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Ⅰ IntroductionIn electronics, the transient interference of voltage and current source is the main cause of damage to circuits and equipment, and it often causes immeasurable losses to the society. These disturbances usually come from the  the starting and stopping operation of power equipment, instability of the AC grid, lightning interference and electrostatic discharge, etc. They are almost ubiquitous and always present. Therefore, scientists have developed a high-efficiency circuit protection device TVS to effectively suppress transient interference.TVS (transient voltage suppressor) is a new product developed on the basis of Zener diode technology. Its circuit symbol is the same as that of ordinary Zener diode. As a common circuit protection component, it is widely used in various fields: automotive electronics, consumer electronics, power drives, industrial power distribution, renewable energy, telecommunications, home appliances, measuring instruments, medical electronics, industrial control, lighting, security systems, building control and automation, audio / video equipment, computers, etc. Learn more about tvs diode, let's check the following transient voltage suppressor tutorial. TVS Diode Tutorial: Transient Voltage SuppressorCatalogⅠ IntroductionⅡ Terminology2.1 Basic Characteristics2.2 Electrical Characteristics2.3 Main ParametersⅢ TVS SelectionⅣ TVS vs Varistor, CapacitorⅤ Application Examples5.1 Lighting Protection5.2 Transistor Protection5.3 Electric Relay Protection5.4 Silicon Control Protection5.5 Integrated Op Amp Protection5.6 Integrated Circuit (IC) Protection5.7 Microcomputer System Protection5.8 DC Regulated Power Supply Protection5.9 Suppression of Electromagnetic Pulse Interference Ⅱ Terminology2.1 Basic CharacteristicsTVS diode, under the specified reverse application conditions, when subjected to a high-energy transient overvoltage pulse, due to it has a very fast response time (sub-nanosecond) and a very high  surge absorption ability, its working impedance can be immediately reduced to a very low on value, allowing large currents to pass, and clamping the voltage to a predetermined level, thereby effectively protecting precision components in electronic circuits from damage. TVS can withstand instantaneous pulse power up to kilowatts, and its clamp response time is only 1ps (10-12S). The forward surge current allowed by TVS can reach 50 ~ 200A under the conditions of TA = 250C and T = 10ms.TVS diodes work similarly to common Zener diodes. If the breakdown voltage is higher than the mark, the TVS diode will conduct. Compared with the Zener diode, the TVS diode has a higher current conduction capability. When the two poles of a TVS diode are subjected to reverse transient high-energy shocks, the high impedance between the two poles of the TVS diode becomes low at a speed of the order of 10 ^ -12S, while absorbing surge power of up to several kilowatts. The clamped voltage between the two poles is at a safe value, which effectively protects precision components in electronic circuits from being damaged by surge pulses. Figure 1. Working Characteristic Curve of TVS DiodeWhen the reverse voltage of the two poles of the TVS is greater than the maximum reverse voltage, it starts to conduct reversely; after the reverse voltage is greater than the breakdown voltage, it begins to be broken down, while the current starts to change suddenly; when the reverse voltage is greater than the maximum clamping voltage, the tube is in an avalanche breakdown state. At this time, the current flowing through the tube increases sharply, and the voltage difference across the tube does not change much (the voltage is clamped).Under specified reverse application conditions, the TVS diode will provide a low-impedance path, and the instantaneous current flowing to the protected component will be shunted to the TVS diode through a large current method, while the voltage across the protected component will be limited to the clamping voltage of TVS. When the overvoltage condition disappears, the TVS diode returns to a high impedance state.  Note: Unidirectional and Bidirectional TVS DiodesUnidirectional TVS SymbolBidirectional TVS Symbol1) Look at the signs: for unidirectional tvs diode, there is a thin color ring, connected to the positive electrode, and for bidirectional tvs diode, there are two rings in the middle, or there is no sign, no polarity.2) Look at the specifications: bidirectional tvs is bidirectional conduction, and unidirectional tvs is unidirectional conduction.3) Look at the model: The model name of the tvs tube is regular, and most of the tvs diode models can see the parameters on the case. For details, it is necessary to consult the manufacturer.4) Using multimeter tool: the unidirectional has voltage, while avalanche breakdown characteristics are available on the DC side; voltage is on both sides, and the DC side is symmetrical on both sides.Bidirectional tvs diodes can absorb instantaneous large pulse power in both forward and reverse directions and clamp the voltage to a predetermined level. In addition, bidirectional TVS is suitable for AC circuits, and unidirectional TVS is generally used for DC circuits. 2.2 Electrical Characteristicsa. Unidirectional TVS (V-I characteristic) Figure 2. Unidirectional TVS DiodeThe unidirectional tvs diode has the same forward characteristics as ordinary Zener diodes, and the reverse breakdown inflection point is approximately “right angle” as a hard breakdown and is a typical PN junction avalanche device. The curve segment from the breakdown point to the VC value indicates that when there is a transient overvoltage pulse, the current of the device increases sharply while the reverse voltage rises to the clamped voltage value and remains at this level.b. Bidirectional TVS (V-I characteristic) Figure 3. Bidirectional TVS DiodeThe V-I characteristic curve of the bidirectional tvs diode is similar to the two back-to-back unidirectional tvs diodes combination. Its forward and reverse directions have the same avalanche breakdown characteristics and clamping characteristics. The symmetry relation of the breakdown voltage on both sides of the positive and negative is as follows: 0.9≤ VBR(positive)/(inverse) ≤1.1, once the interference voltage at both ends of it exceeds the clamping voltage will be immediately suppressed, thus the bidirectional tvs are very convenient for ac loop application.  2.3 Main Parameters1) breakdown voltage V(BR)In the region where the device breaks down, the voltage across the device is measured at the specified test current I (BR), which is called the breakdown voltage. In this area, the tvs diode becomes a low impedance path.2) maximum reverse pulse peak current IPPIn reverse operation, IPP refers to the maximum pulse peak current allowed by the device under specified pulse conditions. The product of IPP and the maximum clamping voltage VC (max) is the maximum value of the transient pulse power.The TVS should be properly selected during use, so that the rated transient pulse power PPR is greater than the maximum transient surge power that may occur in the protected device or wires.When the instantaneous pulse peak current appears, the TVS is broken down and its breakdown voltage value rises to the maximum clamping voltage value. As the pulse current decreases exponentially, the clamping voltage also decreases and returns to the original state. Therefore, TVS diode can suppress the impact of possible pulse power to effectively protect the electronic circuits.The test waveform of the TVS peak current uses a standard wave (exponential waveform), which is determined by TR / TP.Peak current rise time TR: The time from when the current reaches 0.9 IPP from 0.1 IPP.Half-peak current time TP: The time after the current passes through the maximum peak from zero and then drops to 0.5 IPP.The TR / TP values of typical test waveforms are listed below:A. EMP wave: 10ns / 1000nsB. Lightning wave: 8μs / 20μsC. Standard wave: 10μs / 1000μs3) Maximum reverse working voltage VRWMWhen the device operates in reverse, the voltage across the device is called the maximum reverse operating voltage VRWM under the specified IR, usually VRWM = (0.8 ~ 0.9) V(BR). At this voltage, the power consumption of the device is small. When used, VRWM should not be lower than the normal working voltage of the protected device or circuits.4) Maximum clamping current VC(max)The maximum voltage value across the device under the peak pulse current IPP is called the maximum clamping voltage. When used, VC (max) should not be higher than the maximum allowable safe voltage