Abstract:
A feed line switching circuit includes a relay connected between feed lines, a normally close contact of the relay, and a resistor that constitutes a series circuit with the normally close contact, which series circuit is connected in parallel with the relay. The resistance of the coil of the relay and the resistance of the resistor are determined to satisfy the condition that the current flowing through the relay after energizing the relay can prevent the chattering of the relay. Thus, the feed line switching circuit can positively carry out the feed line switching.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a feed line switching circuit for switching a feed line by energizing relays by feed current control, and more particularly to a feed line switching circuit suitable for a branching unit constituting an optical submarine cable system. 
     2. Description of Related Art 
     FIG. 7 is a circuit diagram showing a configuration of a conventional feed line switching circuit disclosed in Japanese patent application publication No. 7-34550/1995, for example. In FIG. 7, reference numerals  1   a - 1   c  each designate a feed line for feeding repeaters and the like with a current;  2   a - 2   c  each designate a power supply such as a DC constant current source; and the reference numeral  3  designates a ground. 
     The reference numeral  4  designates a feed line switching circuit for switching the feeding from the feeding across the feed lines  1   a  and  1   b  to the feeding across the feed lines  1   a  and  1   c  or vice versa by feed current control. In the feed line switching circuit  4 , reference numerals  5   a - 5   c  designate terminals connected to the feed lines  1   a - 1   c.    
     The reference symbol K 1  designates a relay connected between the feed lines  1   a  and  1   b;  K 2  designates a relay connected between the feed lines  1   a  and  1   c;  reference numerals K 3  and K 4  designates relays connected in series across the feed lines  1   b  and  1   c.  The relays K 1 -K 4  each consist of a vacuum relay or the like that operates in response to a feed current. The reference symbol k 1 - 1  designates a contact set of the relay K 1 ; k 2 - 1  designates a contact set of the relay K 2 ; reference symbols k 3 - 1  and k 3 - 2  each designate a contact set of the relay K 3 ; and k 4 - 1  and k 4 - 2  each designate a contact set of the relay K 4 . 
     Next, the operation of the conventional feed line switching circuit will be described. 
     First, to achieve the feeding across the feed lines  1   a  and  1   b,  the power supplies  2   a  and  2   b  are driven so that the feed current flows from the power supply  2   a  to the power supply  2   b  through the feed line  1   a,  terminal  5   a,  relay K 1 , contact k 4 - 2 , contact set k 2 - 1 , terminal  5   b  and feed line  1   b.  As a result, repeaters (not shown) connected to the feed lines  1   a  and  1   b  are supplied by double-end feed. In this state, when the feed current reaches the working current of the relay K 1 , the relay K 1  actuates its contact set k 1 - 1  to open the connection across the feed lines  1   a  and  1   c,  and to close the connection between the relay K 3  and the feed line  1   c.  Furthermore, the power supply  2   c  is driven to pass the feed current through the ground  3 , relay K 3 , contact set k 1 - 1 , terminal  5   c,  feed line  1   c  and power supply  2   c.  As a result, the repeaters connected to the feed line  1   c  are supplied by single-end feed. When the feed current reaches the working current of the relay K 3 , the relay K 3  closes its contact k 3 - 1  and opens its contact k 3 - 2 , thereby self-holding the connection between the relay K 3  and the feed line  1   c.    
     Likewise, to achieve the feeding across the feed lines  1   a  and  1   c,  the power supplies  2   a  and  2   c  are driven so that the feed current flows from the power supply  2   a  to the power supply  2   c  through the feed line  1   a,  terminal  5   a,  relay K 2 , contact k 3 - 2 , contact set k 1 - 1 , terminal  5   c  and feed line  1   c.  As a result, the repeaters connected to the feed lines  1   a  and  1   c  are supplied by double-end feed. In this state, when the feed current reaches the working current of the relay K 2 , the relay K 2  actuates its contact set k 2 - 1  to open the connection across the feed lines  1   a  and  1   b,  and to close the connection between the relay K 4  and the feed line  1   b.  Furthermore, the power supply  2   b  is driven to pass the feed current through the ground  3 , relay K 4 , contact set k 1 - 1 , terminal  5   b,  feed line  1   b  and power supply  2   b.  As a result, the repeaters connected to the feed line  1   b  are supplied by single-end feed. When the feed current reaches the working current of the relay K 4 , the relay K 4  closes its contact k 4 - 1  and opens its contact k 4 - 2 , thereby self-holding the connection between the relay K 4  and the feed line  1   b.    
