Branching unit for underwater telecommunications systems

A branching unit for an underwater communications system is arranged so as to provide remote hot switching for electrical power. The unit has three terminations (1, 2, 3) each for a different one of three line cables and one or more power demand circuits (20) requiring an electrical power feed from the line cables. A first switch is actuable to complete a current path between an appropriate pair of the three terminations via one or more of the power demand circuits depending upon relative line voltage. A second switch is actuable after actuation of the first switch depending upon current supplied to the power demand circuit(s) rising above that which occurs upon action of the first switch. The second switch completes a circuit between the non-paired termination and a fourth termination for a sea-earth.

TECHNICAL FIELD 
This invention relates to branching units for underwater telecommunications 
systems. 
BACKGROUND OF THE INVENTION 
Underwater cable systems originally were such as to connect two land 
terminals which were, for example, on opposite sides of the Atlantic 
ocean. A later development involved having two land terminals on one side 
and a third on the other side, there being a trunk cable (current supply 
cable) extending between the third land terminal and a branching unit (y 
interconnect) and spur cables extending from the branching unit to the two 
other terminals. Repeaters may be disposed in the trunk cable and the spur 
cables and power feed provision must thus be made. The repeaters can be 
powered by supplying current between the land terminal (terminal station) 
at one end of a branch (trunk or spur) cable and a distant earth (single 
end feeding) or between any two terminal stations (double and feeding). 
One known type of branching unit power feed of repeaters in the trunk 
cable and one spur cable is by double end feeding, whereas that of 
repeaters in the other spur cable is by single end feeding, the earth 
being provided by a sea earth integral with the branching unit. The 
branching unit may include relays by means of which the power feeding can 
be changed (switched) in the event of fault conditions in one of the 
branches, in order to isolate that branch whilst continuing to power the 
other branches. 
Optical fibre underwater cable systems for long haul applications are now 
being designed with many landing points, and thus many branching units, 
and complicated traffic routing requirements. A basic branching unit 
suitable for multiple branching unit systems is disclosed in GB Published 
Patent Application No. 2252686A. This basic unit is however a passive 
unit, i.e. it does not involve repeaters/generators for the optical 
signals, which is designed to terminate three line cables and also to 
provide a sea earth for power feeding. A subsequent active design includes 
optical repeaters (regenerators) which require power feeding, as disclosed 
in GB Patent Application No. 9304328.9 in which there is a facility to 
power auxiliary circuitry, such as repeaters, in view of a zener diode 
chain and a full wave rectifier bridge. 
A particular problem that has been experienced with active branching units 
is that of "hot" switching during power-up of the system. The active unit 
described in GB application No. 9304328.9 addresses this problem by 
providing an auxiliary switching relay which performs the hot switching, 
thus preventing damage to the main relays. The present invention can be 
utilized in the detailed circuits of that application, the whole contents 
of which are incorporated herein by reference. 
SUMMARY OF THE INVENTION 
The present invention is concerned with an alternative approach to this 
problem. 
According to one aspect of the invention there is provided a branching unit 
for an underwater communications system, the unit being so arranged as to 
provide remote hot switching for electrical power. 
According to another aspect of the invention there is provided an undersea 
communications system incorporating one or more switching units and shore 
stations associated therewith, wherein means are provided for performing 
hot switching within the shore stations. 
The branching unit may comprise three terminations each for a different one 
of three line cables, one or more power demand circuits requiring an 
electrical power feed from the line cables, a first switching circuit 
actuable to complete a current path between an appropriate pair of the 
three terminations, via said one or more power demand circuits, based upon 
relative line voltage, and a second switching circuit actuable after 
actuation of said first switching circuit based upon current supplied to 
the power demand circuit(s) rising above that which occurs upon actuation 
of said first switching circuit and effective to complete a circuit 
between the non-paired termination(s) and a fourth termination for a sea 
earth. 
The first switching circuit may comprise three electrical relays each 
having an operating coil and a normally closed pair of contacts arranged 
in series and forming one side of a delta network, each side of the delta 
network being connected between a different pair of the three 
terminations, each relay also having a pair of change-over contacts which 
normally connect a different one of the terminations to the delta network, 
but which when actuated, permit connection of the terminations to the 
fourth termination via the second switching circuit. 
The second switching circuit may comprise a relay, the coil of which is 
connected in series with the power demand circuit(s).

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to the drawings, the branching unit is arranged to go through the 
power-up procedure with no hot switching. This is achieved by a two stage 
switching scheme, the two stages being performed each at a predetermined 
current level as the current rises during power-up. 
