Surge Protection Circuit

Abstract

To provide a surge protection circuit that can improve the surge allowance.SOLUTION: A surge protection circuit includes a power input line, an L-G connection line, a first varistor, a first gas arrester, and a first branch line, The power input line includes an ungrounded wire (L), a grounded wire (N), and a grounding wire (G). The L-G connection line connects the ungrounded wire to the grounding wire. The first varistor is arranged on the L-G connection line. The first gas arrester is arranged on the L-G connection line in series with the first varistor. The first branch line is branched from between the first varistor and the first gas arrester on the L-G connection line and is connected to the grounded wire.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] More particularly, the present invention relates to a surge protection circuit for protecting electrical equipment from an abnormal increase in current or voltage. [Background technology]

[0002] As a conventional surge protection circuit 100, an electric circuit incorporated in a lightning arrester installed as shown in Fig. 3(a) is known. The surge protection circuit 100 is connected in parallel to a power input line 200. That is, as shown in Fig. 3(b), the surge protection circuit 100 includes a varistor 300 and a gas arrester 400, or an equivalent circuit, arranged on an LN connection line connecting an ungrounded electric wire L and a grounded electric wire N. For this reason, even after the varistor 300 is destroyed or broken down by the application of a surge and the function of the surge protection circuit 100 is impaired, electricity can still flow between the main power supply and the electric device, making it difficult to protect the electric device from a surge. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-97215 [Patent Document 2] JP 2020-38861 A Summary of the Invention [Problem to be solved by the invention]

[0004] In order to solve the above problems, Patent Document 1 (JP Patent Publication No. 11-97215) discloses a protection device including a varistor and a thermistor arranged adjacent to the varistor. In the device described in Patent Document 1, the thermistor detects that the varistor is in an abnormally high temperature state, thereby interrupting the current. In addition, Patent Document 2 (JP Patent Publication No. 2020-38861) discloses a lightning arrester including a varistor and a thermal fuse arranged adjacent to the varistor. Patent Document 2 discloses that the thermal fuse is melted by the heat generated by the varistor when the varistor is destroyed, thereby interrupting the current.

[0005] Both Patent Document 1 and Patent Document 2 use heat generated by varistor destruction when a surge is applied to cut off the circuit. However, varistors are larger than thermistors and thermal fuses, and varistor destruction occurs instantly at any position on the varistor surface. For this reason, there are cases where abnormally high temperatures cannot be detected accurately, for example, when varistor destruction occurs on the side not in contact with the thermistor or thermal fuse. In other words, there are cases where the current is not cut off even if the varistor is destroyed.

[0006] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a surge protection circuit that can reliably cut off current flow when a varistor fails due to application of a surge. [Means for solving the problem]

[0007] The surge protection circuit of this embodiment comprises a power input line having an ungrounded electric wire (L) and a grounded electric wire (N), a first varistor arranged in an LN connection line connecting the ungrounded electric wire and the grounded electric wire and absorbing surges, and a first fault detection means arranged in the LN connection line in series with the first varistor and cutting off current to at least one of the ungrounded electric wire and the grounded electric wire in response to a current flowing in the LN connection line due to a fault in the first varistor.

[0008] Preferably, the first fault detection means includes a first resistor arranged in the LN connection line and generating heat in response to a flow of a current, and a first temperature fuse provided in an intermediate position of at least one of the non-grounded side electric wire and the grounded side electric wire, arranged in close proximity to the first resistor, and melting in response to the heat temperature of the first resistor exceeding a set value.

[0009] Preferably, the power input line includes a ground wire (G). In this case, the surge protection circuit of this embodiment includes a second varistor that is arranged in the LG connection line connecting the non-grounded electric wire and the ground wire and absorbs surges, a first gas arrester that is arranged in the LG connection line in a series relationship with the second varistor, and a second fault detection means that is arranged in a first branch line that branches off from between the second varistor and the first gas arrester on the LG connection line and is connected to the grounded electric wire, and that cuts off the current flowing in the non-grounded electric wire in response to a current flowing in the LG connection line and the first branch line due to a fault in the second varistor.

[0010] Preferably, the second fault detection means includes a second resistor arranged in the first branch line and generating heat in response to a flow of current, and a second temperature fuse provided at a midpoint of the non-grounded electric wire and arranged in close proximity to the second resistor, and melting in response to the heat temperature of the second resistor exceeding a set value.