     In the conventional feed line switching circuit with the foregoing configuration, the impedance of the feed passage can sometimes vary because of the operation of the contacts, thereby varying the feed current flowing through the feed line switching circuit  4 . 
     For example, assume that the feeding across the feed lines  1   a  and  1   b  is carried out by the feed current control when a breaking of the optical submarine cable on the side of the feed line  1   c  takes place, and a shunt fault occurs in which the end of the optical submarine cable on the side of the feed line  1   c  is grounded by seawater. In this case, the feed current passes not only through the passage of the power supply  2   a —relay K 1 —power supply  2   b,  but also through the shunt fault point—relay K 2 —relay K 1 —power supply  2   b  transiently. When the sum of the two feed currents reach the working current of the relay K 1 , the relay K 1  opens the contact set k 1 - 1 , thereby disconnecting the shunt fault point. Thus, at the time when the contact set k 1 - 1  activates, the impedance of the feed passage seen from the feed line  1   b  to the feed line switching circuit  4  increases, so that the feed current flowing through the feed line switching circuit  4  is reduced transiently. 
     Thus, the feed current flowing through the feed line switching circuit  4  is reduced when the feed current reaches the working current of the relay K 1  and the contact set k 1 - 1  is opened. As a result, the feed current flowing through the relay K 1  is reduced at the same time, and hence the relay K 1  can return to unenergized state again. In such a case, the relay K 1  will repeat the energized state and unenergized state (chattering), presenting a problem of disabling the switching of the feed line. 
     SUMMARY OF THE INVENTION 
     The present invention is implemented to solve the foregoing problem. It is therefore an object of the present invention to provide a feed line switching circuit and branching unit capable of positively switching the feed line with preventing the current reduction in the relay after it is energized, and hence capable of preventing its chattering. 
     According to a first aspect of the present invention, there is provided a feed line switching circuit for switching feeding by feed current control between feeding across a first feed line and a second feed line and feeding across the first feed line and a third feed line, the feed line switching circuit comprising: a relay that is connected between the first feed line and the second feed line, and operates in response to a feed current flowing through the relay; a first contact of the relay that closes when the relay is unenergized and opens when the relay is energized;, and a resistor that constitutes a series circuit with the first contact, which series circuit is connected in parallel with the relay, wherein a resistance r of the relay and a resistance R of the resistor are set to satisfy a relationship of 
     
       
         ( R /( r+R ))× Ia&lt;Ib   
       
     
     where Ia is a current value flowing through the second feed line immediately before operation of the relay, and Ib is a current value flowing through the second feed line immediately after the operation of the relay. 
     Here, the relay may further comprise a second contact that is connected between the first feed line and the third feed line, and that closes when the relay is unenergized and opens when the relay is energized. 
     Besides, each of the resistors may consist of any one of a single resistor, a combination of a plurality of resistors and an assemblage of other elements that provides resistance in its entirety. 
     According to a second aspect of the present invention, there is provided a feed line switching circuit for switching feeding by feed current control between feeding across a first feed line and a second feed line and feeding across the first feed line and a third feed line, the feed line switching circuit comprising: a relay that constitutes a series circuit with a first resistor, and operates in response to a feed current flowing through the relay, the series circuit being connected between the first feed line and the third feed line; a first switching section of the relay that closes its first and second contacts and opens its first and third contacts when the relay is unenergized, and that closes its first and third contacts and opens its first and second contacts when the relay is energized; a second switching section of the relay that is connected between the first feed line and the second feed line, and that closes when the relay is unenergized, and opens when the relay is energized; a first line, a first end of which is connected to a connecting point of the relay and the first resistor, and a second end of which is connected to the first contact of the first switching section of the relay; a second resistor, a first end of which is connected to a relay side end of the series circuit, and a second end of which is connected to the second contact of the first switching section of the relay; and a second line, a first end of which is connected to the first resistor side of the series circuit, and a second end of which is connected to the third contact of the first switching section of the relay, wherein a resistance r of the relay and a resistance R of the second resistor are set to satisfy a relationship of 
     
       
         ( R /( r+R ))× Ia&lt;Ib   
       
     
     where Ia is a current value flowing through the third feed line immediately before operation of the relay, and Ib is a current value flowing through the third feed line immediately after the operation of the relay. 
