Signal Cables Installation
To avoid electromagnetic noise caused by circulating currents, the conductive shield and cable armor is to be grounded only at one end of the cable.
To avoid possible signal interference, signal cables occupying the same cable tray, trunk or conduit with power cables are to be effectively shielded.
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The Basics
A ship's wiring cables form an integral part of the Electrical System on board a ship. Hence they also have to withstand a wide variety of environmental conditions e.g., extremes of temperature, humidity and salinity of atmosphere, as mentioned in Chapter 1. Improved materials have led to obtaining cables of a fairly standard design that are safe, durable and efficient under all conditions.
The normal voltage on ships is 440V and cables for use at this voltage are designated 600 / 1000V - 600V to earth or 1000V between conductors. Higher voltages require cables rated at 1900 / 3300V for 3-phase, earthed neutral systems and cables rated at 3300 / 3300V for 3-phase insulated neutral systems.
Conductors
Many factors determine the type of electrical conductor used to connect components.
Some of these factors are the physical size of the conductor, the type of material used for the conductor, and the electrical characteristics of the insulation. Other factors that can determine choice of a conductor are the weight, the cost, and the environment where the conductor is be used.
Although silver is the best conductor, it is costly and hence is only used in special circuits; it is used where a substance with low resistivity is needed (its resistivity known to beas low as 1.64 x 10-8 ohm-metres at 20°C).
Copper is the next option for the following reasons:
· It possesses low resistivity (for annealed copper it is 1.72 x 10~8 ohm-metres at 20°C).
· It is more ductile (can be drawn out into fine strands).
· It has relatively high tensile strength (the greatest stress a substance can bear along its length without tearing apart) and it can also be easily soldered.
The copper conductor is tinned or alloy-coated to ensure compatibility with its insulation, conductors are normally made of annealed stranded copper, which may be circular or 'ally shaped. Cables with shaped-conductors and cores are usually smaller and lighter cables with circular cores. (Refer Figure 14.1)
Figure 14.1- Cable Cores
Temperature Coefficient
Pure metals, such as silver, copper, and aluminium, have positive temperature coefficient! i.e., their respective resistances increase along with a rise in temperature. The resistance of some alloys, such as constantan and manganin, changes very little as the temperature changes. Measuring instruments use these alloys because the resistance of the circuits must remain constant in order to achieve accurate measurements.
The positive temperature co-efficient of a material maybe defined as the rise in resistance per ohm original resistance per °C rise in temperature. It is denoted by the symbol &, whichis equal to 1/234.5 = 0.00427 for copper at 0°C. Though it is expected to remain constant it does vary at different temperatures i.e., it is 0.00393 at 20°C, 0.00378 at 30°C and 0.00352 a 50°C and so on. This and more is taken into account when designing the electrical distribution system of the vessel. A wire is not just any wire; there is a reason and a purpose for the entire electrical system. The only changes in the electrical system should be for expedient repairs and approved modifications; do not modify electrical systems without proper authority.
Cable Testing
Cables should be periodically tested and checked, ideally when checking their connected appliances. Cable insulation resistance should be measured and the value logged. Cables in exposed and damp situations, e.g., for deck lighting, may develop a low insulation resistance. Usually this is a result of mechanical damage, or a faulty gland permitting the ingress of water.