of the protected device. And the ratio of the maximum clamping voltage to the breakdown voltage is called the clamping coefficient.Clamping coefficient = VC(max) / V(BR), the general clamping coefficient is about 1.3.5) Reverse pulse peak power PPRThe PPR of TVS depends on the pulse peak current IPP and the maximum clamping voltage VC (max). In addition, it is also related to the pulse waveform, pulse time and ambient temperature.When the pulse time Tp is constant, PPR = K1‧K2‧VC (max) ‧IPP(K1 is the power coefficient, and K2 is the temperature coefficient of the power) The typical pulse duration tp is 1MS. When the pulse time tp applied to the transient voltage suppression diode is shorter than the standard pulse time, its peak pulse power will increase as tp is shortened.Figure 4. Peak Pluse Power vs Pluse TimeTVS reverse pulse peak power PPR is related to the pulse waveform subjected to surge, expressed by the power coefficient K1: E=∫i(t)‧V(t)dt     i (t) is the pulse current waveform, and V (t) is the clamping voltage waveform.This rated energy value is not reproducible to TVS in a very short time. However, in practical applications, surges often occur repeatedly. In this case, even if the single pulse energy is much smaller than the pulse energy that the TVS device can withstand, if repeat, these single pulse energy will accumulated, in some cases, it will exceed the pulse energy that the TVS device can withstand. Therefore, the circuit design must carefully consider and select the TVS device, so that the accumulation of pulse energy repeatedly applied within the specified interval does not exceed the pulse energy rating of the TVS device.6) Capacitance CPP  Figure 6. The Capacitance of TVS CircuitThe capacitance of TVS is determined by the area of the silicon sheet and the bias voltage. In the case of zero bias, the capacitance value decreases with the increase of the bias voltage. The value of the capacitance will affect the response time of the TVS device.7) Leakage current IRWhen the maximum reverse working voltage is applied to the TVS, the TVS tube has a leakage current IR. When the TVS is used in a high impedance circuit, the leakage current is an important parameter. In practice, especially in automotive electronics, this parameter affects static current.  Ⅲ TVS SelectionWhen selecting tvs diode, the specific conditions of the circuit must be considered, and generally the following principles should be followed:1) The clamping voltage VC (max) is not greater than the maximum allowable safe voltage of the circuit.2) The maximum reverse working voltage VRWM is not lower than the maximum working voltage of the circuit. Generally, VRWM can be selected to be equal to or slightly higher than the maximum working voltage of the circuit.3) The rated maximum pulse power must be greater than the maximum transient surge power present in the circuit. Ⅳ TVS vs Varistor, Capacitor1) TVS diode and varistor do not have switching characteristics like switching elements, but have voltage regulation characteristics like zener diodes.2) The varistor can withstand a larger surge current, and the larger the varistor can withstand the larger surge current, which can reach tens of kA to hundreds of kA at the maximum; but the non-linear characteristics of the varistor are poor and the limiting voltage is higher at large current, and the leakage current is larger at low voltage.3) The non-linear characteristics of TVS diodes are the same as those of Zener tubes. Leakage current before breakdown is very small. After the breakdown, it is in a standard voltage stabilization. Compared with varistors, the maximum clamping voltage of TVS diode is smaller, but its current capability is poor than a varistor. Since the breakdown voltage VBR and the clamping voltage VC of the varistors are relatively high, the current flow capability is relatively strong, and the surge pulse absorption capability is stronger, so it is more suitable for ESD or surge protection of the power interface.4) For the reaction speed, the response speed of the TVS is fast (ps level), while the varistor’s is slow ( ns level). In addition, the capacitance of both is large (ps: TVS also have low capacitance products).5) The TVS tube has high reliability, and a long service life, while the varistor has poor reliability, easy aging and short service life.Other ViewCompared with ceramic capacitors, TVS diodes can withstand a voltage of 15 kV, but ceramic capacitors have a lower ability to withstand high voltages. A 5 kV shock will cause about 10% of the ceramic capacitor to fail, and by 10 kV, its damage rate will reach 60%. Ⅴ Application Examples5.1 Lighting ProtectionIn thunderstorm-prone areas, lightning-induced voltage often breaks down some of the integrated circuits in a computer network. The reason is that cables are damaged due to transient high voltage caused by lightning induction, by installing tvs diodes in the microcomputer, it is useful to reduce damages and commercial loss. And the result shows that it is very practical, and it can improve the reliability of the whole machine application.TVS also have many other applications, for example, for VMOS high power transistors, the tvs diodes between the gate and the source and the machine can prevent gate breakdown and improve the reliability of the VMOS power tube application. 5.2 Transistor ProtectionVarious transient voltages can cause damage to the EB junction or CE junction of the transistor. Especially when the collector of the transistor has an inductive load (coil, transformer, motor), a high-voltage back-EMF can be generated, which often causes the transistor to be damaged. It is necessary to use a tvs diode as a protector. 5.3 Electric Relay ProtectionRelay contacts often use large currents to switch on and off high-current inductive loads such as motors, and the inductor has a high back electromotive force when switching, and has a large amount of energy. What’s more, the contacts are burned or broken to produce an arc, and the surge current generated by the arc is very large. To protect the contacts by suppressing the occurrence of arcs to protect the relays, adding a tvs diode is more effective. In the past, a capacitor or a capacitor series resistor, a diode or a diode series resistor and other suppression methods were used. 5.4 Silicon Control ProtectionThe thyristor may has wrong trigger and cause malfunction. The control electrode current cannot be too large and the voltage cannot be too high, in order to do it, TVS can be used for protection. 5.5 Integrated Op Amp ProtectionIntegrated op amps are very sensitive to external electrical stress. In the process of using op amps, if having excessive voltage or current due to operating errors or abnormal working conditions, especially surges and electrostatic pulses, it is easy to damage the op amp. In the integrating circuit, if the capacitor is charged and discharged to a high potential, and then the power supply voltage is cut off, a transient voltage will be generated at the input terminal, and a large discharge current will occur, resulting in damage to the operational amplifier. At this time, tvs protection method adopted at the input terminal of op amp to avoid device damage. If the capacitance value is large (such as greater than 0.1μF), the protecting effect will be very significant. 5.6 Integrated Circuit (IC) ProtectionAs integrated circuits become more integrated, their withstand voltages are getting lower and lower, and they are easily damaged by transient voltages. Protective measures must be taken, for example, adding tvs diode in the circuit, the CMOS circuit has a protection network at its input and output ends. 5.7 Microcomputer System ProtectionIn a typical microcomputer system, various interference or transient voltages entering through the power line, input line, and output line may cause the microcomputer to malfunction and fail, especially from the switching power supply. The on-off motor near the microcomputer, voltage surges and transients of AC power, electrostatic discharges, etc. may cause the system to fail, and in severe cases may damage the device. Connecting the tvs diode to the input and output lines of the power supply of the microcomputer can prevent the transient voltage from entering the “microcomputer” bus, strengthen the microcomputer's resistance to external interference, ensure the normal operation, and improve its reliability. 