     Here, each of the resistors may consist of any one of a single resistor, a combination of a plurality of resistors and an assemblage of other elements that provides resistance in its entirety. 
     According to a third aspect of the present invention, there is provided a branching unit comprising a feed line switching circuit, the feed line switching circuit comprising: a relay that is connected between the first feed line and the second feed line, and operates in response to a feed current flowing through the relay; a first contact of the relay that closes when the relay is unenergized and opens when the relay is energized; and a resistor that constitutes a series circuit with the first contact, which series circuit is connected in parallel with the relay, wherein a resistance r of the relay and a resistance R of the resistor are set to satisfy a relationship of 
     
       
         ( R /( r+R ))× Ia&lt;Ib   
       
     
     where Ia is a current value flowing through the second feed line immediately before operation of the relay, and Ib is a current value flowing through the second feed line immediately after the operation of the relay. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing a configuration of a feed line switching circuit of an embodiment 1 in accordance with the present invention; 
     FIG. 2 is a circuit diagram showing a principle of a parallel circuit of the embodiment 1 in accordance with the present invention; 
     FIG. 3 is a circuit diagram showing a configuration of a feed line switching circuit of an embodiment 2 in accordance with the present invention; 
     FIG. 4 is a circuit diagram showing a configuration of a feed line switching circuit of an embodiment 3 in accordance with the present invention; 
     FIG. 5 is a circuit diagram showing a principle of a parallel circuit of the embodiment 3 when the relay K 2  is in the unenergized state; 
     FIG. 6 is a circuit diagram showing a principle of a parallel circuit of the embodiment 3 when the relay K 2  is in the energized state; and 
     FIG. 7 is a circuit diagram showing a configuration of a conventional feed line switching circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be described with reference to the accompanying drawings. 
     Embodiment 1 
     FIG. 1 is a circuit diagram showing a configuration of a feed line switching circuit of an embodiment 1 in accordance with the present invention. In FIG. 1, reference numerals  1   a - 1   c  each designate a feed line for feeding repeaters and the like with power;  2   a - 2   c  each designate a power supply such as a DC constant current source; and the reference numeral  3  designates a ground. 
     The reference numeral  4  designates a feed line switching circuit for switching the feeding across the feed lines  1   a  and  1   b  to the feeding across the feed lines  1   a  and  1   c  or vice versa by feed current control. In the feed line switching circuit  4 , reference numerals  5   a - 5   c  designate terminals connected to the feed lines  1   a - 1   c.    
     The reference symbol K 1  designates a relay connected between the feed lines  1   a  and  1   b;  K 2  designates a relay connected between the feed lines  1   a  and  1   c;  reference numerals K 3  and K 4  designates relays connected in series across the feed lines  1   b  and  1   c.  The relays K 1 -K 4  each consist of a vacuum relay or the like that operates in response to a feed current. The reference symbol k 1 - 1  designates a contact set of the relay K 1 ; k 2 - 1  designates a contact set of the relay K 2 ; reference symbols k 3 - 1  and k 3 - 2  each designate a contact of the relay K 3 ; and k 4 - 1  and k 4 - 2  each designate a contact of the relay K 4 . 
     Furthermore, the reference symbol k 1 - 2  designates a contact of the relay K 1 ; and k 2 - 2  designates a contact of the relay K 2 . The reference symbol R 1  designates a resistor that constitutes a series circuit with the contact k 1 - 2 , which series circuit is connected in parallel with the relay K 1 ; and R 2  designates a resistor that constitutes a series circuit with the contact k 2 - 2 , which series circuit is connected in parallel with the relay K 2 . Reference symbols ZD 1 -ZD 4  designate Zener diodes connected in parallel with the relays K 1 -K 4 , respectively. 
     Next, the operation of the present embodiment 1 will be described. 
     First, the principle of the present embodiment 1 will be described. 
     FIG. 2 is a circuit diagram illustrating the principle of a parallel circuit of the embodiment 1 in accordance with present invention. In FIG. 2, the reference symbol K 1  designates the relay, k 1  designates a normally close contact of the relay K 1 ; and R 1  designates a resistor that constitutes a series circuit with the contact k 1 , which series circuit is connected in parallel with the relay K 1 . 
     It is assumed here that the resistance of the relay K 1  is r, and that of the resistor R 1  is R, and that the contact resistance of the contact k 1  is much smaller than the resistances r and R. 
