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Stray current is a flow of electricity through equipment, buildings or the ground due to imbalances in electrical supply systems or because of wiring damage. Electrical systems are grounded to the earth at regular intervals, both on the neutral and ground phases or wires. Power is supplied through the hot phases, with different voltages available depending on location. Unused electrical current returns to the supplier through the neutral wire or phase, and electrical codes in many areas require a separate ground wire that is connected to a rod placed in the earth.
When an electrical system is improperly installed or maintained, electrical current can flow into the ground, or through the building or equipment itself. Stray 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 hazard of electrocution, small stray currents can also cause damage by corroding metals in the ground.
Direct current (DC) systems are used for rail, subway and some power distribution systems. Stray currents can exist where the rail systems contact the ground, particularly in wet areas. The presence of stray current can cause accelerated metal corrosion, because the electrical flow causes the metal to break down into its ions and enter the ground. Left uncorrected, metal pipes and structures can be destroyed in a short period.
Stray current corrosion is a widespread problem in marine systems, particularly in marinas or ports where large numbers of vessels are docked. A boat having bad 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 the stray current can enter other boats through underwater fittings or propeller shafts. With the electrical flow now established with the defective boat system, accelerated corrosion can occur and destroy metal fittings quite rapidly.
In the 20th century, it was common for homes to ground their electrical systems to copper drinking water pipes entering the homes. Defects in wiring created electrical flows through the copper piping systems and caused widespread corrosion of public water systems. An understanding of these issues led to better grounding systems using metal ground rods driven deep in the ground to provide a path for current flow.
Underground piping used for oil, gas or water supplies can be damaged by stray currents. Many piping systems use isolators, non-conductive connectors or gaskets, that separate the piping into smaller sections to prevent conducting current over longer distances. Coating the exterior of the pipe with plastic or polymer coatings can reduce corrosion by separating the pipe from the nearby soil. Sacrificial anodes, which are rods made from zinc or other metals that corrode more easily than the pipe, can be attached at regular intervals to protect the piping from electrical corrosion. These anodes are also used on boats to protect underwater boat components.
Stray current is an especially large concern in big cities. With the amount of electricity being used, there is a lot of energy being piped around these cities. Many large metropolitan cities are starting to age and so is the electrical infrastructure. Even low instances of stray current from insulation failure can be hazardous to the populace.
In New York City there is a web site showing maps of stray voltage locations in the city. This site was created by a group founded because of the death of Jodie S. Lane who was killed by stray current in Manhattan.
Boston had a series of dog deaths in the 1990s because of stray voltage.
In 2009 Toronto Hydro pulled all employees off their regular duty to deal with stray current issues. This coming after a series of reports with children being shocked.