Submarine branching unit

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a submarine branching unit (herein after simply called branching unit) which branches a transmission line in two different directions in a communication system using the submarine cables, or more particularly to an improvement of the structure of the branching unit.

[0003] In order to further accelerate the development of information communication including the Internet via such submarine cables, it is in demand to shorten the time required to deliver communication equipment to customers after a contract has been made. In fact, submarine equipment such as a repeater has recently come to be manufactured within about half the period required several years ago To install or lay a repeater and a submarine cable underseas, a dedicated ship is used and therefore, development of the submarine equipment which is easy to install and lay in the sea is in demand.

[0004] There is a similar demand for a branching unit. Especially, a small-sized and light-weight branching unit which is easy to lay in and draw out of the sea and also easy to transport and store. Also, the demand arises since a dedicated strong cable is used with the branching unit and since a sheave having large a diameter as 3 meters is used with the cable.

[0005] 2. Description of the Related Art

[0006]FIG. 1 shows an external appearance of a conventional branching unit with cable terminations. FIG. 2 is a sectional side view of the main body of the branching unit as shown in FIG. 1.

[0007] The branching unit <b>1</b> is composed of an inner unit <b>3</b>, cable containers <b>4</b>, a submarine cable <b>5</b>, cable terminations <b>6</b> and gimbal joints <b>7</b>. The inner unit <b>3</b> is accommodated in a pressure-tight cylinder <b>2</b>. The cable containers <b>4</b> located at both ends of the pressure-tight cylinder <b>2</b>, contain pigtail cables including an optical fiber for signal transmission and a feeder line for power supply. The cable terminations <b>6</b> connect the submarine cable <b>5</b> to the cables within the branching unit. The gimbal joint <b>7</b> bendably couples the cable containers <b>4</b> to the cable terminations <b>6</b>.

[0008] As shown in the FIG. 3—side view, FIG. 4—top plan view and FIG. 5—bottom plan view, the inner unit <b>3</b> has vacuum relays <b>8</b> and circuit units <b>9</b>-<b>1</b> to <b>9</b>-<b>4</b> arranged respectively on top and bottom sides of the circuit board. The terminals (not shown in the figure) of the vacuum relays <b>8</b> project to the side of the vacuum relays <b>8</b>. Also, feeder lines (not shown in the figure) connecting to the vacuum relay terminals and the respective circuits project in parallel with the vacuum relay terminals. Therefore, a space is required that is large enough to withstand 15 kilovolts across the terminals of adjacent vacuum relays <b>8</b> and accordingly, the vacuum relays <b>8</b> are mounted separately on both (upper and lower) sides of the circuit board. Four circuit units <b>9</b>-<b>1</b> to <b>9</b>-<b>4</b> are each mounted in a box case such that they are superposed on each other. Thus, the inner unit <b>3</b> is finished to an outside diameter (D) of 300 millimeters and a length (L) of 550 millimeters.

[0009] With the main body of the thus-constructed branching unit as shown in FIG. 2, the pressure-tight cylinder <b>2</b> has an outside diameter D<b>1</b> of about 400 millimeters. The cable container <b>4</b> has an outside diameter D<b>2</b> of about 480 millimeters. The main body has a length L<b>1</b> of about 1600 millimeters and a weight of about 8 kilonewtons. The total length L<b>2</b> of the branching unit as measured between the cable terminations <b>6</b> on both sides (see FIG. 1) is about 4500 millimeters.

[0010] With an ever increasing demand for the communication via the submarine cables, more and more communication capacity is required and accordingly, larger electric power and broader bandwidth are required. As a result, power consumption increases to about 150 watts, i.e., three times as large as the current 50 watts and it is required to increase the withstand voltage characteristics from the current 15 kilovolts to 18 kilovolts.

[0011] Accordingly, it is necessary to design a branching unit having improved withstand voltage and heat radiation characteristics, while minimizing the size of the unit. The conventional branching unit had following structural problems when laying/drawing out the unit in/from the sea.

[0012] First, since the branching unit is large in size, the unit does not wind well around a sheave whose diameter is about 3 meters and the laying/drawing work had to use a crane and a special rope and therefore, a such-equipped ship or work barge was necessary for that work.

[0013] The branching unit, which is about twice as heavy as a repeater, required a submarine cable which can tolerate a high tension applied to the unit in the deep sea. Therefore, a special cable had to be manufactured, taking into consideration the maintenance work after laid in the sea.