     While the relay K 1  is in the unenergized state, the feed current is divided to flow through the relay K 1  side and the resistor R 1  side. In this case, the current values Ir and IR flowing through the relay K 1  side and the resistor R 1  side are expressed by the following equations. 
     
       
           Ir= ( R /( r+R ))× I   
       
     
     
       
           IR= ( r /( r+R ))× I   
       
     
     where I is the feed current. 
     In contrast, while the relay K 1  is energized, Ir=I. 
     Assume that the current value flowing through the feed line  1   b  temporarily drops from Ia to Ib because of the contact operation at a start of the feeding. In this case, the current Ioff flowing before the operation of the relay K 1  is expressed as follows: 
     
       
           I off=( R /( r+R ))× Ia   
       
     
     In contrast, the current Ion flowing after the relay K 1  is energized is expressed as follows: 
     
       
           I on= Ib   
       
     
     Accordingly, by setting the resistances r and R such that the relationship Ioff&lt;Ion, that is, (R/(r+R))×Ia&lt;Ib is satisfied, the current flowing through the relay K 1  after the actuation will not reduce, thereby preventing the chattering of the relay K 1 . 
     Next, the operation of the present embodiment 1 will be described with reference to FIG.  1 . 
     First, to achieve the feeding across the feed lines  1   a  and  1   b,  the power supplies  2   a  and  2   b  are driven so that the feed current flows from the power supply  2   a  to the power supply  2   b  through the feed line  1   a,  terminal  5   a,  parallel circuit of the relay K 1  with the resistor R 1  and contact k 1 - 2 , contact k 4 - 2 , contact set k 2 - 1 , terminal  5   b  and feed line  1   b.  As a result, the repeaters (not shown) connected to the feed lines  1   a  and  1   b  are supplied by double-end feed. In addition, when the feed current reaches the working current of the relay K 1 , the relay K 1  operates its contact set k 1 - 1  to open the connection across the feed lines  1   a  and  1   c,  and to close the connection between the relay K 3  and the feed line  1   c.  Furthermore, the relay K 1  opens its contact k 1 - 2 , thereby disconnecting the resistor R 1  connected in parallel with it. Moreover, the power supply  2   c  is driven to pass the feed current through the ground  3 , relay K 3 , contact set k 1 - 1 , terminal  5   c,  feed line  1   c  and power supply  2   c.  As a result, the repeaters connected to the feed line  1   c  are supplied by single-end feed. When the feed current reaches the working current of the relay K 3 , the relay K 3  closes its contact k 3 - 1  and opens its contact k 3 - 2 , thereby self-holding the connection between the relay K 3  and the feed line  1   c.    
     Likewise, to achieve the feeding across the feed lines  1   a  and  1   c,  the power supplies  2   a  and  2   c  are driven so that the feed current flows from the power supply  2   a  to the power supply  2   c  through the feed line  1   a,  terminal  5   a,  parallel circuit of the relay K 2  with the resistor R 2  and contact k 2 - 2 , contact k 3 - 2 , contact set k 1 - 1 , terminal  5   c  and feed line  1   c.  As a result, the repeaters connected to the feed lines  1   a  and  1   c  are supplied by double-end feed. In addition, when the feed current reaches the working current of the relay K 2 , the relay K 2  actuates its contact set k 2 - 1  to open the connection across the feed lines  1   a  and  1   b,  and to close the connection between the relay K 4  and the feed line  1   b.  Furthermore, the relay K 2  opens its contact k 2 - 2 , thereby disconnecting the resistor R 2  connected in parallel with it. Moreover, the power supply  2   b  is driven to pass the feed current through the ground  3 , relay K 4 , contact set k 2 - 1 , terminal  5   b,  feed line  1   b  and power supply  2   b.  As a result, the repeaters connected to the feed line  1   b  are supplied by single-end feed. When the feed current reaches the working current of the relay K 4 , the relay K 4  closes its contact k 4 - 1  and opens its contact k 4 - 2 , thereby self-holding the connection between the relay K 4  and the feed line  1   b.    
     The Zener diodes ZD 1 -ZD 4  connected in parallel with the relays K 1 -K 4  are provided for the purpose of protecting the relays K 1 -K 4  by bypassing the reverse current and excessive forward current of the relays K 1 -K 4 . 
     Next, test results of the configuration as shown in FIG. 1 will be described. 