5.8 DC Regulated Power Supply ProtectionA DC regulated power supply with a transistor that expands the current output, adding a tvs diode to its regulated output can protect the equipment, and can also absorb peak voltage from the collector to the emitter in the circuit to protect the transistor. In a word, adding a tvs diode at the output end of each voltage stabilization source can greatly improve the reliability of the whole operation. 5.9 Suppression of Electromagnetic Pulse InterferenceA nuclear explosion will cause a strong electromagnetic pulse, which causes induced voltage in the wire. If the induced voltage exceeds the breakdown voltage of the device, it may cause the breakdown of the component, especially for long-term transmission, it is more easily to cause high voltage.TVS diodes are connected in parallel to the signal and power lines, which can absorb the induced voltage caused by electromagnetic pulses, ensure the reliability of the system, and avoid radiation damage to components. Frequently Asked Questions about Transient Voltage Suppression Diode1. How does a transient voltage suppressor work?Transient Voltage Suppressor Diode is a clamping device, so whenever the induced voltage exceeds the avalanche breakdown voltage, it absorbs the excess energy of the overvoltage event, and then it automatically resets after overvoltage condition. 2. What does a transient suppressor do?A transient voltage suppressor or TVS is a general classification of an array of devices that are designed to react to sudden or momentary overvoltage conditions. ... This makes TVS devices or components useful for protection against very fast and often damaging voltage spikes. 3. What is a transient voltage surge suppressor?A transient voltage surge suppressor is a device which is installed on an AC or DC power line to act as a cutoff if there is a momentary surge of electrical power, also known as a “transient.” TVSS devices are considered crucial to the protection of sensitive equipment which would result in circuitry damage or data. 4. What is a voltage transient?A transient voltage is a temporary unwanted voltage in an electrical circuit that range from a few volts to several thousand volts and last micro seconds up to a few milliseconds. ... Faulty contactors and lightning are the most common source of transients. 5. How does a transient voltage suppressor diode work?Transient Voltage Suppressor Diode is a clamping device, so whenever the induced voltage exceeds the avalanche breakdown voltage, it absorbs the excess energy of the overvoltage event, and then it automatically resets after overvoltage condition. 6. Which device can be used as a transient suppressor?Transient voltage suppression diodeOne such common device used for this purpose is known as the transient voltage suppression diode that is simply a Zener diode designed to protect electronics device against overvoltages. 7. What does a suppression diode do?A transient-voltage-suppression (TVS) diode, also transil or thyrector, is an electronic component used to protect electronics from voltage spikes induced on connected wires. 8. What is the difference between Zener and TVS diode?Zener diodes are used to make the voltage more stable. They act as a regulator as well as a protective device. TVS diode is intended to prevent high voltage transients such as Surge and ESD damaging. 9. Where are transient voltage most dangerous?Transient voltages are most dangerous while taking measurements on equipment. Should someone turn something off it could cause a transient voltage spike. 10. What is transient protection?Transients (momentary spikes in voltage or current) can disrupt or damage the products connected to signal or power lines. The most common transient protection schemes limit the voltage amplitude, current amplitude or transition times on the circuit they are protecting.

CatalogⅠ The Role of the FuseⅡ Working Principle of the FuseⅢ Classification of the FuseⅣ The Terminologies of the FuseⅤ Safety Standards and Signs for Fuse TubesⅥ Factors Affecting Fuse Life and Evaluation of Fuse Life6.1 Factors Affecting the Life of the Fuse6.2 Effect of the Use of the Fuse After Aging6.3 Test Evaluation of Fuse LifeⅦ Fuse Suitable CircuitⅧ Precautions for Using the Fuse TubeⅨ Selection of Fuse TubeⅩ FAQⅠ The Role of the Fuse• Under normal circumstances, the fuse acts as a connection circuit in the circuit.• In the case of abnormal (overload), the fuse acts as a safety protection element in the circuit, and safely cuts off and protects the circuit by blowing itself. Figure 1.Ⅱ Working Principle of the FuseWhen the fuse is energized, the heat converted by the electrical energy causes the temperature of the meltable item to rise. When the normal working current or the allowable overload current passes, the generated heat is radiated to the surrounding environment through the meltable body and the outer casing, and the heat generated by convection, conduction, etc. is gradually balanced with the generated heat. If the generated heat is greater than the amount of heat dissipated, the excess heat gradually accumulates on the meltable item, causing the temperature of it to rise; when the temperature reaches and exceeds the melting point of the meltable item, it will be melted, blown and the current will be cut off and plays a role as a safety protection circuit. Ⅲ Classification of the Fuse• According to the external size, it is divided into φ2, φ3, φ4, φ5, φ6 and others. • According to the blowing characteristics, it is divided into fast-blown type, medium time-delay blown type, and time-delay type. (it can also be divided into express, strong delay). • According to the breaking capacity, it is divided into low breaking type and high breaking type (it can also be divided into enhanced breaking type). • According to safety standards (or areas of use): UL/CSA (North America) specification, IEC (China, Europe, etc.) specification, MIT/KTL (Japan/Korea) specification, etc. • Other classifications. Ⅳ The Terminologies of the Fuse• Rated current: The nominal operating current of the fuse tube (the maximum current that the fuse maintains normal operation for a long time under normal conditions). • Rated voltage: The nominal working voltage of the fuse (the maximum voltage that can safely withstand when the fuse is disconnected). When a fuse is selected, the rated voltage of the selected fuse should be greater than the input voltage of the protected circuit. • Breaking capacity: When a large overload current (such as a strong short circuit) occurs in the circuit, the fuse can safely cut off (break) the maximum current of the circuit. It is the most important safety indicator for fuses. Safe breaking means that something endangers the surrounding elements, components and even personal safety such as splashing, burning, the explosion will not happen in the breaking circuit.  • Overload capability (load carrying capacity): The fuse can maintain the maximum overload current for working within the specified time. When the current flowing through the fuse exceeds the rated current, the temperature of the meltable item will gradually rise after a period of time and eventually be blown. The UL standard stipulates that the fuse remains in operation for more than 4 hours, and the maximum unblown current is 110% of the rated current (100% for the miniature fuse tube) The IEC standard stipulates that the fuse remains in operation for more than one hour, and the maximum unblown current is 150% of the rated current. • Fuse characteristics (I-T): The relationship between the load current applied to the fuse and the fuse blowing time. Blowing characteristic curve (I-T curve): A curve formed by the average blowing time coordinate point of the fuse under different load currents in a logarithmic coordinate system in which the load current is the X-axis and the blowing time is the Y coordinate. Each type of fuse has a corresponding curve that represents its blowing characteristics, which is a good indication of the fuse's overload performance and it is for reference when selecting the fuse. Blowing characteristics table: A table consisting of several specified representative load current values and corresponding blowing time ranges. All safety standards have clearly stated that this is the most important basis for the acceptance of fuses. For example, fast-blow type such as UL, CSA, MIT/KTLA, is specified as:In 100% 4 hours(minimum)In 135% 1 hour(maximum)In 200% 2 minutes(maximum) • Melting heat value (I2t): The nominal energy value that the cut-off current needed to melt and partially carburate the fuse, which is simply the minimum amount of heat required to blow the fuse. Total I2t=melted I2t+ arcing I2t The melting I2t (corresponding to the pre-arcing I2t in the IEC standard) refers to the energy required from the melting of the fuse to the moment of arcing; the arcing I2t refers to the energy required for the arcing from the moment it starts to the moment it eventually extinguishes. For low-voltage fuses, the arcing time is very short and often negligible. That is to say, the arcing I2t can be calculated as zero. Both UL and IEC do not require I2t, but I2t has some help with fuse selection. The I2t measurement of the fuse is calculated as I2t when the fuse's blowing time is less than 10ms (usually 8ms). • Voltage drop: The voltage difference across the fuse after thermal equilibrium underrated current conditions. • Temperature rise: Under a certain current condition, the difference between the surface temperature of the fuse and the initial temperature of the energization (which can be understood as the ambient temperature) after the heat balance is reached, that is, the temperature rise = the surface temperature of the fuse - the ambient temperature. Figure 2.