[0011] When the power input line includes an earth wire (G), another embodiment of the surge protection circuit comprises a third varistor arranged in an NG connection line connecting the grounded electric wire and the earth wire, which absorbs surges, a second gas arrester arranged in the NG connection line in a series relationship with the third varistor, and a third fault detection means arranged in a second branch line branching off from between the third varistor and the second gas arrester on the NG connection line and connected to the non-grounded electric wire, which cuts off the flow of electricity to the grounded electric wire in response to a current flowing in the NG connection line and the second branch line due to a fault in the third varistor.

[0012] Preferably, the third fault detection means includes a third resistor arranged in the second branch line and generating heat in response to a flow of current, and a third temperature fuse provided at a midpoint of the ground side wire and positioned close to the third resistor, and melting in response to the temperature of the third resistor exceeding a set value.

[0013] A surge protection circuit according to another embodiment of the present invention comprises: a power input line having a non-grounded electric wire (L), a grounded electric wire (N), and an earth wire (G); a varistor arranged in an earth connection line connecting either the grounded electric wire or the non-grounded electric wire to the earth wire and absorbing a surge; a gas arrester arranged in the earth connection line in a series relationship with the varistor; and fault detection means arranged in a branch line branching off between the varistor and the gas arrester on the earth connection line and connected to the other of the grounded electric wire or the non-grounded electric wire, for cutting off the flow of current to either the grounded electric wire or the non-grounded electric wire in response to a fault in the varistor causing an electric current to flow in the earth connection line and the branch line.

[0014] A surge protection circuit according to another embodiment is incorporated in a three-phase AC circuit having three input lines. The surge protection circuit includes a varistor that is disposed in a connection line connecting any two input lines and absorbs surges, and failure detection means that is disposed in the connection line in series with the varistor and that cuts off the current to one or the other input line in response to a current flowing in the connection line due to a failure of the varistor.

[0015] The surge protection circuit incorporated in a three-phase AC circuit with three input lines (L1, L2, L3) and a ground wire (G) is arranged on the ground connection line that connects one of any two input lines to the ground wire, and is composed of a varistor that absorbs surges and a variator. the gas arrester is arranged in the earth connection line in a series relationship with the varistor, and a fault detection means is arranged in a branch line which branches off between the varistor and the gas arrester on the earth connection line and is connected to the other of any two input lines, and which cuts off the current to one or the other of the input lines in response to a current flowing through the earth connection line and the branch line due to a fault in the varistor. Effect of the Invention

[0016] According to the present invention, when a varistor breaks down due to application of a surge voltage, current flow can be reliably cut off to protect electrical equipment. [Brief description of the drawings]

[0017] [Figure 1] 1A and 1B are diagrams showing a surge protection circuit according to an embodiment of the present invention, where FIG. 1A is a schematic diagram and FIG. [Diagram 2] FIG. 13 is a circuit diagram showing a surge protection circuit according to another embodiment. [Diagram 3] 1A and 1B are diagrams showing a conventional surge protection circuit, where (a) is a schematic diagram and (b) is a circuit diagram. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] (About the composition) The surge protection circuit of this embodiment will be described in detail with reference to the drawings. Note that the same or corresponding parts in the drawings are given the same reference numerals and their description will not be repeated. Also, the configurations labeled "first," "second," and "third" basically have the same or similar performance, and the description of the configuration labeled "second" and subsequent components will not be repeated.

[0019] Fig. 1 is a diagram showing a surge protection circuit 1 according to the present embodiment. Fig. 1(a) is a schematic diagram showing that the surge protection circuit 1 according to the present embodiment is installed between a main power supply and an electric device. Fig. 1(b) is a circuit diagram showing the electric circuit within the dotted line in Fig. 1(a).

[0020] The surge protection circuit 1 includes a power input line 2. The power input line 2 includes a non-grounded electric wire L, a grounded electric wire N, and an earth wire G. The power input line 2 is a line that electrically connects a main power source and an electric device. Note that, although an example in which the earth wire G is provided is described in this embodiment, the provision of the earth wire G is not essential.