In the branching unit of FIG. 1, the A, B and C relays are configured to 
switch at a trunk line current, e.g. about 400 ma. The grounding relay 
(the D relay) is arranged to pass current through its coil and is arranged 
to switch at a higher trunk line current than the A, B and C relays. 
Typically the D relay switches at a coil current of about 800 ma. The 
trunk line current can subsequently rise to the full line current of about 
1.6 a. Thus the A, B and C relays function as configuration relays and the 
D relay functions as a current sensing grounding relay. The trunk line 
current/voltage characteristic illustrated in FIG. 2 shows that ample 
margin can be achieved between the two switching windows. The coil of the 
D relay may be provided with zener diode protection. 
We have found that the arrangement is tolerant of a single zener diode 
failure (resulting in a 25% voltage drop), thus providing inbuilt system 
reliability. 
The branching unit of FIG. 1 is fully symmetrical and is intended for use 
in large multi-spur underwater cable systems. 
In the arrangement shown in FIG. 1, predetermined differential voltage is 
established between two of the three terminations (1, 2, 3). One of the 
relays (A, B or C) switches at the lower (400 ma) current which 
corresponds to a predetermined differential voltage between two of the 
terminations disconnecting the corresponding spur from the trunk line. The 
spur terminal stations (not shown) thus arrange to discharge the 
electrically fluctuating spur cable. Continued trunk powering above the 
higher (800 ma) current threshold allows the disconnected spur to complete 
the ground connection. Accordingly, system configuration is established 
without any hot switching under water. 
Referring now to FIG. 3, this shows a system in which branching units (BU1 
to BU4) are powered each from a respective power feed equipment (PFE1 to 
PFE4) via switches (S1 to S4). Further power feed equipments (PFE5 and 
PFE6) power the ends of the system. 
The system power-up sequence is as follows. 
1. Open switches S1 to S4. 
2. Ramp PFE5 positive and PFE6 negative. 
3. The branching units BU1 to 4 will configure at a current of 400 ma, and 
the ramping is stopped when a current of 600 ma is reached. 
4. The switches S1A to S4A are closed to discharge the spur cables at the 
cable head, i.e. not under water. 
5. Continue ramping PFE5 and PFE6. Earth connection occurs when a current 
of 800 ma is reached. 
6. Once the trunk is powered, close switches S1B to S4B and open switches 
S1A to S4A. 
7. Ramp PFE1 to 4 to the full operating current of 1.6 a. The system is now 
fully powered. 
FIG. 4 shows additional detail of the basic circuit of FIG. 1 and includes 
three rectifier bridges, one around each of the relay coils A, B, C. These 
rectifier bridges ensure that no matter which direction current flows 
(between the terminal) the coil energizing current is always unipolar and 
helps to increase relay dropout time via the flywheel effect. Under fault 
conditions, current flow through the branching unit may suddenly reverse 
as the cable discharges into a short circuit. This reversed process may be 
quite slow for distant faults, thus there is a time when an actuated relay 
is starved of current and, furthermore, it must remagnetize with the 
opposite polarity. In short, the relay may unswitch and connect the spur 
cable to the main trunk cable causing a high current surge that may damage 
the relay, depending on the voltage at the time. Additionally the coil 
bridges act as "flywheels". Removal of current supply will cause a coil to 
generate a back voltage, the polarity of which is conducted by the bridge 
diodes. The diode conduction will further prolong the magnetic field decay 
and thus the dropout time of the relay. This damping effect is sufficient 
to hold a relay in during the line current reversal and so prevent a 
grounded spur being connected to the trunk line. Due to the inclusion of 
the bridge diodes around the relay coils, A, B, C, linearity bypass 
resisters 30 are included across each of them, since at low supply 
voltages these bridge diodes will not conduct. 
There are four power demand circuits in the form of regenerators 20 in the 
illustrated arrangement, although this is not the only possibility, with 
their power feed circuits connected in series. A respective zener diode 21 
is connected across (in parallel with) each regenerator 20 and limits 
supply voltage to the regenerator, the four zener diodes 21 being 
connected in series. The arrangement of four zener diodes 21 is in effect 
disposed in parallel with each relay coil A, B and C. 
