- Resettable Fuses (Polymeric Positive Temperature Coefficient - PTC Fuses): These fuses are also known as PTC fuses or resettable fuses. Unlike traditional fuses, resettable fuses do not need replacement after the trip. They contain a polymeric element that exhibits a high resistance at normal operating temperatures. If an overcurrent condition occurs, the element heats up and its resistance increases significantly, effectively limiting the current flow. Once the fault is cleared, the PTC fuse cools down, and its resistance returns to a low state, allowing current flow again.
- Thermal Fuses: Also called thermal cutoffs or temperature fuses, these safety devices are designed to protect against overheating. They contain a fusible link that melts at a specific temperature, breaking the circuit and disconnecting the power to prevent further overheating.
- High Voltage Fuses: These fuses are specifically designed for high voltage applications, such as in power transmission and distribution systems. They have robust construction to handle high currents and are typically found in oil-filled or gas-filled enclosures.
- Semiconductor Fuses: These fuses are designed to protect sensitive electronic components and semiconductor devices. They have fast-acting characteristics to prevent damage from high-energy transients and short circuits.
Usage and maintenance
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In low-voltage distribution systems, fuses are a type of electrical appliance that plays a safety protection role. Fuses are widely used for power grid protection and electrical equipment protection. When a short circuit fault or overload occurs in the power grid or electrical equipment, they can automatically cut off the circuit to avoid damage to electrical equipment and prevent accidents from spreading.
A fuse is composed of an insulation base (or support component), contacts, melt, etc. The melt is the main working part of the fuse. The melt is equivalent to a special section of wire connected in series in a circuit. When a short circuit or overload occurs in the circuit, the current is too high, and the melt melts due to overheating, thereby cutting off the circuit. Melts are often made into filaments, grids, or sheets. Melt materials have the characteristics of relatively low melting points, stable characteristics, and easy melting. Generally, lead-tin alloy, silver-plated copper sheet, zinc, silver, and other metals are used. During the process of melting and cutting off the circuit, an arc will be generated. In order to safely and effectively extinguish the arc, the melt is generally installed in the fuse housing and measures are taken to quickly extinguish the arc.
Fuses have the advantages of simple structure, convenient use, and low price, and are widely used in low-voltage systems.
note
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Given the excellent short-circuit protection performance of fuses, they are widely used in high and low-voltage distribution systems, control systems, and electrical equipment. Short circuit and overcurrent protectors are one of the most widely used and important protective devices.
In application, attention should be paid to the precautions for using fuses, daily inspections, and maintenance.
Precautions for using fuses:
1. The protective characteristics of the fuse should be adapted to the overload characteristics of the protected object. Considering the possible short-circuit current, a fuse with corresponding breaking capacity should be selected;
2. The rated voltage of the fuse should adapt to the line voltage level, and the rated current of the fuse should be greater than or equal to the rated current of the melt;
3. The rated current of each level of fuse melt in the circuit should be matched accordingly, and the rated current of the previous level of melt must be greater than the rated current of the next level of melt;
4. The melt of the fuse should be matched according to the requirements, and it is not allowed to arbitrarily increase the melt or use other conductors to replace the melt
Inspection of fuses:
1. Check whether the rated values of fuses and melts match the protected equipment;
2. Check the appearance of the fuse for damage or deformation, and check the porcelain insulation for flashing discharge marks;
3. Check whether each contact point of the fuse is intact, tightly connected, and there is no overheating phenomenon;
4. Whether the fuse signal indicator is normal.
Use and maintenance of fuses:
1. When the melt melts, it is necessary to carefully analyze the cause of the melting. Possible reasons include
(1) short circuit fault or normal melting due to overload operation;
(2) If the melt is used for too long, it may break due to oxidation or high temperature during operation, resulting in changes in the characteristics of the melt;
(3) There is mechanical damage during the installation of the melt, which reduces its cross-sectional area and causes accidental disconnection during operation.
2. When replacing the melt, it is required to:
(1) Before installing a new melt, the cause of melt melting should be identified. If the cause of melting is not determined, do not replace the melt for trial delivery;
(2) When replacing a new melt, check whether the rated value of the melt matches the protected equipment;
(3) When replacing a new melt, it is necessary to check the internal burn condition of the fuse tube. If there are severe burns, the fuse tube should be replaced at the same time. When the porcelain fusion tube is damaged, it is not allowed to replace it with another material tube. When replacing the melt of a filler-type fuse, attention should be paid to filling the filler.
3. The fuse should be repaired simultaneously with the power distribution device:
(1) Clean the dust and check the contact condition of the contact points;
(2) Check the appearance of the fuse (remove the fuse tube) for damage or deformation, and check the porcelain components for discharge flicker marks;
(3) Check whether the fuse, melt, and protected circuit or equipment match, and promptly investigate any issues;
(4) Pay attention to checking the N line in the TN Earthing system and the grounding protection line of the equipment. It is not allowed to use fuses; (5) When maintaining and inspecting fuses, it is necessary to cut off the power supply according to safety regulations, and it is not allowed to remove the fuse tube with electricity.
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