[0021] The surge protection circuit 1 according to this embodiment includes an LN connection line 3 that connects an ungrounded electric wire L and a grounded electric wire N. The surge protection circuit 1 includes a first varistor 32 that is arranged on the LN connection line 3 and absorbs surges, and a first fault detection means 31 that is arranged on the LN connection line 3 in series with the first varistor 32 and cuts off current flow to at least one of the ungrounded electric wire L and the grounded electric wire N in response to current flowing through the LN connection line 3 due to a fault in the first varistor 32.

[0022] On the LN connection line 3, the first varistor 32 is provided on the non-grounded electric wire L side, and the first fault detection means 31 is provided on the grounded electric wire N side. During normal energization, the impedance of the first varistor 32 is high, so that a current flows on the non-grounded electric wire L toward the electric device, and does not flow to the LN connection line 3 side. If a surge is applied here and the first varistor 32 breaks down, the impedance of the first varistor 32 drops, so that a current flows into the LN connection line 3. Note that a varistor breaks down is a state in which the function of the varistor is reduced or lost. In this embodiment, on the LN connection line 3, the first varistor 32 is provided on the non-grounded electric wire L side, and the first fault detection means 31 is provided on the grounded electric wire N side, but these arrangements may be reversed. That is, the first varistor 32 is provided on the grounded electric wire N side, and the first fault detection means 31 is provided on the grounded electric wire N side. may be placed on the non-grounded electric wire L side.

[0023] In this embodiment, when a current flows into the LN connection line 3, the first fault detection means 31 detects this and cuts off the current flow to the non-grounded electric wire L. The first fault detection means 31 may be any means that cuts off the current flow to the power input line 2. The first fault detection means 31 in this embodiment includes a first resistor 33 that is disposed in the LN connection line 3 and generates heat in response to the current flow, and a first temperature fuse 34 that is provided in the middle of at least one of the non-grounded electric wire L and the grounded electric wire N, disposed in a position close to the first resistor 33, and melts down in response to the heat temperature of the first resistor 33 exceeding a set value.

[0024] The first resistor 33 generates heat when electricity is passed through the LN connection line 3. That is, the first resistor 33 does not generate heat during normal electricity passing, but generates heat when the first varistor 32 breaks down due to the application of a surge and a current flows through the LN connection line 3. From the viewpoint of cutting off the electricity passing through the non-grounded electric wire L as quickly as possible, it is preferable to use a first resistor 33 that heats up to 100° C. or higher in about several seconds to several tens of seconds.

[0025] The first thermal fuse 34 in this embodiment is provided at a midpoint of the non-grounded electric wire L. The first thermal fuse 34 and the first resistor 33 are disposed in close proximity to each other. "Close proximity" here typically means that at least a portion of the first thermal fuse 34 and the first resistor 33 are in contact with each other, and the first thermal fuse 34 is disposed in a position where it can be sufficiently affected by the temperature of the first resistor 33. This allows the first thermal fuse 34 to melt down due to the heat generated by the first resistor 33. From the viewpoint of practicality, it is preferable to use a first thermal fuse 34 that melts down at 100°C or higher.

[0026] From the viewpoint of ensuring reliable meltdown, the first resistor 33 and the first thermal fuse 34 may be bound together with tape or the like and tightly attached to each other.

[0027] The surge protection circuit 1 according to this embodiment includes an LG connection line 4. In this case, the surge protection circuit 1 includes a second varistor 42 that is disposed on the LG connection line 4 connecting the non-grounded electric wire L and the earth wire G and absorbs surges, and a first gas arrester 45 that is disposed on the LG connection line in a series relationship with the second varistor 42. The surge protection circuit 1 also includes second fault detection means 41 that is disposed on a first branch line 40 that branches off from between the second varistor 42 and the first gas arrester 45 on the LG connection line 4 and is connected to the grounded electric wire N.

[0028] On the LG connection line 4, the second varistor 42 is provided on the non-grounded electric wire L side, and the first gas arrester 45 is provided on the earth wire G side. During normal energization, the impedance of the second varistor 42 and the first gas arrester 45 is high, so that a current flows to the non-grounded electric wire L and does not flow to the LG connection line 4 side. When a surge is applied here, the second varistor 42 absorbs the surge by changing its impedance, and the first gas arrester 45 absorbs the surge by discharging. When the second varistor 42 breaks down due to the application of a surge, the impedance of the second varistor 42 decreases, and the impedance of the first gas arrester 45 is high, so that a current flows into the LG connection line 4 and the first branch line 40. A second resistor 43 is provided on the first branch line 40, and the second fault detection means 41 functions when a current flows through the first branch line 40.

