Branching unit for an optical telecommunication link

1. A branching unit for an optical telecommunication link, the unit comprising a first, a second and a third terminal for termination of conductors of respective branch cables in use, a fourth terminal for connection to a sea earth in use, and a circuit comprising a plurality of relays comprising at least a first, a second and a third relay associated to respectively the first, second and third terminal, the circuit being structured for connecting together two terminals out of the group consisting in the first, second and third terminal and for connecting to the fourth terminal the remaining terminal of the group consisting in the first, second and third terminal, depending on a sequence of power feeding at the first, second and third terminals, in use, wherein the circuit comprises a sea earth circuit terminated at one end to the fourth terminal and a bidirectional triode thyristor placed along the sea earth circuit, and wherein the branching unit comprises a driving circuit for driving the bidirectional triode thyristor, the driving circuit being configured for triggering the bidirectional triode thyristor in a conduction state after complete switch-over of one relay of the group consisting in the first, second and third relay.

2. The branching unit according to claim 1 , wherein the driving circuit is configured for driving the bidirectional triode thyristor in function of a current across the sea earth circuit.

3. The branching unit according to claim 1 , wherein a first main terminal of the bidirectional triode thyristor is on the side of the fourth terminal and a second main terminal on the opposite side and wherein the driving circuit is directly connected to the sea earth circuit on the side of the second main terminal, so as to receive a current flowing across the sea earth circuit.

4. The branching unit according to claim 1 , wherein the sea earth circuit comprises a first, a second and a third earth line terminated at two respective opposite ends respectively at the fourth terminal and at respectively a first, a second and a third switch of respectively the first, second and third relay, and wherein the bidirectional triode thyristor is placed along a common section of the sea earth circuit common to the first, second and third earth line, the common section being terminated at the fourth terminal.

5. The branching unit according to claim 1 , wherein the circuit comprises a first, a second and a third termination line respectively terminated at one end at the first, second and third terminal, wherein said first, second and third relay have respectively a first, a second and a third switch and respectively a first, a second and a third actuator, wherein the first, second and third switch have a respective first contact belonging respectively to the first, second and third termination line, a respective second contact, a respective third contact belonging to the sea earth circuit and a respective mobile contact, the respective actuator of each relay being configured for actuating the mobile contact so as to connect the first contact with either the second contact or the third contact, and wherein the circuit comprises a first line terminated at two opposite ends to the second contacts of respectively the first and second switch, a second line terminated at two opposite ends to the second contacts of respectively the first and the third switch and a third line terminated at two opposite ends to the second contacts of respectively the second and third switch, and wherein the first, second and third relay are configured for connecting respectively the first, second and third terminal either to the sea earth circuit or to the other two terminals in function of the current flowing across respectively the third, second and first line.

6. The branching unit according to claim 4 , wherein the first, second and third switch are configured so that when the respective actuator is not powered, the respective mobile contact connects the first contact with the second contact.

7. The branching unit according to claim 1 , wherein said plurality of relays comprises a fourth, a fifth and a sixth relay having respectively a fourth, a fifth and a sixth switch and respectively a fourth, a fifth and a sixth actuator, wherein the fourth, fifth and sixth switch are placed along a respective common section of respectively the first and the second line, the first and the third line and the second and the third line so as to either open or close the respective common section, depending on a current along the sea earth circuit, the fourth, fifth and sixth switch being configured so that when the respective actuator is not powered, the respective mobile contact close the respective common section.

8. The branching unit according to claim 7 , wherein the respective actuator of the fourth, fifth and sixth relay is placed along the first, second and third earth line respectively and is not placed along respectively the other two earth lines.

9. The branching unit according to claim 7 , wherein said plurality of relays comprises a seventh, an eighth and a ninth relay having respectively a seventh, an eighth and a ninth switch and respectively a seventh, an eighth and a ninth actuator, the seventh, eighth and ninth actuator being coincident with respectively the fourth, fifth and sixth actuator and wherein the seventh, eighth and ninth switch have a respective first contact in connection with respectively the first, second and third termination line, a respective second contact in connection with respectively the first, second and third earth line, and a respective mobile contact, the respective actuator of each relay being configured for actuating the mobile contact so as to either connect the first and second contact or to isolate the first and second contact, depending on a current along respectively a first, a second and a third sea earth line.

