CN111614072A - Security unit - Google Patents

Abstract

The invention relates to a security unit, comprising an external line terminal, an internal line terminal, a grounding terminal, a signal terminal, a fuse link, an overvoltage protection element and an alarm device, wherein the fuse link, the overvoltage protection element are connected with the external line terminal and the internal line terminal to form a main loop of the security unit; the alarm device is characterized in that a normally open alarm switch and an alarm element are connected in series between the grounding end and the signal end, the alarm switch and the fuse link are of a discrete non-linked structure, the response threshold value of the normally open alarm switch is lower than that of the fuse link, the normally open alarm switch keeps the alarm signal end disconnected with the grounding end, and when the overvoltage protection element is overheated, the fuse link fuses to cut off a main loop and triggers the normally open alarm switch to be closed, so that the signal end is conducted with the grounding end through the alarm element. The normally open alarm switch and the fuse link have no linkage characteristic, response thresholds can be respectively set according to the material types and the corresponding melting points, and the normally open alarm switch is ensured to be closed to alarm earlier than the main circuit cut off by the fuse link in an abnormal state.

Description

Security unit

Technical Field

The invention provides a security unit, relates to a circuit protection device, and particularly relates to a circuit protection device applied to the field of communication equipment.

Background

The safety unit is a circuit protector installed on the safety line bank of Main Distribution Frame (MDF) of switchboard in the communication field, in order to prevent the personal and equipment from being damaged by over-voltage and over-current. The whole line is located between an external line (telephone line) and an internal line (switchboard), wherein the external line is located in an outdoor complex cable environment, the aging and the lapping of an alternating current power line can often happen, and a security unit carries the important role of protecting communication equipment from overvoltage and overcurrent.

The schematic circuit diagram of the conventional security unit is shown in fig. 1, wherein a and b are the external terminals of the security unit, a 'and b' are the internal terminals of the security unit, S is the alarm signal terminal of the security unit, GND is the ground terminal of the security unit, and the overcurrent protection elements R1 and R2 are positive temperature coefficient thermistors (PTC), which can be ceramic materials or organic polymer materials. The overvoltage protection elements GDT1 and GDT2 are gas discharge tubes, but may be semiconductor discharge tubes, and break down at 220 VAC. K1 and K2 are normally closed main circuit switches, K3 is a normally open alarm switch, and the normally closed main circuit switches are linked with K1 and K2. H is a light emitting diode for generating an alarm signal, and the alarm signal end S can be connected with other devices capable of giving an audible and visual alarm.

On the basis of the circuit schematic diagram, the conventional security unit mainly adopts a bimetallic strip switch or a mechanical contact type switch as a normally closed main circuit switch, for example, application No. 200910239659.1 relates to a security unit comprising a housing, an inner terminal, an outer terminal and a ground terminal provided on the housing, and a normally closed main circuit switch located in the casing and connecting the inner line end and the outer line end, wherein the normally closed main circuit switch at least comprises a low melting point alloy block and a heat transfer sheet, the low melting point alloy block is welded at the first end of the heat transfer sheet, the second end of the heat transfer sheet is arranged in close contact with the discharge tube, the normally closed main circuit switch also comprises at least one movable block, and the movable block keeps the elastic element in predeformation under the control of the low-melting-point alloy block, loses control when the low-melting-point alloy block is melted, and is driven by the acting force of the elastic element for recovering deformation to disconnect the normally closed main circuit switch. However, the following 4 defects were found in the application: 1. the main loop switch is of a mechanical contact lap joint type, contact resistance of the main loop switch is directly connected into a communication main loop in series, the transmission of a communication line can be influenced by the contact condition and the capacitance inductance effect, and meanwhile, under the surge test condition that the standard requirement passes, the main loop can not be disconnected due to unstable contact resistance or large contact fusion welding, and great safety risk is caused; 2. the structure is complicated, and the mechanical structure comprises an elastic element, a heat transfer block, a pre-deformation movable block, a lap joint type metal sheet switch and the like, so that the instability is easy to occur in practical application, and the production and the processing are difficult; 3. the alloy block is generally low-temperature alloy under the condition limitation, namely the melting point is within the range of 50-100 ℃, and the alloy block cannot work in a high-temperature environment; 4. the heat conduction distance is long, the response is slow, even under the condition of large leakage current, the discharge tube is likely to be out of order before the main loop is switched off, the discharge tube does not generate heat any more, so that the main loop is not switched off, and the rear-end equipment is directly damaged due to abnormal high voltage. In addition, the first and second substrates are,