     The test was performed in the following conditions: The working current of the relays K 1  and K 2  was 60 mA, and the resistance R of the resistors R 1  and R 2  was made equal to the resistance r of the relays K 1  and K 2 . 
     First, the feed line  1   c  side was placed at a shunt fault state by grounding the terminal  5   c,  and the feed line switching circuit  4  was supplied with 80 mA from the power supply  2   a.  As a result, almost all the feed current flowed through the shunt fault point at the feed line  1   c  side with a small impedance, leaving little current flowing through the relay K 1 . 
     In this state, the feed line switching circuit  4  was supplied with a current of 80 mA flowing from the power supply  2   b  to the ground  3 . As a result, the current flowing through the shunt fault point at the feed line  1   c  side became nearly zero mA, leaving a current of 40 mA flowing through each of the relay K 1  and resistor R 1 . 
     In this state, the current value of the power supply  2   b  was gradually increased from the 80 mA. As a result, the relay K 1  operated when the current value Ia was increased to 120 mA. Thus, the current flowing through the terminal  5   a  was increased to 120 mA after the operation of the relay K 1  with maintaining the current value before the operation at 80 mA. The current value flowing through the relay K 1  was 60 mA before the operation, and the current value Ib immediately after the operation was increased to 80 mA, and then to 120 mA. 
     Therefore, it becomes possible to satisfy the relationship (R/(r+R))×Ia&lt;Ib by setting the current value Ia flowing through the feed line  1   b  immediately before the operation of the relay K 1  at 120 mA, and the current value Ib flowing through the feed line  1   b  immediately after the operation of the relay K 1  at 80 mA, and by setting the resistance r of the relay K 1  equal to the resistance R of resistor R 1 . As a result, the current flowing through the relay K 1  increases with the operation of the relay K 1  instead of being reduced, thereby preventing the chattering. 
     As described above, the present embodiment 1 comprises the resistor R 1  for preventing the reduction in the current value flowing through the relay K 1 . As a result, it can positively switch the feed line without reducing the current value flowing through the relay K 1  after the operation of the relay K 1  even if the shunt fault occurs at the feed line  1   c  side, thereby preventing the chattering of the relay K 1 . 
     In addition, since the resistor R 1  is disconnected at the same time as the contact set k 1 - 1  for switching the feed line is opened, the resistor R 1  can accomplish its function to prevent the reduction in the current value flowing through the relay K 1  at the same time when the feed line is switched. 
     Embodiment 2 
     FIG. 3 is a circuit diagram showing a configuration of a feed line switching circuit of an embodiment 2 in accordance with the present invention, in which the principle as illustrated in FIG. 2 is applied to a double-branching method. In FIG. 3, reference numerals  11  and  12  each designate a trunk station;  13  designates a branch station;  14  and  15  each designate a feed line switching circuit;  14   a - 14   c  and  15   a - 15   c  each designate a terminal;  16  designates a feed line connected across the trunk station  11  and the feed line switching circuit  14 ;  17  designates a feed line connected across the feed line switching circuits  14  and  15 ;  18  designates a feed line connected across the branch station  13  and the feed line switching circuit  14 ;  19  designates a feed line connected across the feed line switching circuit  15  and the trunk station  12 ; and  20  designates a feed line connected across the branch station  13  and the feed line switching circuit  15 . 
     In the feed line switching circuit  14 , the reference symbol K 11  designates a relay connected across the feed lines  16  and  17 ; K 12  designates a relay connected across the feed lines  16  and  18 ; and K 13  designates a self-holding relay. The relays K 11 -K 13  are each composed of a vacuum relay or the like, which operates in response to the feed current. In addition, the reference symbol k 11 - 1  designates a contact of the relay K 11 ; k 12 - 1  and k 12 - 2  each designate a contact set of the relay K 12 ; and k 13 - 1  and k 13 - 2  each designate a contact of the relay K 13 . 
     Furthermore, the reference symbol k 11 - 2  designates a contact of the relay K 11 ; R 11  designates a resistor constituting a series circuit with the contact k 11 - 2 , which series circuit is connected in parallel with the relay K 11 ; and R 12  designates a resistor connected in parallel with the contact k 12 - 2 . Reference symbols ZD 11 -ZD 13  designate Zener diodes connected in parallel with the relays K 11 -K 13 , respectively. 
