System and Equipment Grounding Safety
Grounding is used to provide a safe path for a fault current to flow.
Grounding is an integral part of any properly operating electrical system. In residence, grounding protects the occupants by providing a safe pathway for unwanted electricity that might otherwise create a hazard. Electricity always takes the easiest flow path to earth. A ground is a low-resistance conducting connection between electrical circuits, equipment, and the earth.
Grounding is used to provide a safe path for a fault current to flow. A complete ground path must be maintained when installing switches, light fixtures, appliances, and receptacles. In a properly grounded system, the unwanted current flow blows fuses or trips circuit breakers. Once a fuse is blown or a circuit breaker is tripped, the circuit is open, and no additional current will flow.
Grounding is usually done at two levels: system grounding and equipment grounding. The system ground is a special circuit designed to protect the entire distribution system of a residence. Equipment ground is essentially a circuit designed to protect individual components of an electrical system. Grounded conductors are used to providing a path to the ground for system and equipment grounds.
A grounded conductor is one that has been grounded on purpose. Grounded conductors are typically identified with green or green and yellow markings and may be installed as bare conductors.
The primary function of system grounding is to protect the service entrance wiring and the circuits connected to it. There are several methods of grounding an entire system. The two most popular methods used for grounding an electrical system are electrode grounding and water pipe grounding (see Figure 1). Other grounding methods use a concrete-encased electrode or a ground ring, both of which are less common in residential wiring systems.
Figure 1. System grounding methods include the use of an electrode ground, water pipe ground, concrete-encased electrode, or ground ring. Image courtesy of NI
An electrode is a long metal rod used for grounding that makes contact with the earth. When no satisfactory grounding electrode is readily available, the common practice is to drive one or more metal rods (connected in parallel) into the ground. The electrode and circuit must provide a flow path to the earth with less than 25Ω of resistance.
Water Pipe Grounding
A water pipe ground uses the underground metal pipe that supplies a residence with water and is typically the best electrical ground for a residential electrical system. Water pipes work well as grounds because the large surface area of the pipe is in contact with the earth, as it connects the municipal water main to the water distribution system in residence. This large surface area reduces resistance and allows any unwanted electricity to easily pass through the pipe to the earth. When a water pipe is used for grounding, the water pipe run must never be interrupted by a plastic fitting or have an open section of plumbing. Water meters are a source of an open ground circuit when removed. To provide protection when a water meter is removed, a shunt (or meter bonding wire) must be permanently installed. A shunt is a permanent conductor placed across a water meter to provide a continuous flow path for ground current.
All internal piping systems capable of becoming energized must be bonded and connected. A bonding conductor is a reliable conductor that ensures the electrical conductivity between two metal parts that must be connected electrically. The term "grounding conductor" no longer appears by itself in the NEC. Instead, conductors are referred to by their function, such as "grounding electrode conductor," "bonding conductor," or "equipment grounding conductor."
Equipment grounding's main purpose is to protect individual electrical devices. Equipment grounding safely grounds any devices or appliances attached to an electrical system or plugged into receptacles inside a home. For example, when a refrigerator has not been properly grounded, the electrical current caused by a short will seek the easiest path to earth. Unfortunately, the human body is an electrical conductor and allows current to reach the earth by traveling through the body (electric shock). Proper equipment grounding protects the body by harmlessly conducting unwanted electricity to the ground (see Figure 2).
Figure 2. Equipment grounding's main purpose is to protect individual electrical devices. Image courtesy of VFC
Grounding Small Appliances
Small appliances are easily incorporated into a grounded system. Most small electrical appliances are designed with three-prong grounded plugs that match a standard three-prong grounded receptacle (see Figure 3). The U-shaped blade of the plug and the U-shaped hole in the receptacle are the ground connections. The U-shaped blade of a plug is longer than the current-carrying blades. The added length ensures a strong ground connection while the plug is being inserted or removed from a receptacle. The ground wire is connected to all receptacles and metal boxes to provide a continuous pathway for short-circuit current. The ground wire may be connected to each box using a pigtail, screw, or ground clip.
Figure 3. Small appliances are easily incorporated into a grounded system, as most small electrical appliances are designed with three-prong grounded plugs that match standard three-prong grounded receptacles. Image courtesy of APOGEE
According to the NEC, an equipment grounding conductor (EGC) provides a ground-fault current path and connects the non-current-carrying metal parts of the equipment together and to the system ground and/or grounding electrode in order to establish a direct path to earth.