What is an eddy current? (with photo)

Eddy current corrosion is a problem in marinas where large numbers of vessels are moored.

Eddy current is a flow of electricity through equipment, buildings or the ground due to imbalances in electrical supply systems or due to damage to wiring. Electrical systems are grounded to earth at regular intervals, on the phases or neutral and earth wires. Power is supplied through the hot phases, with different voltages available depending on location. Unused electrical current is returned to the supplier via the neutral wire or the hot wire, and electrical codes in many areas require a separate ground wire that is connected to a rod placed in the ground.

When an electrical system is incorrectly installed or maintained, electrical current can flow into the ground or through the building or the equipment itself. Eddy current can be a nuisance if a small amount is present, but it can electrocute and kill if it reaches unsafe levels. Along with the potential danger of electrocution, small eddy currents can also cause corrosion damage to metals in the ground.

Direct current (DC) systems are used for railways, subways and some power distribution systems. Eddy currents can exist where rail systems come into contact with the ground, especially in wet areas. The presence of eddy current can cause accelerated corrosion of the metal, because the electrical flow causes the metal to break down into its ions and enter the soil. If not corrected, pipes and metal structures can be destroyed in a short period of time.

Eddy current corrosion is a widespread problem in marine systems, particularly in marinas or ports where large numbers of vessels are moored. A boat with poor electrical connections can discharge direct current from its batteries directly into the water. Other boats connected to the marina’s electrical system share common wiring, and eddy current can enter other boats through underwater fittings or propeller shafts. With electrical flow now established with the faulty boat system, accelerated corrosion can occur and destroy metal fittings very quickly.

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In the 20th century, it was common for homes to ground their electrical systems to copper pipes for drinking water that entered the homes. Wiring defects created electrical flows through copper piping systems and caused widespread corrosion of public water systems. Understanding these issues has led to better grounding systems using metal ground rods driven deep into the ground to provide a path for current to flow.

Underground piping used to supply oil, gas or water can be damaged by eddy currents. Many piping systems use insulators, non-conductive connectors, or gaskets, which separate the piping into smaller sections to avoid carrying current over longer distances. Coating the outside of the pipe with plastic or polymer coatings can reduce corrosion by separating the pipe from the nearby ground. Sacrificial anodes, which are rods made of zinc or other metals that corrode more easily than the pipe, can be attached at regular intervals to protect the pipe from electrical corrosion. These anodes are also used on boats to protect underwater boat components.

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