To properly understand how these different grounds fit into the vessel’s electrical system, the word “ground” must be understood as it was originally used. Decades ago, when DC tugboats were designed, the term “earth” was used instead of ground. Still later, the tugboats were called positive ground systems. (Foreign cars used to be termed positive earth systems). Over the years, many individuals and contractors mistook the term “ground” for a negative ground (as referred to on today’s automobiles).
Eventually all types of grounds were incorporated into the tugboats, and the true meaning of the term “ground” was lost. Both earth and ground have their original roots in terra firma. However, today the importance of the earth’s electrical potential as a safety factor is realized.
Nikola Tesla, the father of alternating current, discovered the rotating magnetic field and developed the Tesla coil used in radio and television today.
In 1900, he made one of his most important discoveries. He found that the earth could be used as a conductor and would respond to electrical vibrations. He called this terrestrial stationary waves. In one experiment, he lighted 200 lamps without wires from a distance of 25 miles.
The earth has a continuous electrical current that flows in, over, and around itself. This current seeks out equilibrium. In this way, it tries to establish a balance in its magnetic field. The potential of the earth is due to the transient electric currents that are electromagnetically induced within the earth itself. The electrical potential (voltage) of the earth has been measured. It is possible because of the conditions that form an electric cell-like device due to the electrochemical differences in local conditions.
In effect, the earth is its own low-voltage generator. The current produced is called telluric current. Telluric current, some believe, is caused by the motion of our sphere or the magma, the electrically conductive sphere itself, and the presence of the geomagnetic field. The earth is itself a generator and produces its own difference in potential.
Therefore, it is advantageous to maintain our very precise current-carrying electrical systems separate from the earth. When there is current flow between one electrical terminal and the hull, it is usually due to an unintentional ground, carrying current within our own electrical system.