<P> In single - wire earth return (SWER) AC electrical distribution systems, costs are saved by using just a single high voltage conductor for the power grid, while routing the AC return current through the earth . This system is mostly used in rural areas where large earth currents will not otherwise cause hazards . </P> <P> Some high - voltage direct - current (HVDC) power transmission systems use the ground as second conductor . This is especially common in schemes with submarine cables, as sea water is a good conductor . Buried grounding electrodes are used to make the connection to the earth . The site of these electrodes must be chosen carefully to prevent electrochemical corrosion on underground structures . </P> <P> A particular concern in design of electrical substations is earth potential rise . When very large fault currents are injected into the earth, the area around the point of injection may rise to a high potential with respect to distant points . This is due to the limited finite conductivity of the layers of soil in the earth . The gradient of the voltage (changing voltage within a distance) may be so high that two points on the ground may be at significantly different potentials, creating a hazard to anyone standing on the ground in the area . Pipes, rails, or communication wires entering a substation may see different ground potentials inside and outside the substation, creating a dangerous touch voltage . </P> <Table> <Tr> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> Signal ground </Td> <Td> </Td> <Td> Chassis ground </Td> <Td> </Td> <Td> Earth ground </Td> </Tr> </Table>

What is the purpose of grounding electrical equipment