Classification, Structure, Working Principle and Selection of Fuses

Definition: When the current exceeds a specified value, the heat generated by itself causes the melt to fuse and disconnects the circuit. The fuse is also called a fuse, and the IEC127 standard defines it as a "fuse-link". It is an electrical component installed in a circuit to ensure the safe operation of the circuit. Fuse is actually a kind of short-circuit protector, which is widely used in power distribution systems and control systems, mainly for short-circuit protection or severe overload protection.

1. Introduction: A fuse is a current protector made by using this principle to melt the melt with its own generated heat after the current exceeds a specified value for a period of time, thereby disconnecting the circuit. Fuses are widely used in low-voltage power distribution systems and control systems as well as electrical equipment. As short-circuit and over-current protectors, they are one of the most commonly used protection devices.

A fuse is an overcurrent protector. The fuse is mainly composed of melt, fuse tube and external filler. When in use, the fuse is connected in series to the protected circuit. When the current of the protected circuit exceeds the specified value and after a certain period of time, the heat generated by the melt itself melts the melt and disconnects the circuit, thereby achieving protection. effect.

Electrical appliances that use metal conductors as melts to break circuits are connected in series in the circuit. When overload or short-circuit current passes through the melt, the melt itself will heat up and fuse, thus affecting the power system, various electrical equipment and household appliances. certain protective effect. It has anti-delay characteristics. When the overload current is small, the fusing time is long; when the overload current is large, the fusing time is short. Therefore, within a certain overload current range until the current returns to normal, the fuse will not blow and can continue to be used. The fuse is mainly composed of three parts: melt, shell and support. The melt is the key component that controls the fusing characteristics.

2. Working principle: A metal conductor is used as a melt in series in a circuit. When an overload or short-circuit current passes through the melt, it will fuse due to its own heat, thus breaking the circuit. Fuses have a simple structure and are easy to use. They are widely used as protection devices in power systems, various electrical equipment and household appliances.

3. Features: The rated current of the melt is not equal to the rated current of the fuse. The rated current of the melt is selected according to the load current of the protected equipment. The rated current of the fuse should be greater than the rated current of the melt and is determined in conjunction with the main electrical appliance.

The fuse is mainly composed of three parts: melt, shell and support. The melt is the key component that controls the fusing characteristics. The material, size and shape of the melt determine the fusing characteristics. Melt materials are divided into two categories: low melting point and high melting point. Low melting point materials such as lead and lead alloys have low melting points and are easy to fuse. Due to their large resistivity, the cross-section size of the melt is larger, and more metal vapor is generated during fusing. They are only suitable for fusing with low breaking capacity. device. High melting point materials such as copper and silver have high melting points and are not easy to fuse. However, due to their lower resistivity, they can be made into smaller cross-section sizes than low melting point melts. Less metal vapor is generated when fusing, so they are suitable for high breaking. Capability fuse. The shape of the melt is divided into two types: filament and ribbon. Changing the shape of the variable cross-section can significantly change the fusing characteristics of the fuse.

The fuse has an inverse delay characteristic, that is, when the overload current is small, the fusing time is long; when the overload current is large, the fusing time is short. Therefore, within a certain overload current range, when the current returns to normal, the fuse will not blow and can continue to be used. Fuses have various fusing characteristic curves, which can be suitable for the needs of different types of protection objects.

4. Classification

(1) Screw type fuse RL

The fuse tube is equipped with quartz sand, and the melt is buried in it. When the melt melts, the arc is sprayed towards the quartz sand and its gaps, which can quickly cool down and extinguish it. In order to facilitate monitoring, one end of the fuse is equipped with a color dot. Different colors represent different melt currents. When the melt melts, the color dot pops out to indicate that the melt has melted. Screw-type fuses have a rated current of 5 to 200A and are mainly used in branch circuits with large short-circuit currents or in places with flammable gases.

(2) With stuffing tube fuse RT

A stuffed tube fuse is a fuse with a current limiting effect. It consists of a porcelain fusion tube filled with quartz sand, contacts and silver-plated copper grid fusion. Stuffing tube fuses are installed on a special base, such as a base with an isolation knife or a base with the fuse as an isolation knife, and are operated by a manual mechanism. Stuffing tube fuses have a rated current of 50 to 1000A and are mainly used in circuits with large short-circuit currents or places with flammable gases.

(3) Packingless tube fuse RM

The fuse tube of unfilled tube fuses is made of fiber material. The melt used is zinc alloy sheet with variable cross-section. When the melt melts, part of the fiber material in the fiber fusion tube is decomposed by heat, producing high-pressure gas, which quickly extinguishes the arc. Filler-free tube fuses have the characteristics of simple structure, good protection performance, and easy use. They are generally used in combination with a knife switch to form a fuse-knife switch.