     In the feed line switching circuit  15 , the reference symbol K 14  designates a relay connected across the feed lines  17  and  19 ; K 15  designates a relay connected across the feed lines  17  and  20 ; and K 16  designates a self-holding relay. The relays K 14 -K 16  are each composed of a vacuum relay or the like, which operates in response to the feed current. In addition, the reference symbol k 14 - 1  designates a contact set of the relay K 14 ; k 15 - 1  and k 15 - 2  each designate a contact of the relay K 15 ; and k 16 - 1  and k 16 - 2  each designate a contact of the relay K 16 . 
     Furthermore, the reference symbol k 14 - 2  designates a contact of the relay K 14 ; R 14  designates a resistor constituting a series circuit with the contact k 14 - 2 , which series circuit is connected in parallel with the relay K 14 ; and R 15  designates a resistor connected in parallel with the contact k 15 - 2 . Reference symbols ZD 14 -ZD 16  designate Zener diodes connected in parallel with the relays K 14 -K 16 , respectively. 
     Next, the operation of the present embodiment 2 will be described with reference to FIG.  3 . 
     First, to achieve the feeding across the trunk station  11  (+) and branch station  13  (−) by the feed current control, the relay K 12  opens its contact k 12 - 1  and closes its contact k 12 - 2 . As a result, the trunk station  11  (+) feeds the branch station  13  (−) with a current bypassing the resistor R 12 . 
     In addition, to achieve the feeding across the branch station  13  (+) and trunk station  12  (−) by the feed current control, the relay K 15  opens its contact k 15 - 1  and closes its contact k 15 - 2 . As a result, the branch station  13  (+) feeds the trunk station  12  (−) with a current bypassing the resistor R 15 . 
     Furthermore, to achieve the feeding across the trunk station  11  (+) and trunk station  12  (−) by the feed current control, the relay K 11  opens the contacts c and a of the contact set k 11 - 1  and closes the contacts c and b thereof. As a result, the relay K 13  opens its contact k 13 - 1  and closes its contact k 13 - 2 , thereby self-holding it. At the same time, the relay K 11  opens its contact k 11 - 2  to disconnect the resistor R 11  connected in parallel with the relay K 11 . Likewise, the relay K 14  opens the contacts c and a of the contact set k 14 - 1  and closes the contacts c and b thereof. As a result, the relay K 16  opens its contact k 16 - 1  and closes its contact k 16 - 2 , thereby self-holding it. At the same time, the relay K 14  opens its contact k 14 - 2  to disconnect the resistor R 14  connected in parallel with the relay K 14 . 
     When achieving the feeding across the trunk station  11  (+) and trunk station  12  (−) by the feed current control, since the contacts k 12 - 2  and k 15 - 2  are open, even if a shunt fault occurs on the feed line  18  or  20 , it is possible for the resistor R 12  or R 15  to prevent unnecessary feeding from the shunt fault point. 
     Next, test results of the configuration as shown in FIG. 3 will be described. 
     The test was performed in the following conditions: The working current of the relays K 11  and K 14  was 60 mA, and the resistance R of the resistors R 11  and R 14  was made three time greater than the resistance r of the relays K 11  and K 14 . 
     First, the feed line  18  and  20  sides were placed at a shunt fault state by grounding the terminals  14   c  and  15   c,  and the trunk station  11  (+) fed a current of 60 mA. As a result, a current of 35 mA flowed through the shunt fault point at the terminal  14   c  side, and a current of 24 mA flowed through the shunt fault point at the terminal  15   c  side, leaving nearly zero mA flowing from the terminal  15   b  to the trunk station  12  (−). In addition, a current of 24 mA flows across the terminal  14   b  and the terminal  15   a,  and a current of 18 mA flows through the relays K 11  and K 14  each. 
     In this state, the trunk station  12  (−) supplied 120 mA. As a result, the current flowing through the shunt fault point at the terminal  14   c  side was 11 mA, the current flowing through the shunt fault point at the terminal  15   c  side was 47 mA, the current flowing across the terminals  14   b  and  15   a  was 71 mA, and the current flowing through the relays K 11  and K 14  was 53 mA. 
     In this state, the current value of the trunk station  12  (−) was gradually increased from the 120 mA. As a result, the relays K 11  and K 14  were actuated at about the same time when the current value reached 135 mA. In the course of this, the current Ia flowing across the terminals  14   b  and  15   a  was reduced from 80 mA to 70 mA, followed by an increase of the current. The current value flowing through the relay K 11  was 60 mA at the operation, and increased to 80 mA immediately after the operation. Subsequently, the current value Ib was reduced to 70 mA, followed by an increase thereof. 