Three pairs, 22, 23 and 24 of rectifier diodes 25A-25F, conducting in the 
same directions, are disposed in parallel with the series connections of 
regenerators and zener diodes. One pair of rectifier diodes is associated 
with each branch cable terminal. The junction 26 to which both coils B and 
C are connected is connected to a point between the rectifier diodes of 
pair 22, the junction 27 between coils A and B is connected to a point 
between the rectifier diodes of pair 23, and the junction 28 to which both 
coils A and C are connected is connected to a point between the rectifier 
diodes of pair 24. As will be appreciated, associated with each coil A, B 
and C, is a respective pair of the pairs of rectifier diodes 22, 23 and 
24. For example, coil C is associated with diode pairs 22 and 24, and the 
four diodes of these two pairs are arranged as a rectifier bridge. The 
line terminals 1 and 3 and the coil C are connected across one pair of the 
bridge's terminals and the regenerators 20 and zener diodes 21 are 
connected across the other pair of the bridge's terminals. 
When a voltage is applied between two stations such that, for example, 
current flows between terminals 1 and 3, terminal 2 is connected to the 
line A1/B1 and when the current flow through the relay coil D is raised 
above 800 ma the relay is actuated and the terminal 2 is connected to the 
sea earth. In addition, current flows from junction 26 through rectifier 
diode 25A, the series connections of regenerators 20 and zener diodes 21, 
and rectifier diode 25F to junction 28 and thence to terminal 3. 
Hence the regenerator powerfeed circuits are effectively put in series with 
the leg through which the current is being supplied and in which terminals 
(1 and 3 in this example) are connected and hence current is always 
supplied to the regenerators. If current flow is between terminals 1 and 2 
and terminal 3 is connected to the sea earth when contacts D1 are changed 
over, current flows through rectifier diode 25A, the regenerators 20 and 
zener diodes 21 and rectifier diode 25D to junction 27 and thence to 
terminal 2. If terminals 2 and 3 are connected and terminal 1 is connected 
to the sea earth when contacts D1 are changed over and current flow is 
from terminal 2 to terminal 3, it also flows from junction 27 through 
rectifier diode 25C, the regenerators 20 and zener diodes 21 and rectifier 
diode 25F to junction 28. If the current flow is reversed e.g. with the 
last case, but flow is from terminal 3 to terminal 2, current flows from 
junction 28, through rectifier diode 25E, the regenerators 20 and the 
zener diodes 21, rectifier diode 25D to junction 27 and thence to terminal 
2. The arrangement is symmetrical and reversible and achieves the 
requirement of supplying current to the regenerators irrespective of which 
two terminals (arms or branches of the system) are powered and 
irrespective of the current flow direction. 
Considering current applied between terminals 1 and 3, relay coil C sees 
double the voltage of the coils A and B, which means that the C relay is 
capable of switching its contacts C1, but the A and B relays are not 
capable of switching their contracts A1 and B1 as they only have half of 
the voltage. The four zener diodes 21 simultaneously offer surge 
protection to the regenerators 20, relay coils A, B, C and the rectifier 
diodes, i.e. protection during current transients. These may occur such as 
when the cable is cut, it shorts out to the seawater and very large 
currents (300-400 amps) can flow. So by having the zener diodes 21 in 
parallel with the regenerators 20 they limit the voltage across the 
regenerators 20, as well as across the relay coils A, B, C. So the system 
is totally internally surge protected. In addition, surge protection coils 
29 are included between the terminals 1, 2, 3 and the junctions 26, 27, 28 
to lengthen the rise time in the event of a nearby short circuit fault and 
to provide a smaller transient over a longer time than otherwise. The 
surge protection coils 29 thus limit inductive voltage spikes between 
regenerators 20 and across the rectifier diodes 21 and relay coils A, B, C 
during current transients. 
For cable fault finding purposes at low line currents, it is necessary to 
have a dc path with defined resistance. The resistors 30 provide such a 
path since at low power feed voltages, the rectifier bridge will not 
conduct significantly. 
Illustrated in FIG. 4 between the sea earth and the contacts A1, B1 and C1 
is the further relay contact D1 referred to above, associated with further 
relay coil D. The D relay isolates the spur from ground until it has been 
discharged by the terminal station. 
The present invention allows the provision of: 
1). A Branching Unit to allow the powering of a multi-spur system, without 
hot-switching of vacuum relays during power-up, thus preventing 
`Arc-Transfer`. 
2). A Branching Unit that has a defined two stage switching process 
controlled by the system line current flowing through the unit. 
3). A Branching Unit that will allow the flow of current through any two 
spurs, automatically connecting the third to a Sea Earth once fully 
powered. 
4). A Branching Unit that is insensitive to current direction for correct 
operation. 
5). A Branching Unit that is tolerant of zener diode component failure by 
short circuit.