Ⅴ Safety Standards and Signs for Fuse Tubes• UL, CSA standards: North American regional safety standards such as the United States, Canada; small current fuse tube standards are UL248-1/14, CSA248-1/14.Safety sign:--- UL/CSA LIST (Listing Sign), the product safety sign passed the test in accordance with UL/CSA248-1/14.--- UL/CSA RECOGNIZED (Approved Sign), the product safety mark passed the test partly in accordance with UL/CSA248-1/14. • JIS Standard: Japanese Electrical Safety Standard. The standard for small current fuse tubes is JIS C6575.Safety sign:--- T--- PSEBoth signs were valid before the end of 2006, after which only the “PSE” mark was valid. • KTL Standard: Korean Electrical Safety Standard.Safety sign:--- K • IEC standards: International Electrotechnical Commission standards and safety standards used in Europe and China. The standard for small current fuse tubes is IEC60 127, GB 9364 (China).Safety sign:CCC --- ChinaSEMKO --- SwedenVDE --- GermanyBSI --- UKIMQ --- Italy Ⅵ Factors Affecting Fuse Life and Evaluation of Fuse Life6.1 Factors Affecting the Life of the Fuse• Working environment temperature:Excessive ambient temperature is detrimental to the life of the fuse. Time-delay (slow-blow) fuses, such as tin ball type, begin to spread to the wire when the temperature is approximately 160℃ (150-170℃); the meltable item (wire) of the fast-blow fuse begins to violently oxidize at a temperature approximately equal to 200℃ (175 to 225℃). As the fuse is oxidized from the outside to the inside, multiple times of diffusion, thermal stress fatigue, etc., the life of the fuse will be gradually shortened. Therefore, it is recommended that the time-delay fuse should not work above 150℃ for a long time, and the fast-blow fuse should not work above 175~225℃ for a long time. • Pulse current:Constant pulse shock will cause thermal cycling, which will cause the diffusion, oxidation, thermal stress, etc. of the fuse to be generated and even accelerated. The fuse will age as the pulse energy and frequency increase. The impact resistance life of the fuse depends on the I2t of the pulse as a percentage of the fuse's own I2t; normally, it should be less than 20%, so that the fuse can withstand more than 100,000 times of impact. • Other:Such as the tube clamp in contact with the fuse, and the length and cross-sectional area of the connecting wire. The contact resistance between the fuse and the pipe clamp is large, which is detrimental to the service life. The UL standard specifies that the contact resistance between the fuse to the tube clamp is less than 3mΩ during the test. When the contact resistance is large, the tube clamp does not dissipate heat but generates heat and transmits it to the fuse. 6.2 Effect of the Use of the Fuse After AgingAfter the fuse has aged, the situation that the current should be cut off and the fuse is not blown will not happen. When the fuse ages, it is equivalent to a drop in the rated value (current) rather than a rise, so there is no safety problem in the circuit, but the circuit is cut off under a small overload current or pulse. 6.3 Test Evaluation of Fuse LifeThe "endurance test method" is specified in the IEC standard, and there is no similar regulation in the UL standard. The durability test in the IEC standard is the life test by using the DC power supply test at normal temperature:• The voltage drop is measured until the temperature is stable under the rated current;• 1.2 times of rated current for 1h, cut off current for 15min and circulate for 100 times;• Power on 1.5In for 1h and measure voltage drop;• Measure the voltage drop with method a. Requirements: The voltage drop change before and after the test should not exceed 10%, and the sign is still clear and identifiable, and the end cap solder joint does not show any deterioration. Figure 3.Ⅶ Fuse Suitable Circuit• Very fast and fast-blow type fuse tubes: Suitable for circuits with relatively constant current, or circuits with low inrush current, and there are shock-resistant fragile components in the circuit. • Medium time-delay and time-delay blown fuse tubes: Suitable for circuits with normal inrush current, and there are no shock-resistant fragile components in the circuit. Lightning-resistant fuse tube for special circuits that need to withstand lightning strikes, such as telephones. • Breaking current fuse tube: Suitable for circuits where large short-circuit current may occur. • Oxygen resin package and plastic case type fuse tube: suitable for installation of dense components or circuits where contact short circuits may occur. • 350V, 300V fuse tube: suitable for electronic rectifiers and other products. Ⅷ Precautions for Using the Fuse Tube1. The rated voltage of the selected fuse should be greater than the input voltage of the protected circuit. 2. The rated current of the UL specification fuse is determined under laboratory conditions and should be used less than 75% of the nominal value in actual use. For example, the circuit operating current is 0.75A, we can select the fuse tube with a minimum rated current of 1A. 3. The rated current of the IEC specification fuse tube can be used at 90% or 100% of the nominal value in actual use. For example, the circuit operating current is 0.9A, and the fuse tube with a minimum rated current of 0.9A or 1A can be selected. 4. Under different operating environment temperatures, the working life of the fuse is different. The higher the temperature, the shorter the working life of the fuse. In actual selection, the rated current of the fuse should be increased according to the coefficient. 5. The breaking capacity of the fuse tube is proportional to its volume and inversely proportional to the rated voltage, that is, the larger the volume or the smaller the rated voltage, the larger the breaking capacity of the fuse tube; the smaller the volume or the larger the rated voltage, the smaller the breaking capacity of the fuse tube. Therefore, if a small-size fuse tube is used, it is necessary to determine that the short-circuit current that may occur in the protected circuit is not too large; if a large short-circuit current may occur in the protected circuit, a larger-size fuse tube with a larger breaking current must be selected. 6. The surge I2t of the protection circuit should be less than 20% of the rated I2t of the fuse tube. The fuse tube can withstand more than 100,000 surges in the protected circuit. Ⅸ Selection of Fuse Tube1. Determine the safety sign: According to the market requirements for the product to be sold, select the safety certification sign and safety standard (UL standard or IEC standard fuse tube) of the fuse tube. 2. Determine the dimensions of the fuse tube: Select the dimensions of the fuse tube according to the installation space and the defined safety certification sign and safety standards. 3. Determine the model number: Select the type of fuse tube based on the current characteristics of the circuit being protected. For example, if the current characteristic of the protected circuit is a constant current, the fast-blow type should be selected. 4. Determine the rated voltage: Determine the rated voltage of the fuse tube according to the input voltage of the protected circuit and the requirements for use. For example, if the input voltage of the protected circuit is 220V, the fuse tube with rated voltage above 220V should be selected, 250V, 300V, 350V, etc. can also be selected; but considering the cost factor, it is not necessary to use the rated voltage which is too high. 5. Determine the minimum rated current: According to the stable operating current of the protected circuit and the relevant use loss factor, the rated current of the fuse tube is initially determined. For example, the stabilized working current of the protected circuit is 1A, the UL standard time-delay fuse tube should be selected, and the working environment temperature is about 80℃. The minimum rated current of the fuse tube is selected as 1A × 1.25 ÷ 0.5 = 2.5A. 6. Determine the minimum I2t of the fuse tube: Determine the I2t of the fuse tube based on the surge I2t of the protected circuit. For example, the surge I2t of the protected circuit is 1 (A2S). To ensure that the fuse tube can withstand more than 100,000 times of impact, the I2t of the fuse tube should be greater than 1÷0.2=5 (A2S). 