[0029] The second fault detection means 41 cuts off the current flow through the non-grounded electric wire L when a surge is applied to the LG connection line 4 and the second varistor 42 breaks down. The second fault detection means 41 of the present embodiment is comprised of a second resistor 43 that is disposed in the first branch line 40 and generates heat in response to a current flow, and a second resistor 44 that is disposed in the middle of the non-grounded electric wire L and in a position close to the second resistor 43. and a second thermal fuse 44 that melts in response to the temperature of the resistor 43 exceeding a set value.

[0030] The second resistor 43 generates heat when a current flows into the first branch line 40. Since the first gas arrester 45 has a high impedance, almost no current flows to the earth wire G. That is, the current that flows into the LG connection line 4 after the second varistor 42 is destroyed does not flow to the earth wire G, but flows to the first branch line 40, which is why the second resistor 43 reliably generates heat and melts the thermal fuse 44.

[0031] The surge protection circuit 1 according to this embodiment includes an NG connection line 5. In this case, the surge protection circuit 1 includes a third varistor 52 that is disposed on the NG connection line 5 connecting the grounded electric wire N and the earth wire G and absorbs surges, and a second gas arrester 55 that is disposed on the NG connection line 5 in series with the third varistor 52. The surge protection circuit 1 also includes third fault detection means 51 that is disposed on a second branch line 50 that branches off from between the third varistor 52 and the second gas arrester 55 on the NG connection line 5 and is connected to the non-grounded electric wire L.

[0032] On the NG connection line 5, the third varistor 52 is provided on the ground side electric wire N side, and the second gas arrester 55 is provided on the earth wire G side. During normal energization, the impedance of the third varistor 52 and the second gas arrester 55 is high, so that a current flows to the ground side electric wire N and does not flow to the NG connection line 5 side. When a surge is applied here, the third varistor 52 absorbs the surge by changing the impedance, and the second gas arrester 55 absorbs the surge by discharging. When the third varistor 52 breaks down due to the application of a surge, the impedance of the third varistor 52 decreases, and the impedance of the second gas arrester 55 is high, so that a current flows into the NG connection line 5 and the second branch line 50. A third resistor 53 is provided on the second branch line 50, and the third fault detection means 51 functions when a current flows into the second branch line 50.

[0033] The third fault detection means 51 cuts off the current to the ground side electric wire N when a surge is applied to the NG connection line 5 and the third varistor 52 fails. The third fault detection means 51 of this embodiment includes a third resistor 53 that is disposed in the second branch line 50 and generates heat in response to a current flowing therethrough, and a third temperature fuse 54 that is provided at a midpoint of the ground side electric wire N and disposed in close proximity to the third resistor 53 and melts in response to the heat temperature of the third resistor 53 exceeding a set value.

[0034] The third resistor 53 generates heat when a current flows into the second branch line 50. Since the second gas arrester 55 has a high impedance, almost no current flows to the earth wire G. That is, the current that flows into the NG connection line 5 after the third varistor 52 is destroyed does not flow to the earth wire G but flows to the second branch line 50, which is why the third resistor 53 generates heat reliably and the third thermal fuse 54 melts.

[0035] (About the effect) A first gas arrester 45 is provided on the LG connection line 4, and a second gas arrester 55 is provided on the NG connection line 5. Here, the first and second gas arresters 45, 55 have the characteristic that they do not conduct electricity due to high impedance at the voltage during normal current conduction. In other words, the first and second gas arresters 45, 55 function to cause the current flowing into the LG connection line 4 and the NG connection line 5 to flow to the first and second branch lines 40, 50. This allows electricity to flow between LN. Therefore, electricity is conducted between LN after the varistor breaks down via the first and second branch lines 40, 50. The second and third resistors 43, 53 provided on the first and second branch lines 40, 50 are connected to the first and second branch lines 40, 50 to detect the current flowing through the branch lines 40, 50. The current generates heat, which ensures that the second and third thermal fuses 44, 54 melt down.

