Calculations on a Subsea Shunt Fault

Fixing a subsea cable issue needs previous activities, like, for example, finding out, with accuracy, where the cable fault (cut or shunt fault) is located, information that will be passed to the cable ship responsible for restoring the cable. Here are the main steps.

1) Verifying the SMS (Submarine Management System) application's alarms & repeater interrogation software, to determine the section of the subsea segment closest to the point of failure.

2) Use the COTDR equipment for a trace of the segment, to get more insights into the failure.

3) Collect from the cable, repeaters' manufacturer enough information (electrical resistance, attenuation, other characteristics of the metal conductor, fiber type) used in the cable.

4) Retrieve logs (voltage, current) from both PFEs of the segment, which will be used for the calculation ahead.

5) Always stay in contact with the technical support of the cable, and PFEs providers.

In the next lines, a summary of the calculations to identify the point of a shunt fault on a subsea repeater will be presented.

Shunt Fault on a Subsea Segment

• The cable has sixty-nine (69) subsea repeaters, distributed among seventy sections (70, S1 to S70), including the two cable stations, with variable lengths. Each repeater amplifies four (4) fibers, dropping 31,75 Volts each. The electrical resistance is 1,38 Ohms per kilometer. S1 stays close to Rio CS, S70 to Fortaleza CS. The System Length is around 3,529 km.
• During the shunt fault, the Rio's PFE indicated 1,952 V, 900 mA. Fortaleza's PFE was showing 4659 V, with the same current.
• Observing the as built of repeaters left as the installation was completed, one can estimate the shunt fault is inside section #21 - between repeater #20 and repeater #21.
• The voltage of the cable faulty section is:

Vfault = Vpfe – Vcable(Section 1 to 20) – Vrep (20 repeaters)

Vfault = 1952 – 1283 – 635 = 34.17V

• The distance of the fault from Repeater 20 inside section #21 is

Dfault = Vfault / cable resistance / Line current

Dfault = 34.17 / 1.38 Ohm/km / 0.9 = 27.49 Km

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• So, the calculation gives the fault at approximately 27.5 km from Repeater 20, and 1053 km from Rio CS.