CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/JP2018/029275 filed Aug. 3, 2018, claiming priority based on Japanese Patent Application No. 2017-152656, filed Aug. 7, 2017.

TECHNICAL FIELD

The present invention relates to a technique of switching the presence or absence of grounding a circuit to be grounded with respect to an earth.

BACKGROUND ART

An undersea cable system includes an undersea cable (hereinafter, referred to as a “cable”), an undersea relay unit, an undersea branching unit (hereinafter, also referred to as a “BU” (branching unit)), a terminal station device (hereinafter, referred to as a “terminal station”), a power supply device, and the like. The undersea relay unit amplifies an attenuated signal. The BU branches a signal toward a plurality of points. The base station performs various data signal transmission and reception. The power supply device supplies electric power to undersea equipment. The power supply device may supply electric power at a high voltage exceeding several kV. Further, the BU may have a function of switching a power supply path to the undersea equipment in order to deal with a ground fault of a cable, corrective maintenance, and the like.

For example, a voltage at a positive electrode is supplied from a terminal station A, and a voltage at a negative electrode is supplied from a terminal station B to the BU. Further, a voltage at a negative electrode is supplied from a terminal station C to the BU. When a power supply path is switched, the BU may ground a part of a circuit of the BU to a sea earth (hereinafter, also referred to as a sea earth (“SE”)), or unground the part of the circuit of the BU from the SE.

One example of a branching unit having a function of switching a power supply path is disclosed in PTL 1. In a power supply path switching circuit in PTL 1, a power supply path is switched by supplied electric current. The power supply path switching circuit in PTL 1 includes a first relay, a second relay, a third relay, and a resistor R1. The first relay, the second relay, and the third relay include, respectively, relay coils RL11, RL12, and RL13, and relay switches (relay contacts) r111, r112, and r113. The relay coils RL11, RL12, and RL13are connected in series, in this order, between a cable terminal41and a cable terminal42. When first predetermined electric current flows through the relay coil RL11, the relay switch r111disconnects circuits among a cable terminal43, the cable terminal41, and the cable terminal42by opening (breaking a contact). When second predetermined electric current exceeding the first predetermined electric current flows through the relay coil RL12, the relay switch r112connects the cable terminal43to an earth via the resistor R1by closing (making a contact). When third predetermined electric current exceeding the second predetermined electric current flows through the relay coil RL13, the relay switch r113short-circuits the resistor R1by closing (making a contact).

As a result of the above-described configuration, the power supply path switching circuit in PTL 1 operates as follows.

(1) First, by flowing the first predetermined electric current through the relay coil RL11, the power supply path switching circuit opens the relay switch r111. Consequently, the power supply path switching circuit disconnects circuits among the cable terminal43, the cable terminal41, and the cable terminal42.

(2) Next, by flowing the second predetermined electric current exceeding the first predetermined electric current through the relay coil RL12, the power supply path switching circuit closes the relay switch r112. Consequently, the power supply path switching circuit gradually discharges accumulated electric charge in a cable connected to the cable terminal43via the resistor R1by connecting the cable terminal43to the earth via the resistor R1.

(3) Next, by flowing the third predetermined electric current exceeding the second predetermined electric current through the relay coil RL13, the power supply path switching circuit closes the relay switch r113. Consequently, the power supply path switching circuit completely discharges the electric charge charged in the cable at the cable terminal43by directly connecting the cable terminal43to the earth.

In other words, the power supply path switching circuit in PTL 1 suppresses, when the cable terminal43is connected to the earth, large current flowing through the relay switch r112by the resistor R1.

The power supply path switching circuit in PTL 1 operates as follows, when current flowing through a circuit between the cable terminal41and the cable terminal42is lowered after the above operations (1) to (3).

(4) First, when electric current flowing through the relay coil RL13becomes lower than the third predetermined electric current, the power supply path switching circuit opens the relay switch r113.

(5) Next, when electric current flowing through the relay coil RL12becomes lower than the second predetermined electric current, the power supply path switching circuit opens the relay switch r112.

In the above operation (5), arc discharge occurs in the relay switch r112. As a resistance of the resistor R1increases, arc discharge is likely to occur in the relay switch r112. Therefore, there is a problem that the power supply path switching circuit in PTL 1 cannot compatibly achieve suppressing large current flowing through the relay switch r112when being grounded, and suppressing arc discharge in the relay switch r112when being ungrounded.