7. Determine the rated current of the fuse tube: According to the minimum rated current and the minimum I2t value, check the corresponding model specifications, and take the primary rated current specification that is greater than the minimum rated current value and whose I2t value is also greater than the minimum I2t value as the rated current of the selected fuse tube. For example, based on the above minimum,(1) If the I2t of the rated current of 2.5A is 4.3A2S and the I2t of 3A is 5.4A2S, take 3A as the rated current of the selected fuse tube;(2) If the I2t of rated current of 2A is 5.3A2S and the I2t of 2.5A is 7.6A2S, take 2.5A as the rated current of the selected fuse tube. Ⅹ FAQ1. What is Fuse?A Fuse or an Electric Fuse is an Electrical / Electronic device that protects the circuit from different electrical faults like over-current and overload. Fuses can be considered sacrificial elements in the circuit as they act as a weak link in the entire circuit. 2. What is the working principle of fuse?An electric fuse is based on the principle of the heating effect of electric current. It is made up of thin metallic wire of non-combustible material. A fuse is always connected between the ends of the terminal in a series connection with the circuit. 3. What is the application of fuse?Used to protect transformers, motors and power systems from over-current conditions. In feeders, power transformers, and solar circuits. Electrical appliances and house distribution boards use fuse for domestic purposes. 4. What is the type of fuse?Fuses can be divided into two major categories, AC fuses, and DC fuses. The below block diagram illustrates the different types of fuse under each category.  5. Are fuses AC or DC?Generally, fuses have a DC voltage rating that is half of the maximum AC voltage rating. 6. Why fuse is not used in the neutral wire?Because the fuse can disconnect the circuit only when the excess current flows completely through the neutral. ... Since, neutral is not a live conductor coming from the source, disconnecting a neutral line can only open the current path through neutral. But, the live phase still carries the charge. 7. How do I choose a fuse size?In order to select the right amperage of the fuse, you first need to know the full-load steady-state current of the circuit at an ambient temperature of 25º C (68º F). Once the current value is determined, then a fuse rating should be selected to be 135% of this value (taken to the next standard value). 8. How do you use fuses in a circuit?Fuses should always be connected to the hot wire and should be placed before any other component in the circuit. In most projects, the fuse should be the first thing the hot wire connects to after it enters your project enclosure. 9. How long do fuses last?Fuses never need to be replaced unless they are tripped/activated by a failing component or any other even with the circuits of the car. They are encapsulated in plastic and are in a vacuum inside the piece. As long as the current limit isn't reached, that wire will not burn out. 10. Do fuses reduce voltage?The voltage rating of a fuse must be at least equal to or greater than the circuit voltage. It can be higher but never lower. ... If a fuse is used with a voltage rating lower than the circuit voltage, arc suppression will be impaired and, under some overcurrent conditions, the fuse may not clear the overcurrent safely. 

IntroductionThermal relay is a protective device.  are protective devices. It is used in conjunction with a contactor to protect electric motors. The basic working principle of thermal relay is that, when a bimetallic strip is heated up by a heating coil carrying over current of the system, it bends and makes normally open contacts. Getting to know more about the thermal basics just check the following note. CatalogIntroductionⅠ The Working Principle and Structure of Thermal Relay  1.1 The Role and Classification of Thermal Relay  1.2 Protection Characteristics and Working Principle of Thermal RelayⅡ Selection and Setting Principle of Thermal Relay  2.1 Thermal Relay Selection Overview  2.2 Type Selection of Thermal Relay  2.3 Selection of Rated Current of Thermal Relay  2.4 Selection of Thermal Element Setting Current  2.5 Reliable and Reasonable Protection Characteristics of The Thermal Relay  2.6 Other ConsiderationsⅢ Other Matters Needing Attention  3.1 Installation Direction  3.2 Selection of Connecting Wires  3.3 Use Environment  3.4 Adjustment of Thermal RelayⅣ Frequently Asked Questions about Thermal RelayⅠ The Working Principle and Structure of Thermal Relay1.1 The Role and Classification of Thermal RelayIn the electric drag control system, when the three-phase AC motor runs under abnormal conditions such as long-term under-load and under-voltage operation, long-term overload operation, and long-term single-phase operation, it will cause the motor winding to overheat and even burn out. In order to give full play to the overload capacity of the motor, to ensure the normal start and operation of the motor, and once the motor is overloaded for a long time, it can automatically cut off the circuit, so that there are electrical appliances that can change the operating time with the degree of overload and that is thermal relay.  Obviously, the thermal relay is used for overload protection of the three-phase AC motor in the circuit. It must be pointed out that, due to the thermal inertia of the heating elements in the thermal relay, instantaneous overload protection cannot be done in the circuit, and short circuit protection cannot be done either. Therefore, it is different from overcurrent relays and fuses. According to the number of phases, there are three types of thermal relays: single-phase, two-phase, and three-phase. Each type has different specifications and model numbers according to the rated current of the heating element. Three-phase thermal relays are often used in three-phase AC motors for overload protection. Divided by function, there are two types of three-phase thermal relay. One is without phase failure protection and the other one is with phase failure protection.1.2 Protection Characteristics and Working Principle of Thermal Relay1) Protection Characteristics of Thermal Relay Because the contact action time of the thermal relay is related to the overload of the motor being protected, before analyzing the working principle of the thermal relay, the relationship between the motor's overload current and the motor's energizing time must be clarified under the condition that the motor does not exceed the allowable temperature rise. This relationship is called the overload characteristic of the motor. When an overload current occurs during the motor operation, it will inevitably cause the winding to heat up. According to the thermal equilibrium relationship, it is not difficult to draw the conclusion that under the condition of allowable temperature rise, the motor energizing time is inversely proportional to the square of its overload current. According to this conclusion, it can be concluded that the motor's overload characteristics have inverse time characteristics, as shown by curve 1 in Figure 1. Figure 1. Overload Characteristics of the Motor and Protection Characteristics of the Thermal Relay and Their Coordination In order to adapt to the overload characteristic of the motor and play the role of overload protection, it is required that the thermal relay should also have the inverse time characteristic as the motor overload characteristic. For this reason, the thermal relay must have a resistance heating element. The thermal effect generated by the overload heating current through the resistance heating element causes the sensing element to act, thereby driving the contact to complete the protection function.  