The impedance of the ground conductor can be seen as the pathway that a faulted current will have to take to reach the earth ground system. When insulation fails, a short circuit occurs. Protective devices like fuses or breakers open to stop the fault current, but before these devices can act, the ground conductor must carry the fault current to the ground rod in the earth.
The effective ground resistance needs to be as low as possible to quickly and safely dissipate the fault current for two reasons. First, the fault current has to quickly exceed the rating of the protective device, or exposed metal will be energized and potential for catastrophe exists. If the fault current does not exceed the rating of the protective device, the current will continue to leak on the ground continuously, until a complete failure occurs. Second, the fault current and resistance of the ground system can be multiplied together to calculate the effective voltage at the ground. Imagine that a ground system has an effective resistance of 50W. A person standing in mud on a job site may have a lower pathway to ground, say 30W. If that person is unlucky enough to touch the bare ground conductor when a five amp fault current is present, the five amp current at 150 volts may well pass through his body rather than the ground itself, since he has a lower effective resistance than the earth ground. If the effective resistance of the earth ground were at the NEC recommended 25W, virtually all of the fault current would flow through the intended pathway to ground, not through the person’s wet feet. This is a good reason to treat exposed grounds as if they are always energized. You do not want to become a ground conductor.
A similar condition occurs in a building when splices or bonds on ground conductors are not low resistance, or when the neutral and ground is connected at a sub-panel. A high resistance bond on the ground will produce high impedance to fault current. This naturally causes heat as well as increasing the likelihood that the fault current will find a path to ground other than the ground conductor. Here again, someone unfortunate enough to become a lower pathway to ground could suffer the consequence.
This high resistance bond can be a source of power quality problems as well. Modern digital electronics work at five-volt levels or less, switching, communicating and controlling our automated industrial processes. Imagine the problems fault currents cause when they produce voltage on the groundside of solid-state circuits. This common problem can be resolved completely by providing low resistance pathways for fault current to follow to earth ground.
Another related power quality issue is stray voltage. Commonly caused by connecting the ground and neutral conductor in a sub-panel, stray voltage can energize all exposed metal and building steel. Stray voltage in dairy farms causes cows to eventually stop producing milk and in hospitals will cause many problems with high-tech diagnostic equipment and patient-connected equipment. In our modern electrical environment, non-linear loads cause high neutral currents.
The neutral conductor can carry substantial current back to the earth ground system. The ground conductor is not considered an electrical conductor and is present to provide a low resistance pathway for fault current. The neutral must be carried back to the service entrance and can only be bonded to the ground conductor at the main neutral buss, where a large copper conductor carries all the return and faulted current back to the earth. Sometimes through error or ignorance, the neutral and ground are connected upstream from the service entrance. This is called a false or bootleg ground. If the neutral and ground are connected anywhere else in the building, all grounded metal becomes part of the neutral conductor, constantly energized and creating various voltage potentials on electronic equipment. This causes many nuisance problems with automated equipment and computers, but can also create a hazardous and expensive electrical environment.
The solution to these problems is to include complete ground pathway testing as part of the standard procedures in your facility and to choose test equipment which will help you locate and identify high resistance ground paths.
Questions to Ask