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.