Patent Publication Number: US-9837236-B2

Title: High-voltage direct-current thermal fuse

Description:
TECHNICAL FIELD 
     The invention relates to a high-voltage direct-current thermal fuse, particularly for a high-voltage direct-current thermal fuse used for cutting off arcs in high-voltage direct-current circuits. 
     BACKGROUND 
     The thermal fuse is also called a thermal fusible cutout, which is usually mounted in electrical appliances that are prone to generating heat. If the appliance fails and generates heat, and if the temperature exceeds normal temperature ranges, the thermal fuse will automatically fuse to cut off the appliance&#39;s power supply to prevent the electric appliance from catching fire. In recent years, the thermal fuse has been mounted on most household appliances that primarily operate as sources of heat, such as rice cookers, electric irons, and electric furnaces. When internal parts stop working, the thermal fuse can cut off the power supply to prevent the appliance from further internal damage to the appliance, and to avoid causing a fire of the appliance and anything surrounding the appliance. Fuses are well known in the prior art, and thermal fuses work as a path of the power in the circuit. The thermal fuse does not melt until the current in the circuit exceeds thermal fuse&#39;s rated value. The thermal fuse has a relatively lower resistance, a relatively smaller power loss and a relatively lower surface temperature. In case of an abnormal temperature due to a failure of an electrical appliance, the thermal fuse melts and thus cuts off the power supply of the circuit. 
     The thermal fuse plays a role in protection from excessive temperatures in the power supply circuit. If the temperature of the region where the thermal fuse is located reaches the fusing-off temperature of the fusible alloy wire inside the thermal fuse, then, the fusible alloy wire shrinks towards the leads at both ends to cut off the circuit with the help of the fusing agent. The current circuit is thereby cut off to prevent other components in the circuit from being further damaged by the excessive temperature. Thus, the thermal fuse is applied in many circuits that need protection from excessive temperatures. Different circuits have different requirements for the thermal fuse. 
     In a direct current circuit with a high voltage level of 400V or above, during the process of fusing the fusible alloy wire of the traditional thermal fuse, an arc is generated due to the low shrinking speed of the fusible alloy wire and the small gap between the two leads, so that the circuit cannot be cut off in time. The circuit may be destroyed and/or catch fire due to the occurrence of the arc, as well as the high-temperature of the circuit. Thus, if the existing thermal fuse is used in a direct current circuit with a voltage level of 400V or above, the existing thermal fuse not only fails to cut off the circuit in time to protect the circuit, but also may cause additional problems. 
     SUMMARY OF THE INVENTION 
     The embodiment of the invention is aimed at the problem that the existing thermal fuse cannot be directly used in a high-voltage circuit, the invention therefore provides a high-voltage direct-current thermal fuse to solve the problem of cutting off the circuit and preventing an arc in time. The high-voltage direct-current thermal fuse can be directly used in the high-voltage direct-current circuit. 
     The specific solution is as follows: a high-voltage direct-current thermal fuse at least comprising a high-voltage low-current thermal fuse connected into a high-voltage direct-current circuit. The high-voltage low-current thermal fuse comprises a casing, a fusible alloy wire encapsulated in the casing, and two leads extending outside the casing. The fusible alloy wire is connected between the two leads. One of the leads is sequentially sleeved with an arc extinguishing sleeve and a spring. One end of the arc extinguishing sleeve contacts the fusible alloy wire, and the other end of the arc extinguishing sleeve contacts the spring. One end of the spring is connected to the internal end face of the casing, and the spring is in a compressed state. 
     The high-voltage low-current thermal fuse has the functions of high-voltage low-current arc extinguishing and cutting-off protection. Since the fusible alloy wire has a certain stiffness under normal temperature, the arc extinguishing sleeve pushes against the fusible alloy wire under the effect of the compressing spring. The elasticity of the compressing spring in the compressed state is not sufficient to destroy the welding strength of the fusible alloy wire and leads. Thus, the fusible alloy wires retain good fluidity in a liquefied state at temperatures above the fusing temperature of the fusible alloy wires when the high-voltage low-current thermal fuse is connected into the high-voltage direct-current circuit The arc extinguishing sleeve moves along the axis under the effect of the elasticity of the compressing spring to cut off the fusible alloy wire and to cover one lead, such that the discharging gap between the two leads is insulated to avoid the generation of a high-voltage arc. 