[0036] According to the surge protection circuit 1 of this embodiment, the power supply to the electric device is reliably cut off, so that the failure can be easily recognized. In addition, since the electric device will not continue to be used after the varistor has failed, damage caused by the next surge can be prevented.

[0037] Conventional surge protection circuits detect heat generation from a varistor when the shape of the varistor is destroyed and heat sensing is unstable. The surge protection circuit of this embodiment differs from such conventional configurations in that it causes the resistor to heat up due to the current passing through a destroyed or broken varistor, and the heat generated by this resistor reliably melts the thermal fuse and cuts off the power input line.

[0038] The surge protection circuit 1 of this embodiment only needs to include at least one fault detection means. However, if two or more, particularly the second and third fault detection means 41 and 51, are included, further effects can be obtained. When a surge is applied to the power input line 2, a surge current also flows through the first branch line 40 and the second branch line 50. As a result, the surge protection circuit 1 of this embodiment forms three parallel circuits connecting the LNs, and the varistors and / or gas arresters provided on the LN connection line 3, the LG connection line 4, and the NG connection line 5 can absorb the distributed surge. That is, the surge protection circuit 1 of this embodiment can improve the surge tolerance.

[0039] In the above explanation, the surge protection circuit 1 of this embodiment is described as having an LN connection line 3, an LG connection line 4, and an NG connection line 5, but the effects of the present invention can be achieved simply by providing a fault detection means in any one of these connection lines. Therefore, a surge protection circuit 1 in another embodiment of the present invention comprises: a power input line 2 having a non-grounded electric wire (L), a grounded electric wire (N), and an earth wire (G); a varistor 42, 52 that is arranged on an earth connection line 4, 5 connecting either the grounded electric wire or the non-grounded electric wire to the earth wire and absorbs surges; a gas arrester 45, 55 that is arranged on the earth connection line 4, 5 in a series relationship with the varistor 42, 52; and fault detection means 41, 51 that is arranged on a branch line 40, 50 that branches off from between the varistor 42, 52 and the gas arrester 45, 55 on the earth connection line 4, 5 and is connected to the other of the grounded electric wire L or the non-grounded electric wire N and that cuts off the current to either the grounded electric wire N or the non-grounded electric wire L in response to a current flowing through the earth connection line 4, 5 and the branch line 40, 50 due to a fault in the varistor 42, 52.

[0040] (Other embodiments) Next, a surge protection circuit 1A according to another embodiment will be described with reference to Fig. 2. The surge protection circuit 1 according to the present invention is adopted in a single-phase AC electric circuit, but the surge protection circuit 1A according to another embodiment is a surge protection circuit 1A adopted in a three-phase AC electric circuit. A three-phase AC circuit is essentially a combination of three single-phase AC circuits, and the above-mentioned surge protection circuit 1 for a single-phase AC circuit can also be applied to a three-phase AC circuit.

[0041] The varistors and fault detection means of the surge protection circuit 1A shown in Fig. 2 are provided between any two of the three input lines L1, L2, L3. That is, varistors 32A, 42A, 52A for absorbing surges and fault detection means 31A, 41A, 51A are provided on the L1-L2 connection line 3A, the L2-L3 connection line 4A, and the L3-L1 connection line 5A, respectively. A current flows in each connection line 3A, 4A, 5A due to a fault in the varistor 32A, 42A, 52A. In response to the flow of the current, a fault detection means The means 31A, 41A, 51A cut off the current flow to one input line or the other input line.

[0042] The fault detection means 31A, 41A, 51A includes resistors 33A, 43A, 53A which generate heat in response to a current flowing through the connection lines 3A, 4A, 5A, and includes thermal fuses 34A, 44A, 54A which are provided at a midpoint on one or the other of any two input lines, are positioned close to the resistors 33A, 43A, 53A, and melt in response to the heat temperature of the resistors 33A, 43A, 53A exceeding a set value.

[0043] Furthermore, when the power input line 2A includes an earth wire G, the surge protection circuit 1A may have an L1-G connection line 6A, an L2-G connection line 7A, and an L3-G connection line 8A, and a varistor 62A, 72A, 82A and a gas arrester 65A, 75A, 85A may be provided on each of the earth connection lines 6A, 7A, 8A. Even in this case, a current flows through the branch line 60A, 70A, 80A that branches off between the varistor 62A, 72A, 82A and the gas arrester 65A, 75A, 85A of each earth connection line 6A, 7A, 8A and connects to the corresponding line of the power input line 2A, and the fault detection means 61A, 71A, 81A cuts off the current.

