fig6 illustrates a power feed path switching circuit 40 according to a first embodiment of the present invention . the power feed path switching circuit 40 shown in fig6 is provided in a branching unit . first , second and third relay coils rl11 , rl12 and rl13 are connected in series between cable ends 41 and 42 coupled to the stations a and b , respectively . normally , a first relay coil rl11 connects the cable end 41 to cable ends 42 and 43 . the cable end 43 is connected to the station c . the first relay switch rl11 is opened so that the cable end 43 is disconnected from circuits connected between the cable ends 41 and 42 when a first driving current flows in the first relay coil rl11 . strictly speaking , a current equal to or greater than the first driving current flows in the first relay coil rl11 , the first relay switch rl11 is opened . the above holds true for other currents described in the specification . a second relay switch rl12 is provided between the cable end 43 and a resistor rl . normally , the second relay switch rl12 is maintained in the open state . when a second driving current greater than the first driving current flows in the relay coil rl12 , the second relay switch rl12 is closed so that the cable end 43 is grounded via the resistor rl . in this state , a charge stored in the cable having the cable end 43 is allowed to flow to the sea ground via the resistor rl , so that the cable is gradually discharged . a third relay switch rl13 is connected across the resistor rl , and is normally maintained in the open state . when a third driving current greater than the second driving current flows in the third relay coil rl13 , the third relay switch rl13 is closed so that the cable end 43 is directly grounded . that is , after the discharging of the cable having the cable end 43 is substantially completed , the cable end 43 is grounded . hence , the switching between the power feed paths can be performed by increasing the current flowing in the cable between the cable ends 41 and 42 over only three steps , rather than setting the branching unit having the circuit 40 to the ground potential . fig7 illustrates a power feed path switching circuit 60 according to a second embodiment of the present invention . the circuit 60 shown in fig7 is provided in a branching unit . the relay coil rl11 7 is connected between the cable ends 41 and 42 . the relay switch rl11 associated with the relay coil rl11 is connected between the cable ends 42 and 43 . normally , the relay switch rl11 is maintained in the closed state . when a predetermined driving current flows in the relay coil rl11 , the relay switch rl11 is opened and hence the cable end 43 is disconnected from circuits provided between the cable ends 41 and 42 . a power supply circuit 15 , which is connected to the cable between the cable ends 41 and 42 , starts power supply on the basis of the current flowing in the cable between the cable ends 41 and 42 . a first switch circuit 12 is provided between the cable end 43 and the resistor rl , and is normally maintained in the open state . the first switch circuit 12 is closed so that the cable end 43 is connected to the resistor rl when a first period t 1 has elapsed after the power supply circuit 15 starts to supply power . thereby , the cable terminal 43 is grounded via the resistor , and the cable extending from the cable end is gradually discharged via the resistor rl . a second switch circuit 13 is connected across the resistor rl , and is normally maintained in the open state . the second switch circuit 13 is closed so that the cable end 43 is directly grounded when a second period t 2 longer than the first period t 1 has elapsed after the power supply circuit 15 starts the power supply . hence , the cable end 43 is grounded after the discharging of the cable is substantially completed . the switching process of the second embodiment of the present invention is simpler than that of the first embodiment thereof because the switching between the power feed paths can be performed by only driving the relay coil rl11 . fig8 is a circuit diagram of a variation 40a of the first embodiment of the present invention shown in fig6 . in fig8 those parts that are the same as parts shown in fig6 are given the same reference numbers . the variation shown in fig8 includes a fourth relay coil rl14 and two relay switches rl14 1 and rl14 2 associated with the fourth relay coil rl14 . the cable end 41 connected to the station a is coupled to the cable end 42 connected to the station b via the relay coils rl11 , rl12 and rl13 connected in series . the cable end 42 is also coupled to the cable end 43 connected to the station c via the relay switches rl14 2 and rl11 connected in series . the cable end 43 is connected to one end of the relay switch rl14 1 and one end of the relay switch rl12 . the other end of the relay switch rl14 1 is grounded via the relay coil rl14 via the resistor rl having a large resistance enough to gradually discharge the cable having the cable end 43 . the other end of the relay switch rl14 1 is also connected to one end of the relay switch rl13 . the other end of the relay switch rl13 is connected to the other end of the relay switch rl14 1 . the relay switches rl11 - rl13 are driven when the first , second and third driving currents i 1 , i 2 and i 3 ( i 1 & lt ; i 2 & lt ; i 3 ) flow in the relay coils rl11 , rl12 and rl13 , respectively ( strictly speaking , when currents greater than the first , second and third currents i 1 , i 2 and i 3 flow in the delay coils rl11 , rl12 and rl13 ). the relay switches rl14 1 and rl14 2 are driven when a current equal to or greater than a predetermined current i flows in the relay coil rl14 . a description will now be given , with reference to fig8 of a case where the two - end power feed is performed between the stations a and b and the single - end power feed is performed between the station and the ground . the first step of a procedure for realizing the above power feed is to set the cable end 43 to the open state at the station c . the second step is to increase the current flowing in the cable between the stations a and b to the first driving current i 1 and to thereby close the associated relay switch rl11 . the third step is to increase the current flowing in the cable between the stations