the application number 200610061526.6 provides a security unit, which comprises an overcurrent protection element, an overvoltage protection element and an alarm element, wherein the overcurrent protection element is connected between an external line end and an internal line end in series, the overvoltage protection element is connected between the external line end and a grounding end in parallel with the overcurrent protection element, the alarm element is connected with an alarm signal end, the alarm open-circuit single switch is connected with the grounding end and is positioned between the overvoltage protection element and the alarm element, the alarm open-circuit single switch keeps the overvoltage protection element and the grounding end closed, the alarm signal end is disconnected with the grounding end, and when the overvoltage protection element generates heat through continuous discharge, the alarm open-circuit single switch cuts off the connection between the overvoltage protection element and the grounding end and is communicated with the alarm signal end and the grounding end. The invention sets a single alarm open-circuit switch in the branch circuit, the contact resistance of the switch does not affect the main communication circuit, the main circuit is recovered to be smooth after the harmful voltage lap joint is eliminated or the security unit malfunctions, and the communication of the user is not interrupted.

This design has cancelled the major loop switch, adopts the mode of suspension protection to set up the single switch that opens an emergency and asks for help or increased vigilance in the branch road, and the contact resistance of switch does not influence the communication major loop, nevertheless this design still can't avoid the problem except that problem point 1, and has newly-increased some problems, if: 1. the overvoltage protection element is overheated to trigger an alarm to open a circuit single switch, the connection between the overvoltage protection element and a grounding end is cut off, at the moment, the overvoltage protection element loses the function, and the PTC with high resistance bears high voltage and continuously generates heat under the conditions that the abnormality is not relieved and the GDT does not respond, and even the PTC is burnt; 2. the alarm open-circuit single switch uses the bimetallic strip, the cost is higher, the reset is realized after the abnormity is relieved, the alarm is relieved when the communication of the main circuit is recovered to be normal, and the maintenance personnel can not find and relieve the hidden trouble of the circuit.

Disclosure of Invention

In order to solve the above technical problems, the present invention aims to provide a security unit.

It is a further object of the present invention to provide a method of making the security unit.

The purpose of the invention is realized by the following technical scheme: a security unit comprises an external line terminal, an internal line terminal, a grounding terminal, a signal terminal, a fuse link, an overvoltage protection element and an alarm device, wherein the fuse link, the overvoltage protection element and the external line terminal are connected to form a main loop of the security unit; the normally open alarm switch and the fuse link are in a discrete non-linked structure, the response threshold value of the normally open alarm switch is lower than that of the fuse link, the normally open alarm switch keeps the alarm signal end disconnected with the grounding end, and when the overvoltage protection element is overheated, the fuse link fuses to cut off a main loop and triggers the normally open alarm switch to be closed, so that the signal end is conducted with the grounding end through the alarm element.

The normally open alarm switch and the fuse link are simple non-linked structures, the normally open alarm switch keeps an alarm signal end disconnected with a grounding end, when the overvoltage protection element is overheated, the fuse link is fused to cut off a main loop, and the normally open alarm switch is triggered to be closed, so that the signal end is conducted with the grounding end through the alarm element, and the normally open alarm switch is ensured to be closed to alarm earlier than the fuse link to cut off the main loop in an abnormal state. The fuse link is placed in close proximity to the discharge tube, which can improve response speed.

On the basis of the scheme, the overvoltage protection element is a three-electrode open-circuit failure type gas discharge tube, a first fuse link and a second fuse link form a basic structure of a security unit, the first ends of the first fuse link and the second fuse link are respectively and electrically connected with an external line interface end a and an external line interface end b, the second ends of the first fuse link and the second fuse link are respectively and electrically connected with a first electrode and a third electrode of the discharge tube, an internal line interface end a 'and an internal line interface end b' are respectively and electrically connected with the first electrode and the third electrode of the discharge tube, and the second electrode of the discharge tube is connected with a ground terminal GND (ground;

the normally open alarm switch and the fuse link one and two are in a discrete non-linked structure, the response threshold value of the normally open alarm switch is lower than that of the fuse link one and two, the normally open alarm switch keeps the alarm signal end disconnected with the grounding end, and when the overvoltage protection element is overheated, the fuse link fuses to cut off a main loop and trigger the normally open alarm switch to be closed, so that the signal end S is conducted with the grounding end through the alarm element.