     As a preferable embodiment, in order to better prevent arcing in the high-voltage direct-current circuit, the embodiment of the invention also provides a high-voltage direct-current thermal fuse. The high-voltage direct-current thermal fuse includes a second thermal fuse, which is connected in series into the high-voltage direct-current circuit. The high-voltage low-current thermal fuse is connected in parallel to the second thermal fuse. The fusing temperature of the high-voltage low-current thermal fuse is higher than that of the second thermal fuse. 
     As a preferable embodiment, the high-voltage low-current thermal fuse is connected in series with a current fuse to form a primary branch. The primary branch is connected in parallel to the other thermal fuse. The resistance of the current fuse is then more than that of the high-voltage low-current thermal fuse. 
     According to the above arrangements, when the circuit to be protected is a high-voltage, high-current circuit, after the temperature reaches the melting point of the second thermal fuse to fuse the circuit, the current will go through the primary branch in parallel. Since the resistance of the current fuse is more than that of the high-voltage low-current thermal fuse, the current fuse fuses off first, and cuts off the primary branch in parallel. When the circuit to be protected is a high-voltage, low-current circuit, after the temperature reaches the melting point of the second thermal fuse to fuse the circuit, the current will go through the primary branch in parallel. Then, since the low current cannot make the current fuse in the primary branch fusing, the temperature continues to increase till the melting point of the high-voltage low-current thermal fuse, the high-voltage low-current thermal fuse cuts off the primary branch in parallel as a way of cutting-off high voltage circuits and preventing excess temperatures. 
     As a preferable embodiment, the current fuse is a tube fuse, which includes a metal fusing wire inside the tube and a tube body with both ends having a metal connecting terminal. Preferably, the current fuse is the N-type current fuse, which includes a fuse-link and the two leads connecting to both ends of the fuse-link. The two leads extend from the top of the N-type fuse-link, and have segments in parallel to each other. When the high-voltage low-current thermal fuse is connected in parallel to the N-type current fuse, the breaking current of the high-voltage low-current thermal fuse is less than that of the N-type current fuse. As a preferable embodiment, the N-type fuse-link is encapsulated inside the casing, and the casing is filled with arc extinguishing material, such as quartz sand. The N-type current fuse has the function of high-voltage high-current arc extinguishing. Compared to the product with a linear cavity structure, the parallel leads of the current fuse with the N-type fuse-link generate much higher electric field at the moment of fusing off. The diffusion and recombination process of charged particles is more rapid as electric field intensity increases, allowing the gap between the electrode leads to quickly return to the insulation state, so as to achieve the aim of extinguishing the arc. Thus, the arc extinguishing protection function of the new fuse is multiple times more effective than that of the normal fuse. 
     As a preferable embodiment, the second thermal fuse includes at least one fusible alloy wire. The fusible alloy wire is provided between the two leads. Specifically, it is welded between the two leads by soldering. 
     The second thermal fuse in the embodiment of the invention includes an insulated casing and a base. The fusible alloy wire and two leads are arranged inside the cavity formed by the insulated casing and the base. Specifically, the fusible alloy wire is welded between the two leads. The ends of both leads extend outside the base. One or more pieces of fusible alloy wires can be provided between the two leads according to actual needs. The number thereof is not limited. 
     As a preferable embodiment, the second thermal fuse in this embodiment includes two pieces of fusible alloy wires. The two pieces of fusible alloy wires are welded in parallel or crossways between the two leads to form a bridge-type connection. The opposite ends of the two leads are outside the base. The symmetrical structure of two L-type leads contributes to the uniformity of the alloy wires in parallel and improves effective utilization of flow capacity in parallel. 
     As a preferable solution, the high-voltage low-current thermal fuse is a square-shell type, a porcelain-tube type thermal fuse, or other alloy thermal fuse usually used in this field. The working principle of the alloy thermal fuse is the same. Different types of thermal fuses can be selected according to the actual needs of the circuit. 
