Current Affairs: More About Ground
The NEC makes a distinction between three types of ground conductors. Understanding their differences is important to designing, installing, and maintaining a safe system and avoiding some of the pitfalls that we'll discuss in future columns.
Good EarthThe grounding electrode conductor connects the neutral of the power source to earth. It terminates in one or more grounding electrodes, the devices that establish the electrical connection to the earth. Grounding electrodes are buried conductors, and may include metal water pipes, bare copper wire, driven ground rods, concrete-encased structural steel, or other forms. Typically two or more of these will be interconnected and connected to the service neutral by the grounding electrode conductor.
This conductor's job is to provide a low-impedance connection from earth to the neutral as a return path for current should a circuit supplied from the service inadvertently contact earth or a structure connected to earth. The conductor size is specified based on the size of the service entrance conductors and it is required to be installed continuously without splices unless they are of the irreversible compression or exothermic-welded type.
Artificial EarthOnce you leave the service, the role of earth in the grounding system is no more. The equipment grounding conductor is what most of us recognize as the "ground wire," or the fourth wire, color-coded green, that is run with electrical circuits. This conductor is used to connect all metal conduits and equipment enclosures to the service ground point. The equipment grounding conductor inside a building serves the same role as earth outside, providing a low-impedance return path for leakage or fault currents in the event of insulation failure or inadvertent contact of an energized conductor with an enclosure.
The size of the equipment grounding conductor for each circuit is also specified, in this case based on the rating of the fuse or circuit breaker protecting the circuit. If proper connections are used which maintain electrical continuity (bonding), metal conduits themselves may serve as the equipment grounding conductor without the need to provide an additional conductor in the circuit. An important requirement is that the equipment grounding conductor either be the conduit itself or be contained within the conduit along with the circuit conductors; physical separation between conductors increases the impedance of the current path and reduces the effectiveness of the ground. An equipment grounding conductor must be provided for every circuit.
Grounded vs. GroundingGrounded conductor is the term for the neutral circuit conductor. The distinction between grounded and grounding is important. The neutral is a grounded conductor by virtue of the connection at the service, but is not a grounding conductor because it is not used to connect anything else to ground. It is only used to carry the normal load current of lights, outlets, or other devices that are connected from phase to neutral.
The grounded conductor remains isolated from ground everywhere except for the bond at the service. If more than one connection to ground is made, load neutral currents will divide between the grounded conductor and the equipment grounding conductors. This can result in continuous current flow on conduit systems or metal structures and piping, which can cause electrolytic corrosion and interference with sensitive electronic equipment due to radiated magnetic fields.