Communications systems and circuits in buildings must comply with the applicable rules in National Electrical Code (NEC) 2008 Article 800. Even though these systems operate at lower energy levels, improper grounding and bonding can result in severe consequences for equipment, property and people.
Grounding, in its simplest form, is the process of connecting an electrically conductive object to ground (the earth). Bonding is the process of connecting conductive objects together to equalize potential differences between them. When something is grounded, it is connected to the planet and when something is bonded to another, they are connected together to electrically become one potential or as close to the same potential as possible (NEC 250.4). These two processes work in unison to provide safety for communications systems and property.
The definitions in Article 100 provide a foundation on which grounding and bonding requirements are built. The term “ground” is defined simply as “the earth.” The term “bonded” is defined as something being “connected to establish electrical continuity and conductivity.” Grounded and grounding are terms defined as “connected to ground or to a conductive body that extends the ground connection.” The term “grounding conductor” is defined as a conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.”
Connecting to an electrode
Section 800.100(B) requires the grounding conductor for communications systems to be connected to the same grounding electrode that the building electrical system is connected to. This ensures both systems and connected equipment are at the same ground potential. Attempting to install separate grounding electrodes and not bond them to the power system grounding electrode is not permitted by the NEC and creates unsafe conditions for people and property. A revision in the 2008 NEC requires an “intersystem bonding termination” be installed at the service location for connecting systems covered by Chapter 8. It is intended specifically for connecting communications systems’ grounding and bonding conductors. Section 250.94 requires intersystem bonding terminations provide not less than three means of connecting grounding and bonding conductors of communications systems. Intersystem bonding terminations must be connected to the building power grounding-electrode system, so potential differences between both grounding systems are minimized.
Grounding conductor installation
Communications system grounding conductors must be 14 AWG or larger and be made of copper or other corrosion--resistant conducting material. They can be solid or stranded and must be insulated. Grounding conductors for communications systems should be kept short, and for one- and two-family dwelling installations, they must not exceed 20 feet. An exception permits a separate grounding electrode be installed where the grounding conductor length of 20 feet is exceeded. In this case, any separate electrode must be bonded to the power system grounding electrode for the building with a copper or equivalent conductor sized at a minimum of 6 AWG [800.100(A) (4) and 800.100(D)]. Connections to grounding electrodes for communications circuits must meet the requirements in 250.70. Listed connection means must be used and installed according to the manufacturer’s instructions.
Surges and lightning
Using the same grounding electrode as the building electrical service keeps the conductive parts of communications and equipment at or close to the same ground (earth) potential in normal operation. In abnormal events, such as surges related to lightning strikes on or close to the building, the objective is to keep conductive parts of electrical power systems and limited energy communications systems at the same potential while these potentials rise and fall.
This minimizes the possibilities of destructive flashover events within electronic equipment and between electrically conductive parts and equipment. If the grounding conductors of a communications system are connected to an electrode separate from the building power service grounding electrode, a lightning event on or near the building can cause conductive parts of equipment in the power system and the communications system to rise at different potentials, creating possible flashovers that can damage equipment or even start a fire.
JOHNSTON is NECA’s executive director of standards and safety. He is former director of education, codes and standards for IAEI; a member of the IBEW; and an active member of the NFPA Electrical Section, Education Section and the UL Electrical Council. Reach him at firstname.lastname@example.org.