     As a preferable embodiment, the high-voltage direct-current thermal fuse of the embodiment of the invention also includes several (N) secondary branches. The secondary branches include a high-voltage low-current thermal fuse and a current fuse that are connected in series sequentially. Among others, the structure of the high-voltage low-current thermal fuse and that of the current fuse are the same as those of the primary branch, which is not explained again here. When N equals 1, the secondary branch is connected in parallel to the high-voltage low-current thermal fuse in the primary branch. When N is more than 1, the Nth secondary branch is connected in parallel to the high-voltage low-current thermal fuse in the (N−1)th secondary branch. Using the manner of multi-parallel connecting to the high-voltage low-current thermal fuse, high-voltage low-current thermal fuse can be expendably applied in the lightning protection module. Thus, the protected circuit can be cut off more effectively and timely cut off. 
     The invention makes an improvement to the internal structure of the existing thermal fuse to solve the problem that the existing thermal fuse cannot be directly used in high-voltage circuits. The present invention allows a high-voltage low-current thermal fuse to be directly used in a high-voltage direct-current circuit for protection. When the heat generated by the circuit is too high, the fuse can cut off the circuit to avoid further damage to the electronic components and the occurrence of fire. 
     Furthermore, the embodiment of the invention also provides an improved solution of the high-voltage direct-current thermal fuse. The circuitry design in which the high-voltage low-current thermal fuse is connected in series to the current fuse and further connected in parallel to the other thermal fuse, results in extinguishing the voltage arc in a timely manner. As a result, in conditions of both high-voltage low-current, and high-voltage high-current, the arc can be extinguished and the circuit can be cut off in time, to prevent further damage to other components in the circuit resulting from an abnormal increase of temperature or burning caused by the arc. In addition, the high-voltage direct-current thermal fuse of the invention can be expanded using the manner of multi-parallel connecting to the high-voltage low-current thermal fuse, so that the high-voltage direct-current thermal fuse can be used in a lightning protection module. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the following drawings, further descriptions are made to the invention, wherein: 
         FIG. 1  is a perspective partial profile diagram of Embodiment 1 of the invention. 
         FIG. 2  is a perspective explosive view of Embodiment 1 of the invention. 
         FIG. 3  is a circuit schematic diagram of Embodiment 1 of the invention. 
         FIG. 4  is a circuit schematic diagram of Embodiment 2 of the invention. 
     
    
    
     In the text, the same reference numbers denote the same parts. When describing the drawings, not all the parts or components shown need to be discussed together with the corresponding drawings. Among others, the reference numbers are as follows: 
       100 —another thermal fuse/conventional temperature fuse,  101 —insulating base,  102 —small casing,  103 —large casing,  104 —fusible alloy wires,  105 —left lead of the thermal fuse,  106 —right lead of the thermal fuses; 
       200 —current fuse,  201 —casing of current fuse,  202 —cover plate,  203 —fuse,  204 —left lead of the current fuse,  205 —right lead of the current fuse; 
       300 —high-voltage low-current thermal fuse,  301 —casing of high-voltage low-current thermal fuse,  302 —base,  303 —fusible alloy wires,  304 —arc extinguishing sleeve,  305 —compressing spring,  306 —left lead of the high-voltage low-current thermal fuse,  307 —right lead of the high-voltage low-current thermal fuse. 
     DETAIL DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the invention will be described more completely by means of embodiments referring to the drawings. Among others, only some embodiments have been shown. However, in practice, embodiments of the invention can be embodied in many different forms, but not limited to the embodiments in the text. These embodiments are provided for the purpose of better understanding of the invention. 
     Embodiment 1 
       FIG. 1  and  FIG. 2  respectively show the perspective partial profile diagram and the perspective explosive view of Embodiment 1 of the invention. As  FIG. 1  and  FIG. 2  show, the high-voltage direct-current thermal fuse of the embodiment of the invention includes insulating base  101  and a large casing  103  provided thereon. Regular thermal fuse  100 , current fuse  200 , and high-voltage low-current thermal fuse  300  are provided inside a cavity formed between insulating base  101  and large casing  103 . Among others, high-voltage low-current thermal fuse  300  is connected in series to current fuse  200  sequentially to form a primary branch. Then, the primary branch is connected in parallel to thermal fuse  100 . Next, thermal fuse  100  is connected in series into the high-voltage circuit to be protected, to provide the protection for the high-voltage circuit against excessive temperatures. 