SUMMARY OF THE INVENTION

[0014] An object of the present invention is to provide a branching unit which has excellent withstand voltage and heat radiation characteristics and which is the same in size as a submarine repeater.

[0015] Another object of the present invention is to provide a branching unit which has the weight, the distance between gimbal joints on both sides and the outside diameter of the pressure-tight cylinder, reduced to those of a submarine repeater.

[0016] Throughout the above-mentioned drawings, identical reference numerals are used to designate the same or similar component parts.

[0017] To achieve the above and other objects, the present invention provides a branching unit including in a pressure-tight cylinder, a circuit board, at least one circuit unit mounted on the circuit board and a plurality of vacuum relays mounted on the circuit board and each having respective terminals and wirings. The circuit unit is mounted on one side of the circuit board. The vacuum relays are mounted on the other side of the circuit board. The vacuum relays are each accommodated in the insulator covers along with the respective terminals and wirings. Further, the present invention provides a branching unit including a pair of cables for carrying a signal and power, which cables are each spiraled and are engaged with each other in an interleaved and overlapped fashion on the outside of a cover of the pressure-tight cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 shows an external appearance of a conventional branching unit.

[0019]FIG. 2 is a sectional side view of the main body of the branching unit.

[0020]FIG. 3 is a side view of the inner unit.

[0021]FIG. 4 is a top plan view of the inner unit as shown in FIG. 3.

[0022]FIG. 5 is a bottom plan view of the inner unit as shown in FIG. 3.

[0023]FIG. 6 shows an external appearance of the branching unit of the present invention.

[0024]FIG. 7 is a sectional side view of the main body shown in FIG. 6.

[0025]FIG. 8 is a side view of the inner unit.

[0026]FIG. 9 is a top view of the inner unit.

[0027]FIG. 10 shows the inner unit with the pressure-tight cylinder covers and insulator covers removed from that shown in FIG. 8.

[0028] FIGS. <b>1</b>lA and <b>11</b>D show an insulator cover.

[0029]FIGS. 12A and 12B show the inside of the branch-side cover of the pressure-tight cylinder.

[0030]FIG. 13 is a perspective view of the pressure-tight cylinder shown in FIG. 12.

[0031]FIG. 14 is an explanatory view of the pigtail cable.

[0032] FIGS. <b>15</b>A-<b>15</b>C show a process of assembling the outside pigtail cable (part <b>1</b>).

[0033]FIGS. 16A and 16B show a process of assembling the outside pigtail cable (part <b>2</b>).

[0034]FIGS. 17A and 17B show a process of assembling the outside pigtail cable (part <b>3</b>).

[0035]FIG. 18 is a perspective view of the pigtail cable shown in FIG. 17.

[0036]FIG. 19 is a partially sectional side view of the branching unit.

[0037]FIG. 20 is a partially sectional side view of the branching unit with a submarine cable connected.

[0038]FIGS. 21A and 21B are explanatory views of the branching unit wound on the sheave.

[0039]FIG. 22 is a table comparing a conventional branching unit with that of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] Preferred embodiment of the present invention is explained, referring to the drawings. Throughout the above-mentioned drawings, identical reference numerals are used to designate the same or similar component parts. Moreover, the expressions “upper”, “lower”, “right” and “left” are not directed to the real unit, but to the figure for easy understanding.

[0041]FIG. 6 shows an external appearance of the branching unit of the present invention. FIG. 7 is a sectional side view of the main body shown in FIG. 6. The branching unit <b>11</b> is composed of an inner unit <b>13</b>, cable containers <b>14</b>, submarine cables <b>15</b>, cable terminations <b>16</b> and gimbal joints <b>17</b>. The inner unit <b>13</b> is contained in a pressure-tight cylinder <b>12</b>. The cable containers <b>14</b> located at both ends of the pressure-tight cylinder <b>12</b>, contain a pigtail cable consisting of an optical fiber for signal transmission and a feeder line for power supply. The cable terminations <b>16</b> connect the submarine cables <b>15</b> to the cables within the branching unit. The gimbal joint <b>17</b> bendably couples the cable containers <b>14</b> to the cable terminations <b>16</b>.

[0042] As shown in FIG. 8—side view and FIG. 9—top view, the inner unit <b>13</b> has six vacuum relays <b>19</b> and five junction terminals <b>21</b> mounted on top of the circuit board; however, they are entirely covered with and obscured by insulator covers.

