Currents from cables

Ohmic voltage drops resulting in losses cannot be ignored in the connecting cables with long anode cables and high protection currents [28]. Cable costs and losses must be optimized for economic reasons. The most economic calculated cable dimension depends primarily on the lowest cross-section from the thermal point of view. For various reasons the permitted voltage drop usually lies between 1 and 2 V, from which the cross-section of the cable to be installed can be calculated from Eq. (3-36). 

In very built up areas, telephone cables are laid in ducts that mostly consist of shaped duct blocks. This means that cables running parallel to tramway tracks cannot be avoided. Metal-sheathed cables with poor coating, or with none at all, are then heavily exposed to stray currents from the tramway [5,6]. 

The railway lines on bridges are often electrically connected with steel or reinforced steel structures which usually have a very low grounding resistance. In new installations, an electrical separation of the rails from the bridge structure is required according to the grounding resistance of the structure and the type of rail bed. Independent of this, pipelines and metal sheathing of cables are always electrically separated from the structure in order to exclude direct transmission of stray current from the rails in these conductors. 

Electrolytic corrosion may also occur on the inside of cable sheaths by the passage of current from the cable sheath to the wire . 

The corrosion of underground pipes and cables caused by the electrolytic action of stray currents from d.c. electric railway and tramway systems has long been a serious problem. Regulations limiting the maximum potential between tramway rails and neighbouring buried structures and the maximum potential difference between points on the rail systems have been in operation in the UK since 1894 and in Germany since 1910 . 

Nevertheless, special consideration should be given to any measured small positive changes in structure/soil potential on a nearby buried pipe or cable if there is reason to believe that the secondary structure is already corroding because of local soil conditions, or as a result of stray currents from another source. 

For example, one needs shielded and grounded cables, for such tiny currents may be exceeded by stray currents from nearby electrical machinery. 

Apart from corrosion due to differential aeration, corrosion of underground metal structures and pipelines may also arise from stray currents. How this comes about can be seen in the accompanying diagram (Fig. 12.32). The presence of a current-carrying cable in conducting soil results in stray currents passing through the soil. These stray currents may set up a potential difference between two portions of a pipeline, which then develops electron-source (cathodic) and -sink (anodic) areas. Thus, pipelines tend to corrode when they pass near electric lines. 

In order to avoid interfering magnetic noise which rises from cables conducting large currents these wires should be twisted. In case that the current in the two wires is equal and of opposite direction the magnetic field would then cancel. 

Low electrolyte levels can result in a dead battery. Jumper cables conduct current from a car with a good battery to start a car with a dead one. 

Since the cable volt-drop is significant it is necessary to revise the starting time duration and current from the data given by the manufacturer of the motor. 

NEARBY pipeline OR CABLE Fig. 10.35 Stray current from d.c. traction system... 

The stack also includes two half bipolar plates on each side along with end gaskets and end plates. The end plates are used to cover the ends of the stack, house the inlet and outlet ports of the reactant gas streams, interface with the current collectors, and provide the structural support for the stack against any compression loading. The current collector transmits the current from the active area of the MEA to the power cable of the external load circuit. 

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