The relationship between the overload current passed in the thermal relay and the action time of the thermal relay contact is called the protection characteristic of the thermal relay, as shown by curve 2 in figure 1. Considering the effects of various errors, the overload characteristics of the motor and the protection characteristics of the relay are not a curve, but a belt. Obviously, the larger the error, the wider the belt; the smaller the error, the narrower the belt. It can be known from the curve 1 in the figure that when the motor is overloaded, it is safe to work below the curve 1. Therefore, the thermal relay's protection characteristics should be close to the motor's overload characteristics. In this way, if an overload occurs, the thermal relay will operate before the motor reaches its allowable overload limit, cutting off the power to the motor to prevent damage. 2) Working Principle of Thermal Relay The heat-generating heating element in the thermal relay should be connected in series with the motor circuit. In this way, the thermal relay can directly reflect the overload current of the motor. The sensing element of a thermal relay generally uses a bimetal. The so-called bimetallic sheet is to mechanically roll two metal sheets with different linear expansion coefficients into one body. The larger the expansion coefficient is called the active layer, the smaller the expansion coefficient is called the passive layer. The bimetallic sheet undergoes linear expansion when heated. Because the linear expansion coefficients of the two layers of metal are different and the two layers of metal are closely attached together, the bimetallic sheet is bent to the passive layer side, and the mechanical force generated by the bending of the bimetallic piece drives the contact action. There are four types of bimetal heating methods, namely direct heating, indirect heating, composite heating, and current transformer heating. The direct heating type uses the bimetal as a heating element and allows current to pass through it directly; the heating element of the indirect heating type is made of resistance wire or tape, is wound around the bimetal and is insulated from the bimetal; the composite heating type is between the above two methods; the heating element of the current transformer heating type is not directly connected to the motor circuit, but is connected to the secondary side of the current transformer. This method is mostly used in situations where the motor current is relatively large to reduce the current passes through the heating element. Figure 2. Structural Schematic of the Thermal Relay The thermal element 3 is connected in series to the motor stator winding, and the motor winding current is the current flowing through the thermal element. When the motor is running normally, although the heat generated by the thermal element can bend the bimetal 2, it is not enough to make the relay operate; when the motor is overloaded, the heat generated by the thermal element increases, causing the bending displacement of the bimetal to increase.  After a certain period of time, the bimetal is bent to push the guide plate 4, and the contacts 9 and 6 are separated by the compensating bimetal 5 and the push rod 14, the contacts 9 and 6 are normally-closed contacts in which the thermal relay is connected to the contactor coil circuit, and the contactor is de-energized after being disconnected. The normally-open contacts of the contactor disconnect the power supply of the motor to protect the motor. The adjusting knob 11 is an eccentric wheel, which constitutes a lever with the support 12, and 13 is a compression spring. Turning the eccentric wheel and changing its radius can change the contact distance between the compensating bimetal 5 and the guide plate 4 so that the purpose of adjusting the setting action current is achieved. In addition, the position of the normally-open contact 7 is changed by adjusting the reset screw 8 so that the thermal relay can work in two working states: manual reset and automatic reset. When debugging the manual reset, after the fault is excluded, button 10 must be pressed to restore the movable contact to the contact position of the static contact 6. 3) Thermal Relay with Open Phase Protection One of the main reasons for a three-phase asynchronous motor to burn out is that wiring of a three-phase motor is loosened or a phase fuse is blown. If the motor protected by the thermal relay is Y connection method when one phase power failure occurs in the line, the current of the other two phases will increase a lot. Since the line current is equal to the phase current, the current flowing through the motor windings and the current flowing through the thermal relay are increased by the same proportion, so ordinary two-phase or three-phase thermal relays can protect this.  If the motor is △ connection method, the phase current and line current of the motor will not be the same when the phase failure occurs, the current flowing through the motor windings and the current flowing through the thermal relay will increase in different proportions, and the thermal element is connected in series with the power supply line of the motor, and it is set according to the rated current of the motor, that is, the line current, and the setting value is relatively large. When the fault line current reaches the rated current, in the motor winding, the fault current of the phase winding with the larger current will exceed the rated phase current, and there is a danger of overheating and burning. Therefore, the △ connection method must use a thermal relay with phase failure protection. The thermal relay with phase failure protection is a differential mechanism added to the ordinary thermal relay to compare the three currents. The structural principle of the differential phase-open protection device is shown in figure 3. The guide plate of the thermal relay is changed to a differential mechanism, which is composed of an upper guide plate 1, a lower guide plate 2 and a lever 5. They are connected by a rotating shaft. Figure 3a shows the positions of the components of the mechanism before power is applied. Figure 3b shows the position during normal energization. At this time, the three-phase bimetals are bent to the left by heating, but the bending deflection is not enough. Therefore, the lower guide plate is moved to the left for a short distance, and the relay does not operate. Figure 3c shows the situation when the three phases are overloaded simultaneously. The three-phase bimetal is bent to the left at the same time, and the lower guide plate 2 is pushed to move to the left. The normally-closed contact is immediately measured by lever 5. Figure 3 shows the disconnection of phase C.  At this time, the phase C bimetal gradually cools down, the end moves to the right and pushes the upper guide plate 1 to the right. While the temperature of the other two-phase bimetals rises, the ends are bent to the left, pushing the lower guide plate 2 to continue to move to the left. Because the upper and lower guide plates move left and right, a differential function occurs, and the normally-closed contacts are opened by the amplification of the lever. Due to the differential function, the thermal relay is accelerated to protect the motor when the phase failure occurs. Figure 3. Schematic Diagram of Differential Relay Phase Failure Protection Mechanism of Thermal Relay Ⅱ Selection and Setting Principle of Thermal Relay2.1 Thermal Relay Selection OverviewThe thermal relay is mainly used to protect the motor from overload. In order to ensure that the motor can obtain both necessary and sufficient overload protection, it is necessary to fully understand the performance of the motor, and assign it with a suitable thermal relay to perform the necessary settings. Generally, conditions related to the motor are the working environment, starting current, load nature, working system, allowable overload capacity and so on. In principle, the ampere-second characteristic of the thermal relay should be as close as possible or even overlap the motor's overload characteristic, or under the motor's overload characteristic, and at the same time, the thermal relay should not be affected (not actuated) at the moment when the motor is temporarily overloaded and started. The correct selection of the thermal relay is closely related to the working system of the motor. When the thermal relay is used to protect the motor for long-term or intermittent long-term operation, it is generally selected according to the rated current of the motor. For example, the setting value of the thermal relay may be equal to 0.95-1.05 times of the rated current of the motor, or the median value of the setting current of the thermal relay is equal to the rated current of the motor, and then adjust. When the thermal relay is used to protect a motor that is repeatedly operated for a short