[0043] Circuit units <b>24</b> are mounted on the bottom side of the circuit board. On the outside of end plates <b>25</b>, <b>26</b>, an extra length of the feeder lines <b>27</b> and optical fiber cables <b>28</b> are held wound. FIG. 10, which shows the inner unit <b>13</b> with the end plates <b>25</b>, <b>26</b> and the insulator covers <b>22</b>, <b>23</b> removed from that shown in FIG. 8, illustrates the connection of the feeder lines <b>27</b> to vacuum relays <b>19</b> and relay terminals <b>21</b>.

[0044] Terminals <b>29</b> stand erect around the vacuum relays <b>19</b> at a constant interval. The feeder lines <b>27</b> extend upward along the vacuum relays <b>19</b> and are connected with the top of the terminals <b>29</b> and are connected with the junction terminals <b>21</b> in the same way. The feeder lines <b>27</b> extend upward along the vacuum relays <b>19</b> and are connected with the top of the terminals <b>29</b> and the junction terminals.

[0045] An insulator cover <b>22</b> for covering the vacuum relay <b>19</b> are shown in FIGS. <b>11</b>A-<b>11</b>D. FIGS. <b>11</b>A-<b>11</b>D show a sectional side view, top view, bottom view and sectional side view taken on the line A-A in FIG. 11C, respectively of the insulator cover <b>22</b>. The insulator cover <b>22</b> is made of e.g., polyethylene resin mold of excellent insulation property and shaped like a cylinder with an end closed. On the inside wall of the insulator cover <b>22</b>, there are cut six vertical guide grooves <b>31</b> at an equal interval and notches <b>32</b> bored at each foot of the guide grooves <b>31</b>. Outside and around the insulator cover <b>22</b>, there are carved three recesses <b>33</b> at an equal interval and fitting halls <b>34</b> bored in the recesses <b>33</b>.

[0046] Covering the vacuum relays <b>19</b> in the state as shown in FIG. 10 with the respective insulator covers <b>22</b> causes the terminals <b>29</b> and feeder lines <b>27</b> to be accommodated in the guide grooves <b>31</b>. Then, inserting and screwing screws through the fitting holes <b>34</b> into the female threads cut on the circuit board <b>18</b> positions the feeder lines <b>27</b> in the notches <b>32</b> carved at the foot of the guide grooves <b>31</b> and connected as shown in FIGS. 8 and 9. Since the junction terminals <b>21</b> are also covered in the same way and vacuum relays <b>19</b> are kept insulated from each other, the distance between the adjacent vacuum relays <b>19</b> and also that between the junction terminals <b>21</b> of the adjacent vacuum relays <b>19</b> can be minimized. Even when more vacuum relays <b>19</b> are used as in the embodiment in order to increase the communication capacity, the present invention, along with the technique to fabricate the circuit unit <b>24</b> in large-scale integrated circuits, can reduce the size of the branching unit compared with the conventional one.

[0047]FIGS. 12A and 12B show schematic inside and side views respectively, of the cover <b>36</b> on the branch side of the pressure-tight cylinder <b>12</b>, shown in FIG. 7. FIG. 13 is a perspective view thereof. Two cable feed-through assemblies <b>37</b> of the same construction are provided on the pressure-tight cylinder cover <b>36</b>. As shown in the figures, an optical fiber cable <b>28</b> leading straight to the center of the cable feed-through assemblies <b>37</b> and a feeder line <b>27</b> leading sideways thereto are combined into a pigtail cable having a coaxial construction.

[0048] The cable feed-through assemblies <b>37</b> are constructed such that they are watertight even with the pigtail cable leading through they the pressure-tight cylinder cover <b>36</b> and endure against a high hydraulic pressure of the sea. As shown in the figures, the optical fiber cables <b>28</b> and feeder lines <b>27</b> are routed loosely inside of the pressure-tight cylinder cover <b>36</b> and fastened to the metal fittings by fastening bands <b>38</b>. The above-mentioned optical fiber cables <b>28</b> and feeder lines <b>27</b> are connected to the corresponding internal ones explained by way of FIG. 8.