     Please refer to  FIG. 2 , thermal fuse  100  specifically includes small casing  102  which is arranged on insulating base  101 . Right lead of thermal fuse  105  and left lead of thermal fuse  106  are fixedly provided on both sides of insulating base  101 . Fusible alloy wire  104  is provided inside the closed cavity formed by insulating base  101  and small casing  102 . Fusible alloy wires  104  are welded between left lead  106  and right lead  105  which in the thermal fuse. As  FIG. 2  shows, in the embodiment, two pieces of fusible alloy wires  104  provided in parallel are included specifically. In other embodiments, one or two or more pieces of fusible alloy wires that are in parallel or crossways can also be provided if necessary. It should be noted that in the specific implementation process, the number of pieces of fusible alloy wires and the specific cross-sectional area of each piece of fusible alloy wires can be adaptively adjusted by one skilled in the art according to various current flow rates of the thermal fuse. In the embodiment, left lead  106  and right lead  105  presents an L-shape, which are arranged along the central vertical axis of fusible alloy wires  104  symmetrically, and are injected to form a whole together with base  101 . Two pieces of fusible alloy wires  104  in parallel are connected between two L-shape left leads  106  and right leads  105  to form a bridge-type connection. Also, the terminals of left lead  106  and right lead  105  reach out of insulating base  101 , extending in the direction which is opposite to fusible alloy wires  104  respectively. Fusible alloy wires  104  are made of low-melting conductive alloy material which is sensitive to temperature, and is coated by the fusing agent. When the temperature reaches the fusing temperature of fusible alloy wires  104 , fusible alloy wires  104  are fused. With the effects of surface tension and fusing agent, fusible alloy wires  104  shrink towards both ends to become a ball and attach to the ends of two leads, so as to be the fusing switch point in the application circuit, cutting off the circuit. 
     Current fuse  200  includes casing of current fuse  201  and cover plate  202 . Fuse  203  is arranged inside the cavity formed between casing of current fuse  201  and cover plate  202 . Among others, fuse  203  is in a shape of bending N-type. Left lead  204  and right lead  205  are connected to both ends of fuse  203  respectively. Left lead  204  and right lead  205  are shaped to extend from the top of the N-type of fuse  203  and have segments in parallel with each other. Left lead  204  and right lead  205  pass through the via holes on casing of current fuse  201  respectively, extending out of casing of current fuse  201  and exposed to the outside, so as to be an electric connection point connecting fuse  203  to an external environment. Fuse  203  is suspended in the N-type cavity, without contacting the internal cavity walls of the N-type cavity. Since fuse  203  inside current fuse  200  is in a shape of bending N-type, current fuse  200  is called N-type current fuse. In order to improve the effectiveness of extinguishing arcs, the N-type cavity also can be filled with arc extinguishing materials such as quartz sand, to stabilize the heat balance of fuse  203 . Among others, when the high-voltage low-current thermal fuse is connected in series to N-type current fuse, the breaking current of the high-voltage low-current thermal fuse is less than that of the N-type current fuse. 
     When current fuse  200  is powered on, the temperature of fuse  203  will increase because of the heat generated from current conversion. When loading normal working current or allowed over-loading current, the heat generated by the current, and the heat which is dissipated by means of radiating, convecting, conducting, and etc. through fuse  203 , casing of current fuse  201 , and the surrounding environment can reach a balance gradually. If the heat dissipating speed cannot keep up with the heat generating speed, heat will accumulate on the fuse-link to make the temperature of fuse  203  increase. Once the temperature reaches or goes beyond the melting point of fuse  203 , it will be liquefied or vaporized to cut off the circuit. 
     At the fusing moment of fuse  203 , usually, the break is from the center point of the N-type towards both sides. An arc is inevitably generated at the breaking point of fuse  203 , such that a large number of charged particles are generated from the arc. At the same time, the electric field intensity generated by left lead  204  and right lead  205  that are in parallel in the current fuse is more than multiple times. The diffusion and recombination process of charged particles are more rapid under high electric field intensity, making the gap between electrode leads quickly recover to the insulation state, achieving the aim of extinguishing the arc. Thus, the arc extinguishing protection effect which is multiple times more than that of the normal fuse is achieved, and a safety protection for circuit and human is realized. 
