September 2008 Electrical Apparatus

September 2008 Electrical Apparatus

This is a summary of the Electrical Apparatus September 2008 featured technical article,  by Richard L. Nailen, P.E.     

Is the earth a good electrical conductor or a poor one? The answer is: both, depending upon the circumstances. Experiments in England during the 18th Century proved that electric current could readily flow through the earth for long distances. Not until after the work of German Professor Carl Steinheil was publicized in 1838, however, was practical use made of this discovery, when commercial telegraphy was developed. Early systems had used two-wire circuits. Elimination of one wire, using the earth to complete the circuit, greatly reduced system cost.

Today, many a-c power distribution circuits, especially in rural areas, involve a neutral conductor that may carry considerable current and is grounded at multiple points for lightning protection. Consequently, the earth becomes a conductor in parallel with the neutral wire. This can cause voltage to appear between the neutral and the earth, and between different points on the earth’s surface. Dairy cows may be exposed to low “stray” voltages while standing in farm enclosures, their feet on a wet floor and their heads in contact with grounded metal structures. Even half a volt can inhibit milk production or sicken the animals. Higher voltages can be a human shock hazard. All such concerns would be meaningless if the earth were unable to sustain current flow.

As Steinheil and others have recognized, the point of contact between any metallic circuit and earth is where most of the “ground resistance” occurs. It’s the junction of a small, low-resistivity conductor with a very large, high-resistivity conductor. The current flow path through the earth widens rapidly–and its resistance accordingly falls–as distance from that junction increases.

Grasping this relationship is essential to understanding the behavior of current flow in the earth. Equally essential is the recognition that the earth is not a current sink into which all electricity tries to flow. Current does not “seek” the earth; any current flowing towards the earth from any circuit will be matched by equal current flowing away from the earth at some other point.

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