[0049] Referring to the FIG. 14, the pigtail cable <b>44</b> on the outside of the pressure-tight cylinder cover <b>36</b> is comprised of plural optical fiber lines <b>42</b>, a water pressure-resistant copper-tube feeder pipe <b>41</b> for containing the fiber lines <b>42</b> and a polyethylene resin insulator coating layer <b>39</b> for coating the feeder pipe <b>41</b>. The pigtail cable <b>44</b> is flexible enough to be formed in a spiral with a certain radius.

[0050] Next, a process of forming the pigtail cable <b>44</b> on the outside of the pressure-tight cylinder cover <b>36</b> is explained, referring to the perspective views of FIGS. <b>15</b>A-<b>15</b>C. A pair of the pigtail cables <b>44</b> are each bent at the cable feed-through assemblies <b>37</b> in opposite directions as seen in FIG. 15A. Then, the pigtail cables <b>44</b> are spiraled (formed in a spiral) respectively, such that a cable segment does not touch other one but has some gap therebetween as seen in FIG. 15B. Finally, the spiraled pigtail cables <b>44</b> are engaged with each other in an interleaved and overlapped fashion, as seen in FIG. 15C. When the radius of the spiral is, e.g., 130 millimeters, the distance between the far ends of the spirals can be approx. 210 millimeters.

[0051] By using an appropriate tool, the spiral can be made in a correct circle. To engage the spirals with each other as in FIG. 15C, it is desirable not to bend the cables too extremely but to bend smoothly by using a bending tool. Instead of spiraling the cable in the same direction as in FIG. 15C, the cable may be spiraled in opposite directions, thus further minimizing the distance between the axes of the spirals.

[0052] Terminal fittings <b>45</b> for connecting the thus-spiraled and engaged pigtail cables <b>44</b> to the submarine cables, are attached to the tip of the feeder pipe <b>41</b> and kept covered with a cover <b>46</b> until the pigtail cable <b>44</b> is actually connected to the submarine cable, as shown in FIGS. 16A and 16B. A required length of the optical fiber line <b>42</b> is extended from the cover <b>46</b> along with the polyethylene-resin tube <b>43</b>.

[0053] As shown in FIG. 17A—side view, FIG. 17B—front view and FIG. 18—perspective view, two props <b>47</b> with a ring cap attached on the tip, are erected on the pressure-tight cylinder cover. The optical fiber line <b>42</b> and polyethylene-resin tube <b>43</b> are wound and housed in the cap <b>48</b>. This provision prevents the pigtail cable <b>44</b> from deforming and moving and maintains the cable stable in a correct position in a factory manufacturing process. Also, the provision prevents the pigtail cable <b>44</b> when coupled to the pressure-tight cylinder <b>12</b>, from touching other components, since the distance between the far ends of the spiraled pigtail cable pair can be smaller than the diameter of the pressure-tight cylinder cover <b>36</b>.

[0054] The thus-constructed pressure-tight cylinder cover <b>36</b> is engaged and welded with the end of the cylinder portion constituting the pressure-tight cylinder <b>12</b> to form an air-tight (watertight) one-piece construction. Since the same is true with the pressure-tight cylinder cover <b>35</b> on the trunk side except for having only one pigtail cable, figures and explanations are omitted.

[0055]FIG. 19 is a side sectional view of the thus-assembled branching unit body <b>49</b>. A branching joint ring <b>51</b> is provided on the branch side (figure right) of the branching unit body <b>49</b>. One ends of the spiraled pigtail cables <b>44</b> are engaged with each other on the pressure-tight cylinder cover <b>36</b> side. Meanwhile, the other ends are disengaged and introduced to the respective openings of the branching joint rings <b>51</b> with the props <b>47</b> (described via FIG. 18) removed. Each of the other cable ends is covered with a cover <b>46</b> having a terminal metal. The cover <b>46</b> and a cap <b>48</b> at the tip are secured by the protection cap <b>52</b> which covers the opening of the branching joint ring <b>51</b>.

[0056] There is provided a main joint ring <b>53</b> on the trunk side (figure left). The pigtail cable <b>44</b> extending from the cable feed-through assembly <b>37</b> of the pressure-tight cylinder cover <b>36</b> is spiraled and is secured along with the cap <b>48</b> at the tip by the protection cap <b>52</b> covering the opening of the main joint ring <b>53</b>. Further, an earth cable <b>54</b> for supplying the sea earth to the power supply circuit which in turn supplies power to the feeder line <b>27</b>, extends from the cable feed-through assembly <b>55</b> provided on the pressure-tight cylinder cover <b>36</b>.

