Grounding electrodes are required to be installed and connected to electrical services and to some feeders or branch circuits in separate buildings remotely located from the service. Is the grounding electrode conductor permitted to be installed inside the building, and if so, what is the permissible length of the grounding electrode when located inside the building?

Section 250.4 in the 2002 National Electrical Code (NEC) provides the performance requirements for grounding and bonding of both grounded and ungrounded electrical systems. The remainder of Article 250 provides the prescriptive methods or, in other words, the actual methods required for grounding and bonding the electrical system.

To understand the issue of installing a grounding electrode conductor inside a building, one must first understand the purpose and the functions of the grounding electrode and the grounding electrode conductor. The grounding electrode must be connected to earth and to the electrical equipment in such a manner that establishes a zero difference of potential between earth and the electrical equipment. This zero difference of potential will help stabilize voltage for the electrical system. The grounding electrode conductor connects the grounding electrode to the electrical system.

Not only is the grounding electrode system used for stabilizing the voltage for the electrical system but it is additionally used to limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines. If lightning hits the electrical equipment within or on the building or affects the utility company distribution lines in proximity to the electrical system on the premises, connection to a ground plane will provide a path to ground to dissipate the current of a line surge and provide a reference to ground for the high voltage. Providing a zero reference for the electrical system to a ground plane surrounding the system will limit the high-voltage peak by ensuring everything in close proximity to the equipment will be at the same potential. The grounding electrode system will help dissipate the high voltage and associated current from the lightning strike.

Where overhead utility company electrical conductors are installed from pole to pole, high-voltage conductors are usually installed at the top of the poles with lower voltage conductors installed below the high-voltage lines. These different voltage systems are usually kept sufficiently far enough apart to prevent inadvertent touching of the two system conductors. When wind, tree limbs or other influences cause connection between the two different voltage systems, line-voltage surges can occur, causing potential damage to the electrical equipment and the electrical system. Providing a zero reference by connecting the electrical system to a stable ground plane will help mitigate damage to the electrical system from these line surges.

The ground plane surrounding the electrical equipment can become saturated with electrical current due to the longevity of the surges, lightning strikes or unintentional contact with higher voltage lines in the area, thereby causing the spurious current to find an alternate path to ground. Where these alternate current paths are established, heavy arcing can occur with a potential for fire if combustible materials are in proximity to the arcing components. Fortunately, most lightning strikes, surges and unintentional contact with higher voltage systems last for very short periods of time.

Section 250.64 in the 2002 NEC provides grounding electrode conductor installation requirements with information on securing the grounding electrode conductor to a building surface and details on providing physical protection for this conductor. The grounding electrode must be installed in one continuous length without a splice or a joint, unless spliced by an irreversible compression-type connector listed for the purpose or by an exothermic welding process.

Having established that the general purpose of the grounding electrode system is to provide a path to ground for high-voltage surge currents and lightning strikes, keeping the grounding electrode conductor outside the building would be common sense. However, the NEC does not require this conductor to be routed outside of the building.

For example, if a concrete-encased electrode has been installed on one side of a building and the service on the other side, an electrician could install the bare grounding electrode conductor up the side of the building, through the attic, and down to the service equipment on the other side. In this case, any current from surges, lightning and unintentional high voltage would then be routed along the grounding electrode conductor connected at the electrical service, through the attic, and down to ground on the far side of the building. This could create a very dangerous situation with the possibility of fire and destructive voltages in the building.

Understanding the basics for grounding electrode systems should prompt the installation of grounding electrode conductors on the outside of the building or in the ground surrounding the building rather than inside, whenever possible. 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.