10. The branching unit according to claim 9 , wherein the seventh, eighth and ninth switch are configured so that when the respective actuator is not powered, the respective mobile contact isolates the first contact with the second contact.

11. The branching unit according to claim 1 , wherein the circuit comprises a first, a second and a third inductor placed along respectively the first, second and third termination line so as to limit a time rate of change of the current across them and/or a time rate of change of the voltage across them.

12. A branching unit for an optical telecommunication link, the unit comprising a first, a second and a third terminal for termination of conductors of respective branch cables in use, a fourth terminal for connection to a sea earth in use, and a circuit comprising a plurality of relays comprising at least a first, a second and a third relay associated to respectively the first, second and third terminal, the circuit being structured for connecting together two terminals out of the group consisting in the first, second and third terminal and for connecting the remaining terminal of the group consisting in the first, second and third terminal to the fourth terminal, depending on a sequence of power feeding at the first, second and third terminals, in use, wherein the circuit comprises a sea earth circuit terminated at one end to the fourth terminal, wherein the circuit comprises a first, a second and a third termination line respectively terminated at one end at the first, second and third terminal, wherein said first, second and third relay have respectively a first, a second and a third switch and respectively a first, a second and a third actuator, wherein the first, second and third switch have a respective first contact belonging respectively to the first, second and third termination line, a respective second contact, a respective third contact belonging to the sea earth circuit and a respective mobile contact, the respective actuator of each relay being configured for actuating the mobile contact so as to connect the first contact with either the second contact or the third contact in function of a current flowing across respectively the third, second and first line, wherein the circuit comprises a first line terminated at two opposite ends to the second contacts of respectively the first and second switch, a second line terminated at two opposite ends to the second contacts of respectively the first and the third switch and a third line terminated at two opposite ends to the second contacts of respectively the second and third switch, wherein said plurality of relays comprises a fourth, a fifth and a sixth relay having respectively a fourth, a fifth and a sixth switch and respectively a fourth, a fifth and a sixth actuator, wherein the fourth, fifth and sixth switch are placed along a respective common section of respectively the first and the second line, the first and the third line and the second and the third line so as to either open or close the respective common section, depending on a current along the sea earth circuit, the fourth, fifth and sixth switch being configured so that when the respective actuator is not powered, the respective mobile contact close the respective common section, wherein the sea earth circuit comprises a first, a second and a third earth line terminated at two respective opposite ends respectively at the fourth terminal and at respectively the first, second and third switch and wherein said plurality of relays comprises a seventh, an eighth and a ninth relay having respectively a seventh, an eighth and a ninth switch and respectively a seventh, an eighth and a ninth actuator, the seventh, eighth and ninth actuator being coincident with respectively the fourth, fifth and sixth actuator and wherein the seventh, eighth and ninth switch have a respective first contact in connection with respectively the first, second and third termination line, a respective second contact in connection with respectively the first, second and third earth line, and a respective mobile contact, the respective actuator of each relay being configured for actuating the mobile contact so as to either connect the first and second contact or to isolate the first and second contact, depending on a current along respectively a first, a second and a third sea earth line, the seventh, eighth and ninth switch being configured so that when the respective actuator is not powered, the respective mobile contact isolates the first contact with the second contact.

13. The branching unit according to claim 12 , wherein the respective actuator of the fourth, fifth and sixth relay is placed along the first, second and third earth line respectively and is not placed along respectively the other two earth lines.

14. An optical telecommunication link comprising at least one submerged branching unit according to claim 1 , a first, a second and a third land station comprising respectively a first, a second and a third power feeding equipment having respective first terminal, a first, a second and a third cable branch having a respective conductor continuously running along them and having a respective end terminated respectively at the first, second and third terminal of the branching unit and a respective opposite end terminated respectively at the first terminal of the first, second and third power feeding equipment.

15. Method for configuring the branching unit according to claim 1 , the method comprising: sending a current at two terminals out of the group consisting in the first, second and third terminal until the circuit connects together said two terminals therebetween and connects the remaining terminal of the group consisting in the first, second and third terminal to the fourth terminal, by switching of a switch of a relay associated to the remaining terminal.