Supplemental and supplementary grounding electrodes are very similar in name but vastly different in their permitted uses. Understanding the differences in these two electrodes can be critical in providing a safe, Code-compliant installation.

Grounding electrodes provide four basic functions in a grounded electrical system and are extremely important for the safe operation of the system. These electrodes connect the electrical system to earth to help minimize high voltage spikes from lightning strikes, line surges or unintentional contact with higher voltage lines, and to stabilize the voltage of the electrical system to earth during normal operation.

Contrary to popular belief, the grounding electrode and its connecting conductor at the service grounded conductor (or neutral in many cases) are not there to provide a fault current path through the earth to help trip an overcurrent protective device. A fault current path through earth is often a very high impedance path back to the utility company source and should not be relied upon to carry fault currents. Equipment grounding conductors are designed to provide the path for ground fault current back to the source of the electrical supply.

Electrodes provide an equipotential plane that will help stabilize any voltage differences within the electrical supply system by tying the grounded conductor to earth or as close to zero as possible. This zero reference point in the electrical system is similar to the zero reference point used in mathematics. Zero reference between the positive numbers and negative numbers in math is similar to the placeholder that applies between a positive 120V and the negative 120V in electricity. The grounding electrode system provides that reference to zero for the electrical system.

Structural steel that is effectively and substantially connected to earth is an extremely effective electrode. Twenty feet of 4 AWG or larger copper conductor or 20 feet of steel rebar, encased by at least 2 inches of concrete and located within and near the bottom of a concrete foundation or footing in direct contact with the earth, is also an effective electrode. Installing at least 20 feet or more of 2 AWG or larger copper conductor in a continuous ring around a building or structure is also a good grounding electrode. Rods or pipes can be driven into the ground or a flat plate of copper can be installed as an electrode. Finally, large metal tanks and underground pipes (except metal gas pipes) can be used as electrodes.

One of the most common electrodes used at a service is a metal water pipe. Section 250.52(A)(1) requires a metal water pipe, in direct contact with the earth for 10 feet or more and electrically continuous (or made electrically continuous by bonding around any insulating parts) to be used as a grounding electrode. Since metal water piping systems installed in the ground can deteriorate over time, many of these metal systems may eventually be replaced with a nonmetallic water piping system. For this reason, 250.53(A)(2) requires an additional grounding electrode to be used as a supplement to the water piping system electrode. This electrode is called a supplemental electrode and acts as a backup electrode, in case the metal water pipe is replaced with nonmetallic, and combines with all other available electrodes to form the grounding electrode system for the electrical service.

In contrast, supplementary electrodes provide a reference to earth but, unlike supplemental electrodes, this grounding electrode is not located at the electrical service. Supplementary electrodes are located at electrical equipment or structures and are connected to equipment grounding conductors at electrical equipment, such as lighting poles. Electrical equipment may be subject to lightning strikes or other high voltages. Connecting a grounding electrode to the equipment grounding conductor at electrical equipment, for example, may allow a lightning strike to go to ground at the pole, rather than back toward the power source on the equipment grounding conductor.

Unlike grounding electrodes for services, supplementary electrodes are not required to comply with any of the requirements for normal electrodes, such as sizing in accordance with 250.53(C), but can be sized the same as the equipment grounding conductor since these electrodes are a supplement to equipment grounding at the equipment. If a ground rod is used, which is often the case, it is also not necessary to comply with 250.56, which only permits a rod, pipe or plate electrode to have a maximum resistance to ground of 25 ohms. However, unlike a normal service-driven ground rod or plate electrode, additional electrodes are not required if 25 ohms is exceeded.

Supplementary electrodes could be used to provide protection from potential lightning or similar damage in the your next project but full understanding of the concept in Section 250.54 is a must. EC

ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at mark.c.ode@us.ul.com.