     Please refer to  FIG. 2 , high-voltage low-current thermal fuse  300  is a disposable non-resettable fusing device. In the embodiment, the square-shell type thermal fuse is used, which includes the shell consisting of casing of high-voltage low-current thermal fuse  301  and base  302 , temperature sensing member sealed inside the casing (e.g., fusible alloy wires  303  which have a low melting point and a good temperature sensitivity, wherein fusible alloy wires  303  is coated with fusing agent), and two leads extending outside the shell. The reference numbers of the two leads are  306 ,  307  respectively. Among others, fusible alloy wires  303  are welded between left lead  306  and right lead  307 . As  FIG. 2  shows, left lead  306  and right lead  307  are provided in parallel with each other. The axes of two leads are perpendicular to fusible alloy wires  303  respectively. Fusible alloy wires  303  are specifically welded on the top of axes of left lead  306  and right lead  307 . After the left lead  306  and right lead  307  pass through the via holes on base  302  in axial direction, they are bent and extend along the direction which is away from fusible alloy wires  303 . Each extending lead is exposed to outside base  302  as an external electric connection point. 
     A round cavity is further provided inside base  302  where compressing spring  305  and arc extinguishing sleeve  304  are located. Arc extinguishing sleeve  304  and compressing spring  305  are positioned to surround the axis of high-voltage left lead  306 . One end of compressing spring  305  which in a compressed state is connected to internal end face of the round cavity of base  302 , and the other end contacts arc extinguishing sleeve  304 . The end opposite to compressing spring  305  of arc extinguishing sleeve  304  contacts fusible alloy wires  303 . Since fusible alloy wires  303  has a certain stiffness under normal temperature, arc extinguishing sleeve  304  pushes against fusible alloy wires  303  under the effect of compressing spring  305 . The elasticity of the compressing spring, which is configured in the compressed state, is not sufficient to destroy the welding strength of fusible alloy wires  303  and high-voltage left lead  306  and high-voltage right lead  307 . 
     High-voltage low-current thermal fuse  300  mainly functions as excessive temperature and high-voltage cutting off protection. When the temperature of the region where high-voltage low-current thermal fuse  300  is located reaches the fusing temperature of fusible alloy wires  303  inside high-voltage low-current thermal fuse  300 , fusible alloy wires  303  melt. Also, with the help of surface tension and a fusing agent (e.g. special resin), fusible alloy wires  303  shrink towards both ends and become a ball, attaching to the ends of two leads (whose reference numbers are  306  and  307  respectively). Since the circuit where high-voltage low-current thermal fuse  300  is located is a high-voltage circuit, the speed of shrinkage of fusible alloy wires  303  is too slow and the gap between high-voltage left lead  306  and right lead  307  is too short, an arc is likely to be generated. With the generation of a high-voltage arc, liquefied fusible alloy wires  303  has a good fluidity. With the help of the elasticity of compressing spring  305 , arc extinguishing sleeve  304  moves along the axis to cut off fusible alloy wires  303 . Arc extinguishing sleeve  304  covers high-voltage left lead  306  to insulate the discharging gap between high-voltage left lead  306  and high-voltage right lead  307 . Thus, the current circuit is cut off to prevent further damage to other components in the circuit resulting from abnormal increases of temperature or burning caused by the arc. 
       FIG. 3  shows a circuit diagram of Embodiment 1 of the invention. As  FIG. 3  shows, current fuse  200  is connected in series to high-voltage low-current thermal fuse  300 , and is subsequently connected in parallel to regular thermal fuse  100 . Then the left and right leads of regular thermal fuse  100  are connected in series in the high-voltage circuit to be protected to provide protection from excessive temperatures the high-voltage circuit. More specifically, after left lead  204  of current fuse  200  is connected to right lead  307  of high-voltage low-current thermal fuse  300  to form electric connection in series. Right lead  205  of current fuse  200  and left lead  306  of high-voltage low-current thermal fuse  300  are respectively connected to right lead  105  and left lead  106  of thermal fuse  100  to form an electric connection in parallel. Right lead  105  and left lead  106  of regular thermal fuse  100  is connected to the high-voltage circuit, so as to be in series in the circuit which needs protection, so as to provide protection from excessive temperature for the high-voltage circuit. 