     Therefore, it was possible to satisfy the relationship (R/(r+R))×Ia&lt;Ib by setting the current value Ia at 80 mA, and the current value Ib at 70 mA, and by setting the resistance R of resistors R 11  and R 14  at a value three times greater than the resistance r of the relays K 11  and K 14 . As a result, the current flowing through the relays K 11  and K 14  increased with the operation of the relays K 11  and K 14 . Although there was a temporal reduction in the current thereafter, it did not drop below the working current of the relays K 11  and K 14 , thereby preventing the chattering. 
     As described above, the present embodiment 2 comprises the resistors R 11  and R 14  for preventing the reduction in the current values flowing through the relays K 11  and K 14 . As a result, it can positively switch the feed line with preventing the chattering of the relays K 11  and K 14  even if the shunt fault occurs at the feed lines  18  and  20  side. 
     In addition, since the resistors R 11  and R 14  are disconnected at the same time as the contacts k 11 - 1  and k 14 - 1  for switching the feed line are opened, the resistors R 11  and R 14  can accomplish their function to prevent the reduction in the current value flowing through the relays K 11  and K 14  at the same time when the feed line is switched. 
     Furthermore, even if a shunt fault occurs on the feed line  18  or  20  during the feeding across the trunk station  11  (+) and trunk station  12  (−), since the contact k 12 - 2  and k 15 - 2  are open, the resistors R 12  and R 15  can prevent the unnecessary feeding from the shunt fault point, thereby enabling positive feed line switching. 
     Moreover, although the resistors R 12  and R 15  could hinder the feeding during the feeding across the trunk station  11  (+) and the branch station  13  (−) or across the branch station  13  (+) and trunk station  12  (−), since the contacts k 12 - 2  and k 15 - 2  are closed during the operation of the relays K 12  and K 15 , the feeding bypasses the resistors R 12  and R 15 , making it possible to avoid the hindrance of the feeding. 
     Embodiment 3 
     FIG. 4 is a circuit diagram showing a configuration of a feed line switching circuit of an embodiment 3 in accordance with the present invention. In FIG. 4, the reference symbol R 3  designates a resistor constituting a series circuit with a relay K 2 , which series circuit is connected across the feed lines  1   a  and  1   c.  The reference symbol L 1  designates a line, a first end of which is connected to the connecting point of the relay K 2  and resistor R 3 , and a second end of which is connected to a contact a of a contact set k 2 - 3  of the relay K 2 ; R 2  designates a resistor, a first end of-which is connected to a relay K 2  side end of the series circuit, and a second end of which is connected to a contact b of the contact set k 2 - 3  of the relay K 2 ; and L 2  designates a line, a first end of which is connected to a resistor R 3  side end of the series circuit, and a second end of which is connected to a contact c of the contact set k 2 - 3  of the relay K 2 . The reference symbol k 2 - 3  designates the contact set of the relay K 2  that closes the contacts a and b and opens the contacts a and c when the relay K 2  is unenergized, and that closes the contacts a and c and opens the contacts a and b when the relay K 2  is energized. The reference symbol k 2 - 4  designates a contact of the relay K 2  that is connected between the feed lines  1   a  and  1   b,  and that closes when the relay K 2  is unenergized and opens when the relay K 2  is energized. The remaining configuration is the same as that of FIG.  1 . 
     Next, the operation of the present embodiment 3 will be described. 
     First, the principle of the present embodiment 3 will be described with reference to FIGS. 5 and 6 which illustrate the principle of a parallel circuit of the embodiment 3 in accordance with the present invention. Here, FIG. 5 illustrates the state when the relay K 2  is unenergized, and FIG. 6 illustrates the state when the relay K 2  is energized. As illustrated in FIGS. 4 and 5, when the relay K 2  is unenergized, a passage is established starting from the feed line  1   a,  passing through the parallel circuit of the relay K 2  with the resistor R 2 , and the resistor R 3 , and arrives at the feed line  1   c.  Accordingly, even if a shunt fault occurs at the feed line  1   c  side during feeding across the feed lines  1   a  and  1   b,  the resistor R 3  connected between the feed lines  1   a  and  1   c  can prevent unnecessary feeding from the shunt fault point, thereby enabling a positive switching of the feed line. 