[0044] The fault detection means 61A, 71A, 81A includes a resistor 63A, 73A, 83A which generates heat in response to a current flowing through the branch line 60A, 70A, 80A, and a thermal fuse 64A, 74A, 84A which is provided at a midpoint on one or the other of the power input lines, positioned in close proximity to the resistor 63A, 73A, 83A, and which melts in response to the heat temperature of the resistor 63A, 73A, 83A exceeding a set value.

[0045] The embodiments disclosed herein are illustrative in all respects and should not be considered as limiting. The scope of the present invention is defined by the claims, not the above description, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0046] 1 surge protection circuit, 2 power input line, 3 LN connection line, 4 LG connection line, 5 NG connection line, 31 first fault detection means, 32 first varistor, 33 first resistor, 34 first thermal fuse, 40 first branch line, 41 second fault detection means, 42 second varistor, 43 second resistor, 44 second thermal fuse, 45 first gas arrester, 50 second branch line, 51 third fault detection means, 52 third varistor, 53 third resistor, 54 third thermal fuse, 55 second gas arrester, 1A surge protection circuit, 2A power input line, 3A L1-L2 connection line, 4A L2-L3 connection line, 5A L3-L1 connection line, 6A L1-G connection line, 7A L2-G connection line, 8A L3-G connection line, 60A, 70A, 80A Branch line, 31A, 41A, 51A, 61A, 71A, 81A Fault detection means, 32A, 42A, 52A, 62A, 72A, 82A Varistor, 33A, 43A, 53A, 63A, 73A, 83A Resistor, 34A, 44A, 54A, 64A, 74A, 84A Thermal fuse, 65A, 75A, 85A Gas arrester.

Claims

1. A power input line including a non-grounded electric wire (L), a grounded electric wire (N), and a ground wire (G); An L-G connection line that connects the non-grounded electric wire and the earth wire; a first varistor disposed on the LG connection line; a first gas arrester disposed in the LG connection line in series with the first varistor; a first branch line that branches off from between the first varistor and the first gas arrestor on the LG connection line and is connected to the ground side electric wire; a first fault detection means arranged in the first branch line and cutting off current flow to the non-grounded electric wire in response to a current flowing in the first branch line due to a fault in the first varistor.

2. The surge protection circuit comprises: An NG connection line that connects the ground side electric wire and the earth wire; A second varistor disposed on the NG connection line; a second gas arrester disposed in the N-G connection line in series with the second varistor; A second branch line that branches off from between the second varistor and the second gas arrester on the N-G connection line and is connected to the non-grounded electric wire; 2. The surge protection circuit according to claim 1, further comprising: a second fault detection means arranged in the second branch line and cutting off current flow to the ground side electric wire in response to a current flowing in the second branch line due to a fault in the second varistor.

3. A surge protection circuit as described in claim 1, wherein the first fault detection means comprises a resistor disposed in the first branch line.

4. A surge protection circuit as described in claim 2, wherein the second fault detection means comprises a resistor disposed in the second branch line.

5. An L-N connection line that connects the non-grounded electric wire and the grounded electric wire; 3. The surge protection circuit of claim 1, further comprising a varistor disposed on the LN connection line.

6. A surge protection circuit to be incorporated in a three-phase AC circuit having three input lines (L1, L2, L3) and a ground wire (G), an earth connection line connecting one of any two input lines to an earth line; a varistor disposed in the earth connection line; a gas arrester disposed in the earth connection line in series with the varistor; a branch line that branches off from between the varistor and the gas arrester on the earth connection line and is connected to the other of any two input lines; a failure detection means arranged on the branch line for cutting off current flow to the one input line in response to a current flowing through the branch line due to a failure of the varistor.

7. The surge protection circuit of claim 6, wherein the fault detection means comprises a resistor disposed in the branch line.

8. an input line connection line connecting any two input lines; 7. The surge protection circuit of claim 6, further comprising a varistor disposed in the input inter-line connection line.

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