The fuse link can cooperate with the discharge tube, and through discharge tube bleeder current and overcurrent fusing, perhaps, through the high temperature that discharge tube produced and the overheat fusing, embody for higher operating temperature and faster response speed to and its multiple protection characteristic.

On the basis of the scheme, the discharge tube can also adopt two-electrode open-circuit failure type gas discharge tubes to replace a three-electrode open-circuit failure type gas discharge tube.

Furthermore, the fuse link is a high-temperature alloy fuse link with the melting point of 150-400 ℃, has higher melting point and conductivity, and is safely fused in the security unit through overcurrent or overtemperature. The high-voltage-resistant power cable is characterized by strong current-resistant voltage-resistant capability and small size and volume. To meet higher requirements of working environment.

Furthermore, an overcurrent protection element is connected in series between the second end of the high-temperature alloy fuse link and the inner wire end, so that the first electrode and the third electrode of the discharge tube are electrically connected with the inner wire interface ends a 'and b' through the overcurrent protection element respectively.

The overcurrent protection element comprises but is not limited to a positive temperature coefficient thermistor, preferably a surface mounted PTC thermistor I and a surface mounted PTC thermistor II, so that the overall structure is simpler.

Preferably, the alarm element is a light emitting diode.

The working principle of the invention is as follows: normally open alarm switch and fuse-link are discrete parts, have no linkage characteristic, but can set for the response threshold respectively according to its material type and corresponding melting point, normally open alarm switch response threshold is less than fuse-link response threshold, guarantees normally open alarm switch closure under the abnormal condition and reports an emergency and asks for help or increased vigilance earlier than the fuse-link cuts off the major loop, avoids the major loop disconnection under the limit condition but does not report an emergency and ask for help or increased vigilance, is unfavorable for the maintenance personal to look for the change.

On the basis of the scheme, the fuse link I, the fuse link II and the three-electrode open-circuit failure type gas discharge tube can adopt a surface mount component, can realize processing through a surface mount process, and has a simple processing process.

On the basis of the scheme, the normally open alarm switch can be realized by preferably combining an elastic element and an insulating film, the insulating film is clamped between the elastic element contact for storing force and the discharge tube grounding electrode, when the temperature of the discharge tube reaches the response threshold value of the normally open alarm switch, namely the melting point of the insulating film, the insulating film is melted, the elastic element rebounds, and the contact is communicated with the discharge tube grounding electrode to form the normally open alarm switch to be closed.

The invention also provides a preparation method of the security unit, wherein each component is assembled on an assembly plate with a connecting line, one side of the assembly plate is provided with a wiring end part, the other side of the assembly plate is provided with an assembly part, the front surface of the wiring end part is provided with an external line interface end a and an external line interface end b, the positions are longitudinally opposite, and the head part is a grounding end GND; on one side of the wiring end part on the bottom surface of the assembly plate, an inner wire interface end a 'corresponds to the position of an outer wire interface end a, an inner wire interface end b' corresponds to the position of an outer wire interface end b, and a signal end S corresponds to the position of a grounding end GND; in the assembly part on the front side of the assembly plate, a first fuse link, a second fuse link F1 and a first F2 are close to two sides of the three-electrode open-circuit failure type gas discharge tube GDT, two ends of the first fuse link F1 are respectively connected with an outer wire interface end a and a first electrode of the three-electrode open-circuit failure type gas discharge tube GDT, and two ends of the second fuse link F2 are respectively connected with an outer wire interface end b and a third electrode of the three-electrode open-circuit failure type gas discharge tube GDT; the first overcurrent protection element PTC thermistor, the second overcurrent protection element PTC thermistor 1 and the R2 are respectively assembled at the outer sides of the first fuse link, the second fuse link F1 and the F2, wherein the first PTC thermistor R1 is electrically connected between an inner wire interface end a 'and a first electrode of the discharge tube, and the second PTC thermistor R2 is electrically connected between an inner wire interface end b' and a third electrode of the discharge tube; the first, second F1, F2 fuse links in the main circuit of the security unit are used as fuse switches;

the alarm element H is assembled at the tail part and the outer side of the security unit, so that an alarm signal is easy to find, an alarm device formed by serially connecting the alarm element H and the normally open alarm switch K1 is arranged between a ground end GND and a signal end S and is separated from and not linked with a main loop, and the response threshold value of the normally open alarm switch K1 is lower than the response threshold values of the first fuse link, the second fuse link F1 and the first fuse link F2.