time, the thermal relay has only a certain range of adaptability. If there are many operations per hour, a thermal relay with a speed saturation current transformer must be selected. For special working motors with frequent forward and reverse phase on and off, it is not appropriate to use thermal relays as overload protection devices. Instead, use temperature relays or thermistors embedded in the motor windings to protect them.2.2 Type Selection of Thermal RelayThe thermal relay can be divided into the two-pole types and three-pole types from the structural type. The three-pole type is divided into phase-open protection and no phase-open protection, which should be selected according to the stator wiring of the protected motor. When the motor stator winding is in delta connection, a three-pole thermal relay with phase failure protection must be used; for a motor using the star connection method, a thermal relay without phase failure protection is generally used. Because the general motor does not have a neutral wire when using the star connection method, the two-pole or three-pole type of the thermal relay can be used. However, if the motor is set to use a star connection method with a neutral wire, the thermal relay must use a three-pole type. In addition, generally a two-phase structure thermal relay should be selected for light-load starting, long-term working motors or intermittent long-term working motors; when the current and voltage balance of the motor is poor, the working environment is poor, or there are fewer people to look after, three-phase thermal relay can be used.2.3 Selection of Rated Current of Thermal Relay1) Ensure the normal operation and starting of the motorIn the case of normal starting current and starting time and infrequent starting, it must be ensured that the starting of the motor does not cause the thermal relay to malfunction. When the starting current of the motor is 6 times the rated current, the starting time does not exceed 6s, and rarely starts continuously, the thermal relay can generally be selected according to the rated current of the motor. (In practice, the rated current of the thermal relay can be slightly larger than the rated current of the motor) 2) Consider the object of protection-the characteristics of the motorModels, specifications, and characteristics of motors. The insulation materials of motors are classified into A, E, and B grades. Their allowable temperature rises are different, so their ability to withstand overload is also different, which should be paid attention to when selecting a thermal relay. In addition, the open-type motor is easier to dissipate heat, while the closed-type motor is much more difficult to dissipate heat. With a slight overload, its temperature rise may exceed the limit. Although the selection of the thermal relay is based on the rated current of the motor in principle, the rated current of the thermal relay (or thermal element) that it is equipped with should be appropriately small for the motor with poor overload capacity. In this case, the rated current of the thermal relay (or thermal element) can also be taken as 60% -80% of the rated current of the motor. 3) Consider load factorsIf the nature of the load is not allowed to stop, even if the overload will shorten the life of the motor, the motor should not be allowed to trip unexpectedly, so as not to suffer a huge loss many times higher than the price of the motor. At this time, the rated current of the relay can be selected to a larger value (of course, the selection of the motor under this working condition generally also has a strong overload capacity). In this case, it is best to use the protective measures of the organic combination of thermal relays and other protective appliances, and only consider tripping when a very dangerous overload occurs. In short, this is not a dogmatic formula and should be considered comprehensively. 2.4 Selection of Thermal Element Setting CurrentAccording to model number of the thermal relay and the rated current of the thermal element, the adjustment range of the setting current of the thermal element can be found out. Generally, the setting current of the thermal relay is adjusted to the rated current of the motor; for motors with poor overload capacity, the setting value of the thermal element can be adjusted to 0.6-0.8 times of the rated current of the motor; when the motor starts for a long time, drags the impact load or is not allowed to stop, the setting current of the thermal element can be adjusted to 1.1-1.15 times of the rated current of the motor. 2.5 Reliable and Reasonable Protection Characteristics of The Thermal RelaySpecifically, it should have an inverse time characteristic similar to the allowable overload characteristic of the motor, and it should be below the allowable overload characteristic of the motor, and it should have high accuracy to ensure the reliability of the protective action. 2.6 Other Considerations1) Operating frequency: When the operating frequency of the motor exceeds the operating frequency of the thermal relay, such as the motor's reverse braking, reversible operation, and dense on-off, the thermal relay cannot provide protection. At this time, you can consider using a semiconductor temperature relay for protection. 2) It is not necessary to set overload protection for motors with short working hours and long intervals (such as rocker lifting motors for rocker drilling machines, etc.), and motors that have little possibility of overload despite long-term work(such as exhaust fans, etc.). 3) Thermal relays are generally not suitable for motors with a jog, heavy load starting, continuous forward and reverse rotation, and reverse braking. 4) It should have a certain temperature compensation: due to the change in the temperature of the surrounding medium, under the same overload current, the operation of the thermal relay will cause an error. To eliminate this error, temperature compensation measurements should be set up. 5) In general, the principle that the protected motor should not restart automatically even after the thermal relay is automatically reset after the thermal relay protection action, otherwise, the thermal relay should be set to the manual reset state. This is to prevent the motor from being repeatedly restarted several times to damage the equipment before the fault is eliminated.  For example: Generally, for the control circuit using button control to manually start and stop, the thermal relay can be set to the automatic reset form; for the automatic start circuit using automatic component control, the thermal relay should be set to the manual reset form; Any thermal relay that can be automatically reset should be able to be automatically reset reliably within 5 minutes after operation, while the manual reset one should be reset reliably when the manual reset button is pressed by hand within 2 minutes after the operation. Most products generally have both manual and automatic reset methods and can be adjusted to any method with screws to meet the needs of different occasions. 6) The operating current value should be adjustable to meet the needs in production and use, and reduce the specification-grade, so the thermal relay of a certain specification should be able to be realized by adjusting the cam. 7) Since it takes time for the thermal element to deform due to heat, the thermal relay can only be used as overload protection for the motor, not as short circuit protection. Therefore, when using a thermal relay, a fuse should be installed as short circuit protection. For heavy load, frequent-starting large-capacity important motors, overcurrent relays (time-delay action type) can be used for its overload and short circuit protection. Ⅲ Other Matters Needing Attention3.1 Installation DirectionThe installation direction of the thermal relay is easily overlooked. In the thermal relay, there is a current that generates heat through the heating element, which promotes the action of the bimetal. There are three ways of heat transfer: convection, radiation and conduction. Convection is directional, and heat is transferred from the bottom up. During the placement, if the heating element is under the bimetal, the bimetal will heat up quickly and the action time will be short; if the heating element is next to the bimetal, the bimetal will heat slowly and the action time of the thermal relay will be long.  