Grounding Electrode. A typical grounding electrode is the common ground rod—a length of metal rod, 5/8-inch or greater in diameter, and sometimes copper-plated, that is driven about 6 feet (2 meters) into the earth. Since moist earth creates a better ground than dry earth does, multiple ground rods are often needed in typically dry climates like the Southwest. Grounding can also be done with a copper wire or the appropriate length of properly connected coated steel reinforcing bars (rebar) installed inside a concrete foundation; this is known as a Ufer ground (named after H.G. Ufer, an Army consultant in the 1940s) and is said to offer better performance, especially in dry soil locations.
A ground rod is the simplest grounding component. Its job is to connect the electrical system to earth. This reduces the chance of electrical shock by keeping all grounding system components at the same voltage level relative to the earth, and helps prevent the system from developing a high-voltage static charge. Without grounding to dissipate static buildup, the shock that may result can damage sensitive electrical equipment or, at a minimum, scare people into thinking there is a problem with the system.
The critical idea to remember is that if there is more than one ground rod in a system, they all need to be connected together to keep component-to-component voltage at an equal level. This also applies to parts of the system that are themselves effective ground rods—such as solar-electric array mounting poles or wind turbine tower anchors. These all need to be interconnected with the entire grounding system.
Equipment Grounds. The equipment ground is the second part of a grounding system. Confusingly, the NEC refers to the equipment ground simply as “bonding.” The idea is to ensure a reliable interconnection of all metal enclosures and metallic components to each other and to the ground rod. Then, any errant current or ground-fault current can be conducted reliably, causing the circuit breaker to trip and protecting the wire or device on that circuit.
The equipment ground also prevents shock if you happen to touch two different parts of an electrical system by ensuring that they are at the same voltage potential. The equipment ground is often accomplished with an additional wire, but in some situations specific grounding screws and even metallic conduit connecting system components are used.
There can (and should) be multiple equipment ground connections of metal enclosures, raceways, and components in a system. The more equipment grounding connections, the better—they will provide redundant paths for a ground-fault current, guaranteeing that a breaker will trip. For example, most inverters used in battery-based systems will have an AC and DC ground terminal—both should be used. Often the inverter will be mounted on a metal rack or panel, which should also be connected to the grounding system. Redundant equipment grounds can also help to reduce radio and TV interference problems by providing more paths to dissipate the radiated and conducted emissions produced by some system components.
Grounded Conductor. In the United States, the NEC requires nearly all systems to have one of the current-carrying conductors connected to the grounding system. This conductor is then typically called the “neutral” in an AC system and the “negative” in a DC system. Because this conductor is connected to the grounding system, it will be at the same electrical potential in reference to ground if the system is properly wired.
These AC neutral and DC negative wires are called the grounded conductor, and they are not the same as the grounding conductor. The grounded conductor normally carries current, while the grounding conductor only carries current when a problem occurs, which results in a ground fault situation.
In an AC electrical system, the entire grounding system must only be connected to one of the current-carrying conductors at a single point. If an installation has both AC and DC systems, the AC neutral and the DC negative conductors will each be connected to the grounding system at separate points. Under NEC regulations, the AC and DC systems are considered to be separate electrical systems even though they are interconnected. Incorrectly connecting the grounded conductor at two separate points will result in the grounding system carrying current under normal operation. This is referred to as a “ground fault” and can cause equipment failure or damage, and hazards such as energized metallic surfaces and possible electric shock.
There isn’t any special location in, or name for where the grounding system and the current-carrying conductor should be connected. In most AC systems, the connection between the current-carrying conductor and the grounding system is made between the neutral and ground bus bars in the AC breaker panel. In a DC system, it’s usually located in the DC disconnect enclosure or factory-made inside the inverter itself. This connection should not be made at a backup generator or a battery. These are serviceable parts of a system that may be removed or reinstalled improperly, creating a potential fire or electric shock hazard if the system becomes ungrounded as a result.
The next time you discuss the ins and outs of grounding, be sure you don’t confuse the purposes and terms—it’s easy to do! If you first clarify which of the three parts of the grounding system is involved, and if you use the proper terms, I think you’ll find discussing grounding much easier—even productive—for a change. Who knows, you might even find that you can agree with other RE nerds when the topic of grounding comes up, yet again, around the campfire.
Christopher Freitas, OutBack Power Systems Inc., 19009 62nd Ave. NE, Arlington, WA 98223 • 360-435-6030 • Fax: 360-435-6019 • www.outbackpower.com