(4) Filled closed tube type quick fuse RS

The stuffed closed tube fast-acting fuse is a fast-acting fuse, which consists of a fuse tube, a contact base, an action indicator and a melt. The melt is in the form of a silver narrow section or mesh. The melt is one-time use and cannot be replaced by yourself. Due to its fast action, it is generally used to protect semiconductor rectifier components.

(5) Fuses can be divided into high-voltage fuses and low-voltage fuses according to the voltage used.

According to the protection objects, they can be divided into fuses for protecting transformers and general electrical equipment, fuses for protecting voltage transformers, fuses for protecting power capacitors, fuses for protecting semiconductor components, fuses for protecting motors and fuses for protecting household appliances. Fuses, etc. According to the structure, it can be divided into open, semi-enclosed, tube and spray fuses.

(6) Open fuse

The structure is simple, the melt is completely exposed to the air, supported by porcelain pillars, without supports, and is suitable for low-pressure outdoor use. When the current is broken, a loud sound and light are produced in the atmosphere.

(7) Semi-enclosed fuse

The melt is mounted on a porcelain frame and inserted into a porcelain box with metal sockets at both ends, suitable for low-voltage indoor use. When the current is broken, the sound and light generated are blocked by the porcelain box.

(8) Tube fuse

The melt is contained in the fuse body. Plug into the holder or directly connected to the circuit for use. The fuse link is a completely sealed insulating tube with metal caps or contact blades at both ends. If the insulating tube of this fuse is filled with quartz sand, it will have a current limiting effect when breaking current, which can greatly improve the breaking capacity, so it is also called a high breaking capacity fuse. If a vacuum is drawn inside the tube, it is called a vacuum fuse. If the tube is filled with SF6 gas, it is called an SF6 fuse, and its purpose is to improve arc extinguishing performance. Since quartz sand, vacuum and SF6 gas all have good insulation properties, this fuse is suitable for not only low voltage but also high voltage.

(9) Injection fuse

The melt is contained in an insulating tube made of solid gas-generating material. Solid gas-generating materials can be electrically inverted white cardboard or organic glass materials. When the short-circuit current passes through the melt, the melt immediately melts and generates an arc. The high-temperature arc causes the solid gas-producing material to rapidly decompose to produce a large amount of high-pressure gas, thereby ejecting the ionized gas with the arc at both ends of the tube, emitting great sound and light. And when the AC current crosses zero, it extinguishes the arc and breaks the current. The insulating tube is usually mounted on an insulating bracket to form the entire fuse. Sometimes the upper end of the insulating tube is made movable, and it detaches and falls after breaking the current. This type of spray fuse is commonly known as a drop fuse. Generally suitable for outdoor occasions with voltages higher than 6 kV.

In addition, fuses can also be divided into general-purpose fuses, backup fuses and full-range fuses according to the breaking current range. The breaking current range of general-purpose fuses refers to the range from the overload current greater than 1.6 to 2 times the rated current to the maximum breaking current. This type of fuse is mainly used to protect power transformers and general electrical equipment. The breaking current range of the backup fuse refers to the range from the overload current greater than 4 to 7 times the rated current to the maximum breaking current. This kind of fuse is often used in series with a contactor. When the overload current is less than 4 to 7 times the rated current, the contactor implements breaking protection. Mainly protect the motor.

With the needs of industrial development, special fuses suitable for various requirements are also manufactured, such as electronic fuses, thermal fuses and resettable fuses.

The fuse is a simple and effective protective appliance. It mainly plays the role of short circuit protection in the circuit. The fuse is mainly composed of a melt and an insulating tube (insulating base) in which the melt is installed. When used, the melt is connected in series to the protected circuit. When a short-circuit fault occurs in the circuit, the melt is instantly blown and the circuit is disconnected, thus playing a protective role. The function of the fuse is: when a fault or abnormality occurs in the circuit, the current will continue to increase, and the increased current may damage some important or valuable components in the circuit, and may also burn the circuit or even cause a fire. If the fuse is correctly installed in the circuit, then the fuse will blow itself and cut off the current when the current abnormally rises to a certain height and at a certain time, thereby protecting the safe operation of the circuit. The earliest fuse was invented by Edison more than a hundred years ago. Due to the underdeveloped industrial technology at that time, incandescent lamps were very expensive, so it was originally used to protect expensive incandescent lamps.