PTL 2 describes one example of a technique in which suppressing large current flowing through a relay when being grounded, and suppressing arc discharge in the relay when being ungrounded are compatibly achieved. A grounding circuit in PTL 2 includes a resistor connected between a first relay contact and an earth, and a second relay contact connected in parallel to the resistor. Further, when a device to be grounded (circuit to be grounded) is grounded to the earth, electric current flowing through the first relay contact is suppressed by the resistor. Furthermore, when the circuit to be grounded is ungrounded, the resistor is short-circuited by the second relay contact. Since a voltage at the first relay contact becomes 0V, there is no likelihood that a potential difference occurs between first relay contacts instantaneously when the first relay contact is opened, and arc discharge does not occur at the first relay contact. Therefore, the grounding circuit in PTL 2 suppresses electric current flowing through a relay contact, when the circuit to be grounded is grounded by connection to the relay contact, and also suppresses arc discharge between relay contacts, when the circuit to be grounded is ungrounded by disconnection from the relay contact.

CITATION LIST

Patent Literature

[PTL 1] Japanese Unexamined Patent Application Publication No. H5-129991[PTL 2] Japanese Patent Application No. 2016-103851

SUMMARY OF INVENTION

Technical Problem

However, in the grounding circuit in PTL 2, in case that an operator inadvertently short-circuits the resistor by the second relay contact when the circuit to be grounded is grounded to the earth, electric current flowing through the first relay contact is not suppressed by the resistor. Further, in the grounding circuit in PTL 2, in case that an operator inadvertently fails to short-circuit the resistor by the second relay contact when the circuit to be grounded is ungrounded, arc discharge occurs in the first relay contact. In other words, there is a problem that the grounding circuit in PTL 2 does not have a fail-safe function regarding an operation of grounding a circuit to be grounded and an operation of ungrounding the circuit to be grounded.

In view of the above-described issue, a main object of the present invention is to provide a fail-safe function regarding an operation of grounding a circuit to be grounded and an operation of ungrounding the circuit to be grounded.

Solution to Problem

In one aspect of the invention, a grounding circuit includes: a first contact in a first relay connected to a circuit to be grounded; a resistor connected between the first contact and an earth, and configured to suppress, when the first contact is closed, electric current flowing through the first contact; a second contact in a second relay connected in parallel to the resistor; and interlocking control means for inhibiting, when the second contact is in an opened state, opening of the first contact in a closed state, and inhibiting, when the second contact is in a closed state, closing of the first contact in an opened state.

In one aspect of the invention, an electrical apparatus includes: a circuit to be grounded; and a grounding circuit including a first contact in a first relay, connected to the circuit to be grounded, a resistor connected between the first contact and an earth, and configured to suppress, when the first contact is closed, electric current flowing through the first contact, a second contact in a second relay, connected in parallel to the resistor, and interlocking control means for inhibiting, when the second contact is in an opened state, opening of the first contact in a closed state, and inhibiting, when the second contact is in a closed state, closing of the first contact in an opened state.

In one aspect of the invention, a grounding control method of a grounding circuit provided with a first contact in a first relay, connected to a circuit to be grounded, a resistor connected between the first contact and an earth, and configured to suppress, when the first contact is closed, electric current flowing through the first contact, and a second contact in a second relay, connected in parallel to the resistor, includes: inhibiting, when the second contact is in an opened state, opening of the first contact in a closed state, and inhibiting, when the second contact is in a closed state, closing of the first contact in an opened state.

In one aspect of the invention, a grounding control program causes a computer provided with a grounding circuit including a first contact in a first relay, connected to a circuit to be grounded, a resistor connected between the first contact and an earth, and configured to suppress, when the first contact is closed, electric current flowing through the first contact, and a second contact in a second relay, connected in parallel to the resistor, to execute processing of inhibiting, when the second contact is in an opened state, opening of the first contact in a closed state, and processing of inhibiting, when the second contact is in a closed state, closing of the first contact in an opened state.

Advantageous Effects of Invention

The present invention provides an advantageous effect being capable of providing a fail-safe function regarding an operation of grounding a circuit to be grounded and an operation of ungrounding the circuit to be grounded.