[0057]FIG. 19 is a partially sectional side view of the branching unit <b>11</b> in a state before actually connected with the submarine cables. FIG. 20 is a partially sectional side view of the branching unit <b>11</b> connected with the submarine cable <b>15</b>. For easy understanding, the same reference numerals are used to designate the same or similar components on the branch and trunk sides.

[0058] On the branch side, a joint ring <b>57</b> is disposed in each of the two openings of the branching joint ring <b>51</b>. Two joint pins <b>58</b> (not shown in the figure) are inserted into the joint ring <b>57</b> through the opening of the branching joint ring <b>51</b> respectively, from downward to upward and vice versa with respect to the drawing, to allow the joint ring <b>57</b> to rotate in parallel with the drawing. An axis <b>59</b> is inserted in the center of the joint ring <b>57</b>. Two joint pins <b>61</b> are inserted into the axis <b>59</b> through the joint ring <b>57</b> respectively, from left to right and vice versa with respect to the drawing, to allow the axis <b>59</b> to rotate vertically to the drawing. Thus, so-called a gimbal joint <b>17</b> is constructed which allows the axis <b>59</b> to tilt in any directions at the opening of the branching joint ring <b>51</b>, such that the axis <b>59</b> rotates on a conic surface, with the point at which the axis of the joint pins <b>58</b> intersects with that of the joint pins <b>61</b> as the axis of rotation.

[0059] A cylindrical coupling resin (resin with glass fiber mixed) <b>62</b> connected with the submarine cable <b>15</b>, is coupled with the tip of the axis <b>59</b>. The optical fiber lines <b>42</b> and feeder pipe <b>41</b> of pigtail cable <b>44</b> coming through the axis <b>59</b>, are connected, as shown by the dotted line, with the corresponding optical fiber lines <b>64</b> and feeder pipe <b>63</b> (not shown in the figure) of the submarine cable <b>15</b> and then, maintained airtight (watertight). To connect the corresponding optical fiber lines and feeder lines within the coupling resin <b>62</b> in this way, the pigtail cable <b>44</b> spiraled within the branching joint ring <b>51</b> is partially extended and drawn into the axis <b>59</b>. The part having a reference numeral <b>65</b> provided at the tip of the coupling resin <b>62</b> is a terminal for detaining the high-tensile steel line of the submarine cable <b>15</b>.

[0060] On the trunk side, a joint ring <b>57</b> is disposed in the opening of the main joint ring <b>53</b>. Two joint pins <b>58</b> (not shown in the figure) are inserted into the joint ring <b>57</b> through the opening of the main joint ring <b>53</b> respectively, from downward to upward and vice versa with respect to the drawing, to allow the joint ring <b>57</b> to rotate in parallel with the drawing. An axis <b>59</b> is inserted in the center of the joint ring <b>57</b>. Two joint pins <b>61</b> are each inserted into the axis <b>59</b> through the joint ring <b>57</b> respectively, from left to right and vice versa with respect to the drawing, to allow the axis <b>59</b> to rotate vertically to the drawing. Thus, so-called a gimbal joint <b>17</b> is constructed which allows the axis <b>59</b> to tilt in any directions at the opening of the main joint ring <b>53</b>, such that the axis <b>59</b> rotates on a conic surface, with the point at which the axis of the joint pins <b>58</b> intersects with that of the joint pins <b>61</b> as the axis of rotation.

[0061] On the trunk side, too, a cylindrical coupling resin <b>62</b> connected with the submarine cable <b>15</b>, is coupled with the tip of the axis <b>59</b>. The optical fiber lines <b>42</b> and feeder pipe <b>41</b> of pigtail cable <b>44</b> coming through the center of the axis <b>59</b>, are connected, as shown by the dotted line, with the corresponding optical fiber lines <b>64</b> and feeder pipe <b>63</b> (not shown in the figure) of the submarine cable <b>15</b> and are maintained airtight (watertight).

[0062] Further, on the trunk side, also the earth cable <b>54</b> passes through the center of the axis <b>59</b> and by the coupling resin <b>62</b>. Terminal fittings <b>56</b> provided at the earth cable <b>54</b> are connected to an earth electrode <b>66</b>, which is provided around the coupling resin <b>62</b> and contact the sea water.