When the thermal relay is installed with other electrical appliances, it should be installed below the electrical appliances and away from other electrical appliances by more than 50 mm to avoid the influence of other electrical appliances. The installation direction of the thermal relay should be in accordance with the specifications of the product manual to ensure that the thermal relay's operating performance is consistent during use.3.2 Selection of Connecting WiresThe connecting wires at the output end should be selected according to the rated current of the thermal relay. Too thick or too thin will also affect the normal operation of the thermal relay. If the connecting wire is too thin, the heat generated by the connecting wire will be transferred to the bimetallic sheet, and the heat-generating component will dissipate less heat along the wire, which shortens the trip time of the thermal relay.  On the contrary, if the connecting wire is too thick, this will extend the trip time of the thermal relay. For thermal relays with a rated current of 10 A, the cross-sectional area of the connecting wire at the output end is preferably 2.5 mm2 (single-strand copper-core plastic wire), the one of 20 A is preferably 4 mm2 (single-strand copper-core plastic wire), 16 mm2 is suitable for the one of 60 A(multi-strand copper-core rubber flexible wire), and the one of150 A is preferably 35 mm2 (multi-strand copper-core rubber flexible wire).  Because the material and thickness of the wire will affect the heat conduction from the termination of the thermal element to the external heat, if the wire is too thin, the axial thermal conductivity is poor, and the thermal relay may act in advance; if the wire is too thick, the axial heat conduction is fast, and the thermal relay may lag behind. The connecting wire at the output end of the thermal relay is generally a copper core wire. If an aluminum core wire is used, the cross-sectional area of the wire should be increased by 1.8 times, and the end of the wire should be tinned. Reference table for selection of cross-section of connecting wire:Setting current of thermal relay I / ACross-sectional area of connecting wire MM20 < IN ≤81.08 < IN ≤121.512 < IN ≤202.520 < IN ≤254.025 < IN ≤326.032 < IN ≤5010.050 < IN ≤6516.065 < IN ≤8525.085 < IN ≤11535.0115 < IN ≤15050.0150 < IN ≤16070.03.3 Use EnvironmentThis mainly refers to the ambient temperature, which has a greater impact on the speed of the thermal relay. The temperature of the medium surrounding the thermal relay should be the same as the temperature of the medium surrounding the motor, otherwise, the adjusted fit will be destroyed. For example: when the motor is installed in an environment of high temperature and the thermal relay is installed in an environment of lower temperature, the action of the thermal relay will lag (or the action current is large); otherwise, its action will be advanced (or the action current is small). For thermal relays without temperature compensation, they should be used at the place where there is little difference in an ambient temperature between the thermal relay and the motor. For the thermal relay with temperature compensation, it can be used in the place where the environmental temperature of the thermal relay and the motor is different, but the influence caused by the environmental temperature change should be minimized as much as possible. The ambient temperature of the thermal relay and the protected motor should be considered. When the ambient temperature of the thermal relay is lower than the ambient temperature of the protected motor by 15℃, a thermal relay with a larger rated current rating should be used; when the ambient temperature of the thermal relay is lower than the ambient temperature of the protected motor by 15℃, a thermal relay with a smaller rated current rating should be used. In addition, the load of the motor and the adjustment range that the thermal relay may require should also be considered.3.4 Adjustment of Thermal RelayBefore putting it into use, the setting current of the thermal relay must be adjusted to ensure that the set current of the thermal relay matches the rated current of the protected motor. Before the thermal relay is used in the circuit, the specific current of the thermal relay must be adjusted according to the rated current of the motor to meet the requirements of corresponding occasions. For example, for a 10kW, 380V motor with a rated current of 19.9A, a XX20-25 thermal relay can be used. The setting current of the thermal element is 17 ～ 21 ～ 25A. First, set it at 21A according to the general situation. If it is found that it often moves in advance and the temperature rise of the motor is not high, you can change the setting current to 25A and continue to observe; if the motor temperature rises at 21A, and the thermal relay lags, you can change the setting current to 17A and observe to get the best fit. It is used to adjust the rated current when overload protection of the motor is repeatedly operated for a short time. Multiple tests and adjustments in the field can get more reliable protection. The method is: first adjust the rated current of the thermal relay to be slightly smaller than the rated current of the motor. If it is found that it often moves during operation, then gradually increase the rated value of the thermal relay until it meets the operating requirements. There should be motor protection during the special operations. Motors with forward, reverse, and frequent on-off operations should not be protected by thermal relays. The ideal method is to protect it with a temperature relay or thermistor embedded in the winding.  Ⅳ Frequently Asked Questions about Thermal Relay1. What is a thermal relay?A relay that opens or closes contacts with a bending mechanism as a result of the difference in the expansion coefficients of a bimetal, which is heated by the current. ... The thermal relay is combined with a magnetic contactor because it cannot switch the main circuit by itself. The operating point can be changed. 2. How does a thermal relay work?A thermal relay works depending upon the above-mentioned property of metals. The basic working principle of thermal relay is that, when a bimetallic strip is heated up by a heating coil carrying overcurrent of the system, it bends and makes normally open contacts. 3. What is the purpose of thermal overload relay?Thermal overload relays are economic electromechanical protection devices for the main circuit. They offer reliable protection for motors in the event of overload or phase failure. The thermal overload relay can make up a compact starting solution together with contactors. 4. What are the two types of thermal overload relays?Thermal Overload RelaysThermal overloads can be divided into two types: solder melting type, or solder pot, and bimetal strip type. Because thermal overload relays operate on the principle of heat, they are sensitive to ambient (surrounding air) temperature. 5. How do you use a thermal overload relay?Overload relays protect a motor by sensing the current going to the motor. Many of these use small heaters, often bi-metallic elements that bend when warmed by the current to the motor. When the current is too high for too long, heaters open the relay contacts carrying current to the coil of the contactor. 6. How do you test a thermal overload relay?CEP7 Overload Relay test procedures:Measure the normal motor running current (i motor).Turn off the motor and let it cool for about 10 minutes.Calculate the following ratio: i (motor) / i (overload min FLA).Set the overload to its minimum FLA and turn on the motor.Wait for the overload to trip. 7. What is thermal overload relay how it functions?The function of a thermal overload relay, used in motor starter circuits is to prevent the motor from drawing excessive current which is harmful to motor insulation. It is connected either directly to motor lines or indirectly through current transformers. 8. What is the purpose of a thermal overload?Thermal overload relays are protective devices. They are designed to cut power if the motor draws too much current for an extended period of time. To accomplish this, thermal overload relays contain a normally closed (NC) relay. 9. What does a thermal overload relay consist of?Bimetallic thermal overload relays (sometimes referred to as heater elements) are made of two metals, with different coefficients of thermal expansion, that is fastened or bonded together. A winding, wrapped around or placed near the bimetallic strip, carries current. 10. How do I know if my overload relay is bad?Unplug the start relay from the compressor and give it a shake. If you can hear rattling on the inside of the start relay, then the part is bad and will have to be replaced. If it's not rattling and appears to be in good condition, you may have a problem with the actual compressor.