a and b to the second driving current i 2 and to close the associated relay switch rl12 . hence , a charge stored in the cable provided between the station and the branching unit equipped with the power feed path switching circuit 40a is allowed to gradually flow to the ground via the resistor rl . the fourth step is to increase the current flowing in the cable between the stations a and b to the third driving current i 3 and to close the associated switch rl13 . hence , the cable end 43 is grounded and the power feed path between the station c and the sea ground se is established without the hot - switching phenomenon . the fifth step is to gradually start power feed from the station c . when the current flowing in the relay coil rl14 becomes equal to or greater than the predetermined driving current i , the associated relay switch rl14 1 is closed and held by itself . further , the relay switch rl114 2 is opened and the cable end 43 is disconnected from circuits in the cable between the stations a and b . even if the relay switch rl11 returns to the original state ( closed state ) due to a fault of power feed between the stations a and b , the power feed between the station c and the ground is not affected at all . further , it is not necessary to set the branching unit equipped with the power feed path switching circuit to the ground potential at the time of switching the power feed paths . furthermore , the switching of the power feed paths can be simultaneously performed at a plurality of branching units . the driving currents i 1 , i 2 and i 3 , which are different from each other , can be commonly used in the branching units . the predetermined driving current i flowing in the relay rl14 can have a quantity equal to one of the driving currents i 1 , i 2 and i 3 or an arbitrary quantity different from the quantities of these driving currents . fig9 illustrates a power feed system to which the first embodiment of the present invention is applied . the power feed system shown in fig9 includes n cascaded branching units 40 1 - 40 n which are the same as each other . stations b - m are connected to the branching unit 40 1 - 40 n , respectively . the station a is connected to the branching unit 40 1 , and a station n is connected to the branching unit 40 n . the power feed path switching units provided in the branching units 40 1 - 40 n are the same as each other , and the same structural elements as those in the units are given the same reference numbers . symbols including arrows having upward heads denote power sources provided in the stations a - n . the switching can be performed by only increasing the current flowing in the cable between the stations a and n in the order of i 1 & lt ; i 2 & lt ; i 3 , so that the stations b - m can be simultaneously switched to the single - end power feed in which the stations b - m are respectively grounded at the branching units 40 1 - 40 n without the hot - switching phenomenon . even if a fault , such as a grounding fault , has occurred in a cable connected to any of the stations a - n , the stations b - m can be simultaneously switched to the single - end power feed without the hot - switching phenomenon because there is no need to sequentially set the branching units to the ground potential . fig1 illustrates a variation 60a of the second embodiment of the present invention shown in fig7 . in fig1 , parts that are the same as those shown in fig7 are given the same reference numbers . the variation 60a shown in fig1 , has the relay switches rl14 1 and rl14 2 which are the same as those shown in fig8 . a description will now be given , with reference to fig1 , of a case where the two - end power feed is performed between the stations a and b and the single - end power feed is performed between the station c and the sea ground se . the first step of a procedure for realizing the above power feed is to set the cable end 43 to the open state at the station c . the second step is to increase the current flowing in the cable between the stations a and b to the predetermined driving current and to thereby open the associated relay switch rl11 . hence , the cable end 43 is disconnected from circuits provided between the cable ends 41 and 42 . the power supply circuit 15 branches the current flowing in the cable between the stations a and b . when a branched current i exceeds a threshold level , the power supply circuit 15 generates a voltage + v with respect to the ground e of the branching unit equipped with the present power feed path switching circuit . the switch circuit 12 is driven by the voltage + v generated by the power supply circuit 15 , and charging of a capacitor c1 via one of two resistors r is started . the two resistors r are connected in series , and a connection nodes of these resistors is connected to the base of a transistor q1 . when the first period t 1 has elapsed after the charging is started , the transistor q1 is turned on and causes a driving current to flow in a relay coil rl12 connected between the collector of the transistor q1 and the power supply circuit 15 . hence , the relay switch rl12 associated with the relay coil rl12 is closed . then , a charge stored in the cable between the station c and the branching unit 60a is allowed to flow to the ground via the resistor rl , so that the cable is gradually discharged . the voltage + v is also applied to the second switch circuit 13 , and charging of a capacitor c2 via one of two resistors r is started . these two resistors r are connected in series and a connection node of these resistors is connected to the base of a transistor q2 . the capacitor c2 has a capacitance greater than that of the capacitor c1 . when the second period t 2 longer than the first period t 1 has elapsed after the charging is started , the transistor q2 is turned on and causes a driving current to flow in a relay coil rl13 connected between the collector of the transistor q2 and the power supply circuit 15 . hence , the relay switch rl13 associated with the relay coil rl13 is closed . as a result , the cable end 43 is grounded after the discharging of the cable extending from the station c is substantially completed . thereafter , the power supply from the station c is gradually started . when the current flowing in the relay coil rl14 becomes equal to or greater than