Page 208 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

The operating voltage is another consideration in the cable design. It is important to limit volt- age stress on the insulation. If this is too high it can cause the insulation to fail and the electrical energy to exit the conductor before it reaches its objective, which can (to say the least) create a hazardous condition. Further, should the insulation break down (through damage or some other mechanism) and a ground fault develop, the flash grounding could produce an instantaneous high- temperature arcing, thus melting through all parts of the tether. This converts a tethered vehicle to an (expensive) untethered (and unpowered) floating/sinking vessel. This is not comical as it hap- pens all too often. Therefore, it is important for the cable design to address the insulation voltage stress. Also, a separate conductor for an emergency ground is common as a safeguard in case there is a breakdown in the insulation.
8.8.3 Signal requirements
The signal requirements translate to attenuation losses. The signal, whether electrical or optical, attenuates through both the conductor and the insulator. This loss varies with both the signal trans- mission media and the frequency.
Signal transmission can be either analog or digital and either electrical or optical. The system usually dictates the signal transmission type. It is important for the cable designers to understand the media and as many parameters about the signal transmission as possible so they can select the proper conductor for the signals.
Copper conductors with thermoplastic insulation are also common for electrical signals, similar to power conductors. Signal transmission wires frequently require a shield from electromagnetic interference and radio frequency interference. Also, it is common to group the signal transmission wires separate from the power conductors.
As more fully explained in Chapter 13, there are both balanced and unbalanced electrical trans- mission schemes, and the system determines this requirement. Typical balanced lines are twisted pairs, and unbalanced lines are coaxial.
You can also transmit signals over optical fibers. Fiber optics come in various types:
 Multi-Mode  50/125
 62.5/125  Single-Mode
 Dispersion shifted
 Nondispersion shifted
The system requirements determine the optical fiber type. Some parameters to consider in any type fiber optic are as follows:





Attenuation Bandwidth Wavelength
There are two different ways to package the fiber optics in a cable:
Loose-tube buffer Tight-buffer
8.8 Underwater cable design 197