     Furthermore, in order to realize the work of high-voltage direct-current thermal fuse in the embodiment of the invention, the fusing temperature of traditional thermal fuse  100  should be configured to be less than the fusing temperature of high-voltage low-current thermal fuse  300 . The resistance of fuse-link in the current fuse should be configured to be more than that of high-voltage low-current thermal fuse. 
     Thus, when the circuit is a high-voltage high-current circuit, if the outside temperature reaches the fusing temperature of thermal fuse  100 , with the help of surface tension and fusing agent, fusible alloy wires  104  fuse off and shrink towards the left and right leads on both ends. Due to the existence of the parallel circuit, the cutting off of fusible alloy wires  104  will not generate arcing. The current will go through the primary branch which is connected in parallel with thermal fuse  100 , that is, the branch formed by current fuse  200  connected in series with high-voltage low-current thermal fuse  300 . Since the resistance of fuse  203  in current fuse  200  is more than that of high-voltage low-current thermal fuse  300 , fuse  203  fuses off first to cut off the parallel circuit. Since current fuse  200  with respect to the linear type fuse, at the fusing-off moment, the electric field intensity generated by the leads in parallel is more than multiple times, the diffusion and recombination process of the charged particles are more rapid under high electric field intensity, making the gap between the electrode leads quickly recovery to the insulation state, achieving the aim of extinguishing the arc. It has an arc extinguishing protection that is multiple times more than that of the normal fuse. 
     When the circuit is a high-voltage low-current circuit, if the outside temperature reaches the fusing temperature of thermal fuse  100 , after fusible alloy wires  104  fuse off, the current goes through the parallel circuit which is formed by current fuse  200  and high-voltage low-current thermal fuse  300 . Since the current that goes through the parallel circuit is not sufficient to fuse off current fuse  200 , the parallel circuit is not cut off and the outside temperature keeps increasing. When it reaches the fusing temperature of fusible alloy wires  303  of high-voltage low-current thermal fuse  300 , fusible alloy wires fuse off, and shrink towards both ends to become a ball, attaching to ends of two leads  306 ,  307 . Since the circuit is a high-voltage circuit, the speed of shrinkage of fusible alloy wires  303  is too slow and the gap between high-voltage left lead, right lead  306 ,  307  is too short, an arc is likely to be generated. With the generation of the high-voltage arc, liquefied fusible alloy wires  303  has a good fluidity. With the help of the elasticity of compressing spring  305 , arc extinguishing sleeve  304  moves along the axis to cut off fusible alloy wires  303 . Arc extinguishing sleeve  304  covers high-voltage left lead  306  to insulate the discharging gap between high-voltage left lead  306  and high-voltage right lead  307 , so as to cut off the parallel circuit to prevent further damages to the electric appliance resulted from abnormal increasing of temperature or burning caused by the arc. 
     Embodiment 2 
       FIG. 4  shows the circuit schematic diagram of Embodiment 2 of the invention. As an expanded solution, in this Embodiment 2, the high-voltage direct-current thermal fuse is composed of thermal fuse  100 , current fuse  200 , and high-voltage low-current thermal fuse  300  as the same as those in Embodiment 1. Among others, high-voltage low-current thermal fuse  300  is sequentially connected in series to current fuse  200  to form the primary branch. Next, the primary branch is connected in parallel to thermal fuse  100 . Thermal fuse  100  is connected in series to the high-voltage circuit to be protected, so as to provide the over-temperature protection for the high-voltage circuit, which is not explained again here. 
     The differences between Embodiment 1 and Embodiment 2 lie in that: the high-voltage direct-current thermal fuse also includes N secondary branches, and each secondary branch includes the high-voltage low-current thermal fuse sequentially connected in series to the current fuse. Among others, the structure of the high-voltage low-current thermal fuse and that of the current fuse are the same as those of the primary branch, which is not explained again here. When N is equal to 1, the secondary branch is connected in parallel to the high-voltage low-current thermal fuse in the primary branch. When N is more than 1, the Nth secondary branch is connected in parallel to the high-voltage low-current thermal fuse in the (N−1)th secondary branch. As  FIG. 4  shows,  FIG. 4  includes two secondary branches. N is equal to 2. The first secondary branch includes high-voltage low-current thermal fuse  300 ′ and current fuse  200 ′ that are connected to each other in series sequentially. The second secondary branch includes high-voltage low-current thermal fuse  300 ″ and current fuse  200 ″ that are connected to each other in series sequentially. Among others, the first secondary branch is connected in parallel to high-voltage low-current thermal fuse  300  in the primary branch. The second secondary branch is connected in parallel to high-voltage low-current thermal fuse  300 ′ in the first secondary branch. 