[0063] Similarly, the part having a reference numeral <b>65</b> at a tip of the coupling resin <b>62</b> is a terminal for detaining the high-tensile steel line of the submarine cable <b>15</b>. Synthetic rubber bellows <b>67</b> cover the axes <b>59</b> located between the opening of the branching joint ring <b>51</b> and the coupling resin <b>62</b> (on the branch side) and also between the main joint ring <b>53</b> and the coupling resin <b>62</b> (on the trunk side). The bellows <b>67</b> follow the gimbal joint <b>17</b> as it tilts and moves to protect a foreign matter from entering the movable portion.

[0064] With the thus-constructed submarine branch unit, the length L<b>2</b> (see FIG. 6) between the tips of the cable terminations <b>16</b> is 3800 millimeters. The outside diameter D<b>1</b> (see FIG. 7) of pressure-tight cylinder <b>12</b> is 250 millimeters. The length (L<b>1</b>) between the branching joint ring <b>51</b> and the main joint ring <b>53</b> is 1400 millimeters. The maximum width (D<b>2</b>) of the branching joint ring <b>51</b> is 480 millimeters. Thus, the present invention has significantly reduced the size of the branching unit compared with that shown in FIGS. 1 and 2 and also reduced the weight from conventional 8.5 kilo-newtons (see FIG. 2) to 3.8 kilo-newtons (see FIG. 7).

[0065]FIGS. 21A and 21B show the branching unit <b>11</b> of the present invention in relationship to a sheave equipped on a cable wiring ship. FIG. 21A is a side perspective view and FIG. 21B is a front view of the branching unit shown in FIG. 21A with the left and right of the center line of the branching unit body <b>49</b> shown as the horizon. The sheave <b>68</b> is shown in a circumference view. While the branching unit <b>11</b> is being laid in or drawn out of the sea, the branching unit <b>11</b> bends along the sheave <b>68</b> having a diameter of, e.g., 3 meters. Thus, the gimbal joints <b>17</b> allow the pressure-tight cylinder <b>12</b> of the branching unit body <b>49</b> and the cable terminations <b>16</b> (or coupling resins <b>62</b>) to contact the sheave <b>68</b>.

[0066] The angle θ between the center line CL<b>1</b> of the branching unit body <b>49</b> and the center line CL<b>2</b> of the cable termination <b>16</b> is 48.6 degrees for the shallow-sea submarine cable (SA cable) and 49.4 degrees for the deep-sea submarine cable (LW cable). A protection layer is provided for the SA cable to prevent the cable from damages caused by touching anchors and fishing implements, thus making the finished cable thick in outside diameter and accordingly, making the distance from the bending center of the gimbal joint <b>17</b> to the contact point larger. To the contrary, the LW cable without the protection layer is lighter in weight and thinner in diameter.

[0067]FIG. 22 shows a table comparing a conventional branching unit with that of the present invention.

[0068] As for the materials making the branching unit, the metal parts touching the sea water are made of beryllium copper alloy which is known as mechanically strong and highly corrosion-resistant. The present invention can make the branching unit small in size and light in weight. Besides, since the present invention can be make the gimbal joint <b>17</b> the same in shape and size as the submarine repeaters, the cable terminations <b>16</b> and related components can be used in common with the submarine repeaters. Since the circuit unit <b>24</b> can be mounted in contact with the circuit board <b>18</b>, the heat generated from the circuit unit <b>24</b> can be effectively dissipated from the circuit board <b>18</b>.

[0069] As is apparent from the above description, according to the present invention, the vacuum relays are all mounted on one side of the circuit board and are each covered with the insulator covers. Thus, the insulator covers each insulate securely the terminals and wirings of a vacuum relay from those of other relays and therefore, the vacuum relays can be arranged closely adjacent to each other, reducing the relay mounting area significantly.

[0070] Since the circuit units are all mounted on the other side of the circuit board and therefore, the heat generated from the circuits is conducted to the circuit board and dissipated effectively, the circuit board can be reduced in size. Since, on the outside of the pressure-tight cylinder cover, a pair of the pigtail cables are spiraled and engaged with each other in an interleaved and overlapped fashion, the space which the individual cables wound independently occupied conventionally was reduced significantly and thus, minimizing the size of the branching unit. Further, the present invention can have the branching unit in common in configuration with a submarine repeater and when laying/drawing the unit in/out of the sea, can use the same sheave as used for the submarine repeater.