Specifically, the normally open alarm switch K1 selects an elastic element with a contact and a fusible insulating film, the insulating film is laid on the electrode two of the three-electrode open failure type gas discharge tube GDT connected with the ground terminal GND, the elastic element is inserted on the assembly plate, and the contact is abutted against the insulating film.

The invention has the advantages that: the design that a main loop switch and an alarm device are separated is adopted, namely a normally open alarm switch and a fuse link have no linkage characteristic, the threshold value can be separately regulated and controlled aiming at different use scenes, response threshold values can be respectively set according to the material types and corresponding melting points, the adaptability is strong, and the condition that the normally open alarm switch closes the alarm before the fuse link cuts off the main loop under the abnormal state is ensured; the heat conduction distance of the assembly is short, the heat conduction efficiency is high, and the response speed is high; the high-temperature alloy fuse link is used for replacing a mechanical contact type switch, so that higher working temperature and reliability are realized on the overall performance of the security unit, and the influence of contact resistance on a main communication loop is avoided; the structure is simple, patch elements are adopted, the complex linkage structure of the existing linkage type security unit is eliminated under the condition of realizing the same function, the processing and the assembly are convenient, and the cost can be effectively reduced.

Drawings

The invention will be further described with reference to the accompanying drawings and examples, in which:

FIG. 1: a schematic circuit diagram of a prior art security unit;

FIG. 2: a schematic circuit diagram of the security unit of the present invention;

FIG. 3: embodiment 1 the schematic structure of the security unit;

FIG. 4: the structural schematic diagram of a normally open alarm switch K1;

FIG. 5: the bottom structure of the security unit of the embodiment of the invention is shown schematically;

the reference numbers in the figures illustrate:

GDT-three-electrode open failure type gas discharge tube;

1. 2, 3-electrodes one, two, three;

f1, F2-fuse link one, two;

k1-normally open alarm switch;

4-a resilient element; 5-insulating film;

h-alarm element;

6-assembling plate.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

A security unit, as shown in fig. 2, a schematic circuit diagram of the security unit of the present invention, fig. 3, a schematic structural diagram of the security unit of the embodiment of the present invention, and fig. 5, a schematic structural diagram of a bottom surface of the security unit of the embodiment of the present invention, comprising an external interface end a, an external interface end b, internal interface ends a ', b', a ground end GND and a signal end S, a first fuse link, a second fuse link F1, a second fuse link F2, an overvoltage protection element, and an alarm device, wherein,

the overvoltage protection element is a three-electrode open-circuit failure type gas discharge tube GDT, and a basic structure of a security unit is formed by the three-electrode open-circuit failure type gas discharge tube GDT and fuse links I, II F1 and F2;

the first fuse link, the second fuse link F1 and the F2 are two high-temperature alloy fuse links and are close to a three-electrode open-circuit failure type gas discharge tube GDT, the first ends of the first fuse link, the second fuse link F1 and the F2 are respectively and electrically connected with an outer wire interface end a and an outer wire interface end b, the second ends of the first fuse link, the second fuse link F1 and the F2 are respectively and electrically connected with a first electrode, a third electrode 1 and a third electrode 3 of the three-electrode open-circuit failure type gas discharge tube GDT, the inner wire interface ends a 'and b' are respectively and electrically connected with the first electrode, the third electrode 1 and the third electrode 3 of the discharge tube through a PTC thermistor I, a second PTC 1 and a R2, and the second electrode 2 is;

an alarm device consisting of a normally open alarm switch K1 and an alarm element H light emitting diode is connected in series between a ground end GND and a signal end S, and the alarm signal end can also be connected with other devices capable of giving sound and light alarm;

the normally open alarm switch K1 and the first, second and third fuse links F1 and F2 are in a separated non-linkage structure, the response threshold of the normally open alarm switch K1 is lower than the response thresholds of the first, second and third fuse links F1 and F2, the normally open alarm switch K1 keeps the alarm signal end S disconnected with the ground terminal GND, when the three-electrode open failure type gas discharge tube GDT is overheated, the first, second and third fuse links F1 and F2 fuse and cut off the main loop, and the normally open alarm switch K1 is triggered to be closed, so that the signal end S is conducted with the ground terminal GND through the alarm element H.