5. Choose

1) Selection of melt rated current

Since various electrical equipment have certain overload capabilities, they are allowed to operate for a long time under certain conditions; when the load exceeds the allowable value, the protection melt is required to fuse within a certain period of time. There are also some equipments that have a large starting current but a short starting time. Therefore, the protection characteristics of these equipments are required to adapt to the needs of equipment operation. The fuses are required not to blow when the motor is started. Under the action of short-circuit current and exceeding the allowable overload current When, it can reliably fuse and play a protective role. If the rated current of the melt is too large, the load will not fuse in time during a short circuit or long-term overload; if it is too small, it may fuse under the normal load current, affecting normal operation. To ensure the normal operation of the equipment, it must be reasonable according to the nature of the load. Select the melt rated current.

(1) Lighting circuit: The rated current of the melt ≥ the sum of the operating currents of all lighting appliances on the protected circuit.

(2) Electric motor:

①Single direct starting motor: melt rated current = (1.5~2.5)×motor rated current.

② Multiple direct-start motors: Total protection melt rated current = (1.5~2.5) × sum of currents of each motor.

③Reduced voltage starting motor: melt rated current = (1.5~2)×motor rated current.

④Wound motor: Melt rated current = (1.2~1.5)×motor rated current.

(3) Low-voltage side of distribution transformer: melt rated current = (1.0~1.5) × rated current of the low-voltage side of the transformer.

(4) Parallel capacitor bank: rated melt current = (1.43~1.55) × rated current of capacitor bank.

(5) Electric welding machine: melt rated current = (1.5~2.5)×load current.

(6) Electronic rectifier component: melt rated current ≥ 1.57 × rectifier component rated current.

Note: The numerical range of melt rated current is to adapt to the melt's standard component rating.

2) Ampere-second (inverse time limit) characteristics of the fuse

The action of the fuse is achieved by the melting of the melt. When the current is larger, the time required for the melt to melt is shorter. When the current is small, it takes a long time for the melt to fuse, or it may not even fuse. Therefore, for the melt, its operating current and operating time characteristics, that is, the ampere-second characteristics of the fuse, are inverse time characteristics.

Each melt has a minimum melting current. Corresponding to different temperatures, the minimum melting current is also different. Although this current is affected by the external environment, it can be ignored in practical applications. The ratio of the minimum melting current of the melt to the rated current of the melt is generally defined as the minimum melting coefficient. The melting coefficient of commonly used melts is greater than 1.25, which means that a melt with a rated current of 10A will not melt when the current is below 12.5A.

It can be seen from here that the fuse can only play a role in short-circuit protection and cannot play a role in overload protection. If it really needs to be used in overload protection, the rated current must be reduced. For example, an 8A melt is used in a 10A circuit for short-circuit protection and overload protection, but the overload protection characteristics at this time are not ideal.

The selection of fuses mainly depends on the protection characteristics of the load and the size of the short-circuit current. For small-capacity motors and lighting branch lines, fuses are often used as overload and short-circuit protection, so the melting coefficient of the melt is expected to be appropriately small. RQA series fuses with lead-tin alloy melt are usually used. For larger-capacity motors and lighting trunks, short-circuit protection and breaking capacity should be considered. RM10 and RL1 series fuses with higher breaking capacity are usually selected; when the short-circuit current is large, RT0 and RTl2 series fuses with current limiting function should be used.

The rated current of the melt can be selected as follows:

1) When protecting stable loads without starting process such as lighting lines, resistors, electric furnaces, etc., the rated current of the melt is slightly greater than or equal to the rated current in the load circuit.

2) The melt current to protect a single long-term working motor can be selected according to the maximum starting current, or according to the following formula:

IRN ≥ (1.5～2.5)IN

In the formula, IRN--melt rated current; IN--motor rated current. If the motor starts frequently, the coefficient in the formula can be appropriately increased to 3 to 3.5, depending on the actual situation.

3) Protect multiple long-term working motors (power supply trunk lines)

IRN ≥ (1.5～2.5)IN max+ΣIN

IN max - the rated current of the single motor with the largest capacity. ΣIN the rest. The sum of the rated current of the motor.

3) Inter-stage coordination of fuses

In order to prevent over-level fuses from occurring and expanding the scope of the accident, the fuses of the upper and lower levels (i.e. power supply trunks and branch lines) should have good coordination. When selecting, the melt rated current of the upper level (power supply trunk line) fuse should be 1 to 2 steps larger than that of the lower level (power supply branch line). Commonly used fuses include tubular fuses R1 series, spiral fuses RLl series, stuffed enclosed fuses RT0 series and fast-acting fuses RSO, RS3 series, etc.

HZDL130 Primary Current Injection Tester