     In fact, as an expanded solution, the number of the secondary branches is not limited to two in Embodiment 2, and can also be more. The next level of secondary branch is connected in parallel to the high-voltage low-current thermal fuse in the last level of secondary branch. Using the manner of multi-parallel to the high-voltage low-current thermal fuse, the high-voltage low-current thermal fuse can be expendably applied in lightning protection modules. Thus, the protection circuit is separated more effectively and timely to meet effective cutting off of the voltage. 
     In additional, as another application solution, the high-voltage low-current thermal fuse in above Embodiment 1 and Embodiment 2 can both use the porcelain-tube type thermal fuse. The porcelain-tube type thermal fuse includes insulated porcelain tube, inside which fusible alloy wires that melt at a predetermined temperature are encapsulated. The fusible alloy wires are welded between the right lead and left lead that are axisymmetric. The ends of two leads respectively extend outside the insulated porcelain tube in the direction that is away from the fusible alloy wires. Among others, any of the two leads can be sleeved by an arc extinguishing sleeve and a compressing spring. One end of the arc extinguishing sleeve contacts the fusible alloy wires, and the other end contacts the spring. One end of the spring is connected to the internal end face of the insulated porcelain tube in the compressed state. The elasticity of the spring which in configured in a compressed state is not sufficient to destroy the welding strength between the fusible alloy wires and left, right leads. Other settings are the same as those in Embodiment 1 or 2, which is not explained again here. 
     Furthermore, as a basic application solution, high-voltage low-current thermal fuse  300  in the embodiment of the invention can be used in the high-voltage direct-current circuit alone (e.g. connecting in series into the high-voltage direct-current circuit). When the circuit to be protected is the high-voltage direct-current circuit, if the outside temperature reaches the fusing temperature of fusible alloy wires  303  in the high-voltage direct-current thermal fuse  300 , fusible alloy wires  303  fuse off and shrink towards both ends to become a ball, attaching to the ends of the leads whose reference numbers are  306 ,  307  respectively. With the generation of high-voltage arc, liquidized fusible alloy wires  303  has a good fluidity. Arc extinguishing sleeve  304  moves along the axis to cut off fusible alloy wires  303  under the effect of the elasticity of compressing spring  305 . Arc extinguishing sleeve  304  covers high-voltage left lead  306  to insulate the special discharging gap between the high-voltage left lead  306  and the high-voltage right lead  307 , so as to cut off the parallel circuit to prevent further damages to other components in the circuit resulted from the abnormal increasing of temperature or burning caused by the arc. 
     As another expanded solution, the manner of using a regular thermal fuse connected in parallel to a current fuse can also be used to apply in the high-voltage direct-current circuit. Although the effect of the manner may not be optimal, it can realize the function of circuit cutting-off and arc extinguishing. If outside temperature reaches the fusing temperature of thermal fuse  100 , the cutting-off of fusible alloy wires  104  fuse off and shrink towards the right and left leads at both ends. Due to the existence of parallel circuit, the cutting-off of fusible alloy wires  104  will not generate the arc. The current will go through the current fuse connected in parallel to thermal fuse  100 . When the current reaches a certain intensity and a certain temperature, fuse  203  of current fuse  200  will fuse off automatically to cut off the current, so as to achieve the function of protecting the circuit to operate safely. 
     For persons skilled in the art, it is easy to conceive of many modifications and other embodiments of the invention. In the invention, contents shown in the above descriptions and associated drawings have useful technical motivations. Thus, the embodiments of the invention only disclose preferable embodiments, and are not limited to specific embodiments disclosed, but also include various modifications and other embodiments within the scope of the claims. Although in the context, certain specific terms are used, they are only used for a general and descriptive sense, and do not constitute a limitation.