In this embodiment, the first fuse link, the second fuse link F1 and the first fuse link F2 are high temperature alloy fuse links with a melting point of 150 ℃ to 400 ℃, are sheet-shaped structures, and are assembled according to the following steps:

on the strip assembling plate 6 with connecting lines, as shown in fig. 3, one side is a wiring end part, the other side is an assembling part, an external wire interface end a and an external wire interface end b are arranged on the front surface of the wiring end part and are on the same side, the positions are longitudinally opposite, and the head part is a grounding end GND; as shown in fig. 5, on the bottom surface of the mounting board 6, an inner terminal a 'corresponding to the outer terminal a, an inner terminal b' corresponding to the outer terminal b, and a signal terminal S corresponding to the ground GND are provided;

at the fitting portion on the front surface of the fitting plate 6, as shown in fig. 2 and 3:

the first fuse link, the second fuse link F1 and the F2 are close to two sides of the three-electrode open-circuit failure type gas discharge tube GDT, two ends of the first fuse link F1 are respectively connected with the external line interface end a and the first electrode 1 of the three-electrode open-circuit failure type gas discharge tube GDT, and two ends of the second fuse link F2 are respectively connected with the external line interface end b and the third electrode 3 of the three-electrode open-circuit failure type gas discharge tube GDT;

the first and second overcurrent protection elements PTC thermistors R1 and R2 are respectively assembled at the outer sides of the first and second fuse links F1 and F2, wherein the first PTC thermistor R1 is electrically connected between an inner wire interface end a 'and the first electrode 1 of the discharge tube, and the second PTC thermistor R2 is electrically connected between an inner wire interface end b' and the third electrode 3 of the discharge tube.

The first fuse link, the second fuse link F1 and the F2 can be fused through overcurrent generated by leakage current of the three-electrode open-circuit failure type gas discharge tube GDT in the safety unit, and can also be fused through a high-temperature environment generated by heating of the three-electrode open-circuit failure type gas discharge tube GDT, and in the aspect of structural layout, the first fuse link, the second fuse link F1 and the F2 are respectively placed close to the first electrode, the third electrode 1 and the third electrode 3 of the three-electrode open-circuit failure type gas discharge tube GDT, so that higher heat transfer efficiency and higher response speed are reflected.

As shown in fig. 4, the normally open alarm switch K1 of the present embodiment is formed by combining an elastic element 4 and a fusible insulating film 5, the insulating film 5 is laid on the second electrode 2 of the three-electrode open failure type gas discharge tube GDT connected to the ground GND, the elastic element 4 is inserted on the mounting plate 6, and the contact point abuts against the insulating film 5. An insulating film 5 is clamped between a contact of an elastic element 4 with accumulated force and a GDT electrode II 2 of the three-electrode open-circuit failure type gas discharge tube connected with a ground end GND, when the temperature of the GDT reaches a response threshold value of a normally-open alarm switch K1, namely a melting point of the insulating film 5, the insulating film 5 is melted, the elastic element 4 rebounds, the contact of the elastic element is communicated with the electrode II 2 of the GDT, and the normally-open alarm switch K1 is closed.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.

Claims (11)

1. A security unit comprises an external line end, an internal line end, a grounding end, a signal end, a fuse link, an overvoltage protection element and an alarm device, and is characterized in that the fuse link and the overvoltage protection element are connected with the external line end and the internal line end to form a main loop of the security unit, the alarm device formed by a normally open alarm switch and an alarm element is connected in series between the grounding end and the signal end, the normally open alarm switch and the fuse link are of a separated non-linkage structure, the response threshold value of the normally open alarm switch is lower than that of the fuse link, the normally open alarm switch keeps the alarm signal end disconnected with the grounding end, and when the overvoltage protection element is overheated, the fuse link fuses and cuts off the main loop and triggers the alarm switch to be closed, so that the signal end is conducted with the grounding end through the alarm element.

2. The security unit as claimed in claim 1, wherein the overvoltage protection element is a three-electrode open failure type Gas Discharge Tube (GDT), the basic structure of the security unit is composed of a three-electrode open failure type Gas Discharge Tube (GDT) and first and second fuses (F1, F2), the first ends of the first and second fuses are electrically connected with the external line interface end a and the external line interface end b respectively, the second ends of the first and second fuses (F1, F2) are electrically connected with the first and third electrodes (1, 3) of the discharge tube respectively, the interface ends a ', b' are electrically connected with the first and third electrodes (1, 3) of the internal line of the discharge tube respectively, and the second electrode (2) is connected with the ground terminal GND of the discharge tube;

the alarm device is characterized in that a normally open alarm switch (K1) and an alarm element (H) are connected in series between a ground terminal GND and a signal terminal S, the normally open alarm switch (K1) and first and second fuse links (F1 and F2) are of a discrete non-linked structure, the response threshold of the normally open alarm switch (K1) is lower than the response thresholds of the first and second fuse links (F1 and F2), the normally open alarm switch (K1) keeps the alarm signal terminal disconnected with the ground terminal, when the overvoltage protection element is overheated, the main circuit is cut off by the fuse links, the normally open alarm switch (K1) is triggered to be closed, and the signal terminal S is conducted with the ground terminal through the alarm element (H).