the predetermined driving current i , the associated relay switch rl14 1 is closed and held by itself . further , the relay switch rl14 2 is opened and the cable end 43 is disconnected from circuits in the cable between the stations a and b . fig1 is a block diagram of a power feed system to which the variation of the second embodiment shown in fig1 is applied . the power feed system shown in fig1 includes n cascaded branching units 60 1 - 60 n which are the same as each other . stations b - m are connected to the branching unit 60 1 - 60 n , respectively . the station a is connected to the branching unit 60 1 , and a station n is connected to the branching unit 60 n . the power feed path switching units provided in the branching units 60 1 - 60 n are the same as each other , and the same structural elements as those in the units are given the same reference numbers . symbols including arrows having upward heads denote power sources provided in the stations a - n . the switching can be performed by only causing the predetermined driving current to flowing in the cable between the stations a and n . hence the stations b - m can be simultaneously switched to the single - end power feed in which the stations b - m are respectively grounded at the branching units 60 1 - 60 n without the hot - switching phenomenon . even if a fault , such as a grounding fault , has occurred in a cable connected to any of the stations a - n , the stations b - m can be simultaneously switched to the single - end power feed without the hot - switching phenomenon because there is no need to sequentially set the branching units to the ground potential . fig1 is a block diagram of a power feed path switching circuit 70 according to a third embodiment of the present invention . in fig1 , parts that are the same as parts shown in the previously described figures are given the same reference numbers . the power feed path switching circuit 70 includes circuit parts respectively provided for the cable ends 41 , 42 and 43 , each of the circuit parts corresponding to , for example , the circuit shown in fig1 . the station c is set to the open state , and a predetermined driving current ia is made to flow from the station b to the station a . a voltage having polarities shown in fig1 develops across a zener diode zda . the voltage developing across the zener diode zda flows a predetermined driving current in a relay coil rl11a , and an associated relay switch rl11a is opened . further , the above voltage drives a power supply circuit ( pu ) 15a . when the first period t 1 has elapsed after the power supply circuit 15a is driven , a switch circuit 12a is closed and the cable extending from the station c is gradually discharged via a resistor rla . when the second period t 2 longer than the first period t 1 has elapsed after the power supply circuit 15a is driven , a second switch circuit 13a is driven , and the cable end 43 is grounded via a relay coil rl14a . thereafter , power feed from the station is started . when a predetermined driving current flows in the relay coil rl14a , an associated relay switch rl14 1a is closed and an associated relay switch rl14 2a is opened . thereby , the current from the station c holds the single - end power feed by itself . the station a is opened to the open state , and a predetermined driving current ib is made to flow from the station c to the station a . a voltage having polarities shown in fig1 develops across a zener diode zdb . the voltage developing across the zener diode zdb flows a predetermined driving current in a relay coil rl11b , and an associated relay switch rl11b is opened . further , the above voltage drives a power supply circuit ( pu ) 15b . when the first period t 1 has elapsed after the power supply circuit 15b is driven , a switch circuit 12b is closed and the cable extending from the station a is gradually discharged via a resistor rlb . when the second period t 2 longer than the first period t 1 has elapsed after the power supply circuit 15b is driven , a second switch circuit 13b is driven , and the cable end 41 is grounded via a relay coil 14b . thereafter , power feed from the station is started . when a predetermined driving current flows in the relay coil rl14b , an associated relay switch rl14 1b is closed and an associated relay switch rl14 2b is opened . thereby , the current from the station a holds the single - end power feed by itself . the station b is opened to the open state , and a predetermined driving current ic is made to flow from the station a to the station c . a voltage having polarities shown in fig1 develops across a zener diode zdc . the voltage developing across the zener diode zdc flows a predetermined driving current in a relay coil rl11c , and an associated relay switch rl11c is opened . further , the above voltage drives a power supply circuit ( pu ) 15c . when the first period t 1 has elapsed after the power supply circuit 15c is driven , a switch circuit 12c is closed and the cable extending from the station b is gradually discharged via a resistor rlc . when the second period t 2 longer than the first period t 1 has elapsed after the power supply circuit 15c is driven , a second switch circuit 13c is driven , and the cable end 42 is grounded via a relay coil 14c . thereafter , power feed from the station is started . when a predetermined driving current flows in the relay coil rl14c , an associated relay switch rl14 1c is closed and an associated relay switch rl14 2c is opened . thereby , the current from the station b holds the single - end power feed by itself . with the structure shown in fig1 , it becomes possible to disconnect one of the stations a , b and c connected to the cable in which a fault has occurred and to maintain the two - end power feed between the other stations . it is also possible to use the circuits shown in fig6 through 8 instead of the circuits shown in fig1 . it is possible to replace the relay coils rl and relay switches rl by semiconductor switches or the like . it is also possible to define the first and second times t 1 and t 2 by means of circuits other than the circuits shown in fig1 . the present invention is not limited to the specifically disclosed embodiments , and variations and modifications may be made without departing from the scope of the present invention .