What is a Fuse? Fuse Definition & Meaning, A Complete Guide to Fuses

Introduction

The fuse is an electric element that protects electric devices. It is commonly connected in series with a circuit. When the fault current reaches a specific value, the fuse blows, cutting off the circuit and thereby protecting other equipment connected to it. The fuse is the most commonly used component in an over-current protection circuit. The traditional fuse primarily consists of two parts: a tube body with metal connecting terminals at both ends, and a metal melt within the tube body. The majority of fuses are cylinder-shaped, typically featuring a cartridge construction. However, with the rapid development of advanced technology, some special materials are now well applied, and the fuse industry has begun to move forward in the direction of sub-miniature products. A series of new products, including Surface-Mounted Devices (SMD) type fuses, have been developed sequentially.

Fuse's function

• The fuse can withstand overload within a specific range when current fluctuations occur due to the power supply or external interference.
• When a larger overload current appears in the circuit, the fuse should cut off the circuit within the set time to protect other electrical components in the circuit.
• When a huge current appears due to a short circuit, the fuse can safely cut off the circuit to avoid damage from the overload current.

Technical Parameter Term

• Rated current: The rated current values are typically: 100mA, 160mA, 200mA, 315mA, 400mA, 500mA, 630mA, 800mA, 1A, 1.6A, 2A, 2.5A, 3.15A, 4A, 5A, 6.3A, etc.

• Rated voltage: The rated voltage typically ranges from 24V to 250V, with values such as 32V, 63V, 125V, etc. The fuse can be used at a voltage less than or equal to its rated voltage, but it normally can’t be used at a voltage more than its rated voltage.

• Voltage drop: The fuse is operated at the rated current. When the fuse reaches heat balance, namely, the temperature is stabilized, the voltage difference measured at both ends of the fuse is the voltage drop. Because the voltage drop has a particular influence on the circuit, the voltage drop is clearly regulated in the European standard, but not defined in the American standard.

• Overload current: The overload current refers to the current that exceeds the normal operating current of the circuit. If the overload can’t be cut off in time, the electrical components in the circuit may be damaged. The short-circuit current refers to the current generated due to a partial or complete short circuit, and is typically very high, much higher than the overload current.

• Fusing characteristic: Namely, time-current characteristic. Usually, the fusing characteristic is expressed in two ways: the I-T diagram and the test report. The I-T diagram is a curve formed by connecting average operating time point of the fuse under different current loads in a coordinate system in which the overload current is X coordinate and the operating time is Y coordinate. Each fuse model has a corresponding curve that represents its fusing characteristics. While selecting fuses, you can refer to the curve. The test report is a record of the test data obtained by completing the test items required by the standard.

• Interrupting rating: Also called rated short circuit capacity, namely, at the rated voltage, the maximum current value (practical value for AC) at which the fuse can safely cut off the circuit. The interrupting rating is a crucial safety parameter for the fuse.

• Melting heat energy value: Specifically, the heat energy required to melt the fuse is denoted as I²t. The melting heat energy value is the product of the square of the corresponding current when the fuse blows out in 8ms or a shorter time, and the operating time. The limited time is 8ms to ensure that all heat generated by the fuse wire is used to melt the fuse, rather than being dissipated. It is a constant for each type of fuse wire and also a parameter of the fuse wire, which is determined during the design process.

• Temperature rise: The temperature rise is the difference between the temperature that is stabilized when the fuse is charged with a regulated current and the room temperature.

 Construction of a Fuse

The fuse consists typically of three part: The fuse-element part, it is the core of the fuse and has the function of cutting off the circuit when melted, resistance as far as possible small and be consistent, and more importantly, the fusing characteristic must be consistent; The electrode part, it has two parts and is connected with the melt and the circuit, the electrode part must have high electrical conductivity and doesn't generate obvious installation contact resistance; The support part, fuse-element is commonly slim and soft, the fuse support part is used to fix the fuse-element and make the three parts form a complete rigid body, so that the fuse can be easily installed and used, the fuse support part must have high mechanical strength, insulating property, heat resistance and flame resistance, and the phenomena of breakage, deformation, burning and short circuit don't occur while in use.

Working Principle of a Fuse

When the fuse is electrified, due to the fuse's resistance, the electric energy is converted into heat energy, which heats the melt. Simultaneously, the heat energy generated by the current radiates toward the ambient environment through the melt and shell and is dissipated by means of convection and conduction. When the fuse is charged with the allowed working current, the dissipated heat energy and the generated heat energy are balanced, and the heat energy is not accumulated in the melt. As a result, the fuse does not reach its melting point and can continue to operate. When the current passing the fuse reaches a specific value, the heat converted by electric energy increases, the heat dissipation speed can't catch up with the heating speed, the heat energy will be gradually accumulated on the melt, the temperature of the melt increases, when the melting point of the melt is reached, the melting wire starts to be melted and continues to absorb heat energy for further and to be converted into liquid, later, the temperature of the melt further increases to the evaporation point, the electric arc is formed. The electric arc is a gas discharge phenomenon, the strength of the electric arc is related to the voltage of the circuit, the higher the voltage, the stronger the electric arc, the fuse can't be used in the circuit, of which the voltage is higher than the fuse's rated voltage, that's because the electric arc is difficultly extinguished when the circuit voltage is higher than the fuse's rated voltage. In addition, the strength of the electric arc is also related to the circuit current; the bigger the current, the stronger the electric arc. If the electric arc cannot be extinguished in time, not only can the circuit not be cut off, but also the other components in the circuit may be damaged, and fire accidents can easily occur. For the high anti-explosion type fuse, the electric arc is extinguished by adding explosion-proof sand. After the electric arc is extinguished, the fuse can effectively cut off the circuit, thereby achieving the aim of protecting other equipment.

Type of fuse

　　According to the type of protection, the fuse can be divided into overcurrent protection and overheat protection. The fuse used for overcurrent protection is the commonly used fuse (also called a current-limiting fuse). The fuse used for overheat protection is often referred to as a "thermal cutoff fuse." The thermal cutoff fuse is used to protect heating electric devices, such as electric dryers, electric irons, electric cookers, electric furnaces, transformers, and motors. The fuse responds to the temperature rise of the electric devices and is not affected by the working current of the circuit. The working principle of the thermal cutoff fuse is different from that of the current-limiting fuse.

　　According to their range of use, fuses are categorized into electric device fuses, electric power fuses, vehicle fuses, and others.

　　According to volume, the fuse is categorized into four types: large, medium, small, and micro.

　　According to rated voltage, fuses are divided into high-voltage fuses, low-voltage fuses, and safe-voltage fuses.

　　According to shape, the fuse is divided into flat-head tube-shaped fuse, sharp-head tube-shaped fuse, cutting knife type fuse, screw type fuse, blade type fuse, flat plate type fuse and SMD type fuse.

　　According to fusing speed, the fuse is commonly divided into Time-Lag fuse, usually expressed by T, and Fast-Acting fuse, generally described by F.

　　According to the standard, the fuse is divided into European standard fuse (IEC60127 series), American standard fuse (UL248-1/14 series), Japanese standard fuse (METI Ordinance Article 1), etc.

　　According to protection type, the fuse is divided into a current fuse and a thermal cutoff fuse.

　　According to size, the fuse is divided into surface-mounted devices (type: 0603, 0805, 1206, and 1812) and plug-in types (Φ2.4*7, Φ3.6*10, Φ4.5*15, Φ5.2*20, Φ6.35*32, Φ8.5*8, 8.5*8*4, etc.).