3. The security unit according to claim 1 or 2, wherein the fuse link is a superalloy fuse link having a melting point of 150-400 ℃, which is safely fused within the security unit by over-current or over-temperature.

4. The security unit according to claim 3, wherein an overcurrent protection element is connected in series between the second end of the superalloy fuse link and the inner wire end, so that the first, third (1, 3) electrodes of the discharge tube are electrically connected to the inner wire interface ends a ', b' through the overcurrent protection element, respectively.

5. The security unit as claimed in claim 4, wherein the overcurrent protection element is a PTC thermistor I, II (R1, R2).

6. The security unit of claim 1 or 2, wherein the alarm element is a light emitting diode.

7. The security unit according to claim 2, wherein the three-electrode open failure gas discharge tube is replaced by two-electrode open failure gas discharge tubes.

8. The security unit according to claim 2, wherein the fuse link one, two and three electrode open circuit failure type gas discharge tube is a patch type element.

9. The security unit according to claim 1 or 2, wherein the normally open alarm switch is formed by combining an elastic element and an insulating film, the insulating film is sandwiched between the contact of the elastic element with the grounding electrode of the discharge tube, when the temperature of the discharge tube reaches the response threshold of the normally open alarm switch, that is, the melting point of the insulating film is reached, the insulating film melts, the elastic element rebounds, and the contact is communicated with the grounding electrode of the discharge tube, so as to form the closing of the normally open alarm switch.

10. A security unit manufacturing method according to any one of claims 2 to 9, wherein each component is mounted on a mounting plate with a connecting line, one side of the mounting plate is provided with a terminal portion, the other side is provided with a mounting portion, an external interface end a and an external interface end b are arranged on the front surface of the terminal portion, the positions are longitudinally opposite, and the head portion is a ground terminal GND; on one side of the wiring end part on the bottom surface of the assembly plate, an inner wire interface end a 'corresponds to the position of an outer wire interface end a, an inner wire interface end b' corresponds to the position of an outer wire interface end b, and a signal end S corresponds to the position of a grounding end GND; in an assembly part on the front surface of the assembly plate, a first fuse link and a second fuse link (F1 and F2) are close to two sides of a three-electrode open-circuit failure type Gas Discharge Tube (GDT), two ends of the first fuse link (F1) are respectively connected with an outer wire interface end a and a first electrode (1) of the three-electrode open-circuit failure type Gas Discharge Tube (GDT), and two ends of the second fuse link (F2) are respectively connected with an outer wire interface end b and a third electrode (3) of the three-electrode open-circuit failure type Gas Discharge Tube (GDT); the first and second (R1, R2) overcurrent protection elements are respectively assembled at the outer sides of the first and second (F1, F2) fuse links, wherein the first (R1) PTC thermistor is electrically connected between an inner wire interface end a 'and a first (1) electrode of the discharge tube, and the second (R2) PTC thermistor is electrically connected between an inner wire interface end b' and a third (1, 3) electrode of the discharge tube; the first and second fuses (F1, F2) in the main circuit of the security unit are used as the fuse type switches;

the alarm element (H) is assembled at the tail part, an alarm device which is formed by connecting the alarm element (H) with a normally open alarm switch (K1) in series is arranged between a ground terminal GND and a signal terminal S and is not linked with a main loop separately, and the response threshold value of the normally open alarm switch (K1) is lower than the response threshold values of the first fuse link and the second fuse link (F1 and F2).

11. The security unit manufacturing method of claim 10,

the normally open alarm switch (K1) selects an elastic element (4) with a contact and a meltable insulating film (5), the insulating film (5) is laid on a second electrode (2) of the three-electrode open failure type gas discharge tube GDT connected with the ground end GND, the elastic element (4) is inserted on the assembly plate, and the contact abuts against the insulating film (5).

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