     On the other hand, when the relay K 2  is energized as illustrated in FIGS. 4 and 6, a passage is established starting from the feed line  1   a,  passing through the relay K 2  and the bypass circuit of the resistor R 3  consisting of the lines L 1  and L 2 , and arrives at the feed line  1   c.  Thus bypassing the resistor R 3  by the lines L 1  and L 2  makes it possible to avoid the hindrance of the feeding during the feeding across the feed lines  1   a  and  1   c.    
     In addition, achieving these functions by the single relay K 2  and the single contact set k 2 - 3  makes it possible to implement the simple configuration without increasing the number of the relays and contacts, thereby enabling low-cost configuration without increasing a fault factor. 
     Incidentally, the setting conditions imposed on the resistance r of the relays K 1  and K 2  and the resistance R of the resistors R 1  and R 2  are assumed to be identical to those of the foregoing embodiment 1. 
     Next, test results of the configuration of FIG. 4 will be described. 
     The test was performed in the following conditions: The working current of the relays K 1  and K 2  was 60 mA; the resistance r of the relays K 1  and K 2  and the resistance R of the resistors R 1  and R 2  were both 200Ω; and the resistor R 3  was 1000Ω. 
     First, the power supply  2   b  was disconnected from the feed line  1   b,  and the power supply  2   a  fed 60 mA. As a result, a current of 30 mA flowed through the relay K 2 , and a current of 60 mA flowed through the resistor R 3 . In this case, the voltage drop across the terminals  5   a  and  5   c  was 70 V. 
     In this state, the power supply  2   c  fed 120 mA. As a result, the relay K 2  was actuated so that the contact set k 2 - 3  was transferred and the contact k 2 - 4  was opened. In this case, a current of 120 mA flowed through the relay K 2 , and almost no current flowed through the resistor R 3 . In addition, the voltage drop across the terminals  5   a  and  5   c  was 15 V. 
     After that, the feeding was temporarily halted. Then, the power supply  2   b  was connected to the feed line  1   b,  and the terminal  5   c  was grounded to make a shunt fault state, followed by feeding a current of 60 mA from the power supply  2   a.  As a result, a current of 50 mA flowed through the feed line  1   b,  and a current of 10 mA flowed through the shunt fault point of the feed line  1   c.    
     In this state, the power supply  2   b  fed 120 mA. As a result, the relay K 1  was actuated so that the contact set k 1 - 1  was transferred and the contact k 1 - 2  was opened. In this case, a current of 120 mA flowed through the relay K 1 , and a current of 0 mA flowed through the shunt fault point of the feed line 
     As described above, the present embodiment 3 comprises the resistor R 2  for preventing the reduction in the current value flowing through the relay K 2 . As a result, it can prevent the current value flowing through the relay K 2  from being reduced after the relay K 2  is actuated even if the shunt fault occurs at the feed line  1   b  side, thereby preventing the chattering of the relay K 2 , and positively switching the feed line. 
     In addition, since the resistor R 2  is disconnected at the same time as the contact k 2 - 4  for switching the feed line is opened, the resistor R 2  can accomplish its function to prevent the reduction in the current value flowing through the relay K 2  at the same time when the feed line is switched. 
     Furthermore, even if a shunt fault occurs at the feed line  1   c  side during the feeding across the feed lines  1   a  and  1   b,  the resistor R 3  connected between the feed lines  1   a  and  1   c  can prevent unnecessary feeding from the shunt fault point, thereby enabling positive switching of the feed line. 
     Moreover, bypassing the resistor R 3  by the lines L 1  and L 2  during the feeding across the feed lines  1   a  and  1   c  makes it possible to eliminate the hindrance of the feeding. 
     In addition, achieving these functions by the single relay K 2  and the single contact set k 2 - 3  makes it possible to implement a simple configuration without increasing the number of the relays and contacts, thereby enabling low-cost configuration without increasing a fault factor. 
     Incidentally, it is not necessary for each resistor of the foregoing embodiments to be composed of a single resistor. It can be composed a plurality of resistors, or replaced by other elements that have resistance as an assemblage. Any elements with any structure can be utilized as long as they have a resistance that meets the required conditions, making it possible to expand the selection range in the design and manufacturing. 
     In addition, the foregoing embodiments can be integrated into the branching unit as its feed line switching circuit, offering a marked advantage in the branching unit that is likely to suffer from a shunt fault.