Grounding Communications Systems
in Accordance with the 2002 NEC
By Mike Holt, for EC&M magazine.
Edited extract from my illustrated grounding and bonding textbook ... [ View More ]
Grounding is the intentional connection to earth through a connection of sufficiently low impedance. Its purpose as it relates to communications equipment is to assist in preventing the destruction of electrical components, and property damage from superimposed voltage from lightning and voltage transients. In addition, grounding (the connection to the earth) helps in reducing static charges on equipment surfaces to ensure the proper performance of sensitive electronic equipment.
One of the primary purposes of grounding communications equipment to the earth is to reduce high voltage from lightning from entering into the building or structure via metal raceways or cables. If the metal parts of communication equipment are not grounded in accordance with the NEC, much of the high energy from the lightning strike will be dissipated within the structure, which can result in equipment and property damage as well as the potential for electric shock. [ View Sample Graphic ]
Grounding (the connection to the earth) also helps in reducing the build-up of static charges on equipment and material and it establishing a zero voltage reference point to ensure proper performance of sensitive communications equipment.
Ground Resistance. The resistance (actually the impedance) of the ground impacts the effectiveness of shunting high-voltage surges into the earth. The resistance is dependent on the resistance of the electrodes, the termination resistance, the contact resistance of the electrodes to the adjacent earth, and the soils resistivity, which varies throughout the world.
Soil resistivity has the greatest influence on ground resistance. Soil resistivity is significantly influenced by the soils electrolytes, which consist of moisture, minerals and dissolved salts. Because soil resistivity changes with moisture content, the resistance of any grounding system will vary with the seasons of the year. Since moisture becomes more stable at greater distances below the surface of the earth, grounding systems appear to be more effective if the electrode can reach the water table. In addition, having the electrode below the frost line helps to insure less deviation in the systems resistance year round.
The resistance can be lowered by installing multiple electrodes that are properly spaced apart or by chemically treating the earth around the electrode. There are many readily available commercial products for this purpose.
Suitable Ground. The NEC specifies that any of the following nearest accessible locations are considered suitable as ground for communications equipment [800.40(B)(1), 810.21(F), 820.40(B)(1) and 830.40(B)(1)].
The service equipment enclosure, or The building or structure grounding electrode conductor or the grounding electrode conductor metal enclosures.
Any grounding electrode added for communications systems is required to be bonded with a 6 AWG or larger conductor to the building or structure grounding electrode system in accordance with 800.40(D), 810.21(J), 820.40(D), and 830.40(D).
The grounding of power and communications systems to the same single point ground helps in reducing the difference in voltage potentials between the systems. This becomes very important where these different systems are integrated together in sensitive and expensive electronic equipment. Graphic
Ground Termination. The termination of the grounding conductors to the grounding electrode must be by exothermic welding, listed lug, listed pressure connector, or by listed clamp. Termination fittings that are concrete-encased or buried in the earth shall be listed for direct burial and marked DB [800.40(C), 820.40(C) and 830.40(C)].
Telecommunications Systems [Article 800]. The National Electrical Code requires the metallic members of the telecommunication cable sheath, where required to be grounded by 800.33 and primary protectors to be grounded to an electrode as close as practicable to the point of entrance of the cable to the building or structure [800.33]. The grounding conductor shall be insulated and shall not be smaller than 14 copper AWG and its length shall be as short as practicable run in as straight a line as practicable [800.40].
In one- and two-family dwellings, the grounding conductor shall not exceed 20 ft in length. Where it is not practicable to limit the grounding conductor to 20 ft, the primary protector can be grounded to a separate 5 ft communications ground rod [800.40(B)(2)(2)] that must be bonded to the power grounding electrode system with a conductor not smaller than 6 AWG [800.40(D)].
Radio and Television Equipment [Article 810]. The antenna mast that supports radio, HAM, television and satellite receiving antennas must be grounded [810.15]. In addition, each conductor (coaxial, control, and signal conductors) of a lead-in from an "outdoor antenna" must be provided with a listed antenna discharge unit (grounding block). The antenna discharge unit shall be grounded and it must be located outside or inside as near as practicable to the entrance of the conductors to the building and it must not be located near combustible material [810.20].
The grounding conductor for the mast and discharge unit shall not be smaller than 10 copper AWG and its length shall be as short as practicable run in as straight a line as practicable [800.21].
Grounding of the antenna mast and lead-in cables in accordance with the NEC is somewhat effective in protecting receiving equipment from voltage surges, as well as voltage transients from lightning. If the mast is not properly grounded, the Low Noise Block (LNB), as well as the dc rotor motors that control the positioning larger satellite dishes often will be destroyed by voltage surges caused by nearby lightning strikes. If the lead-in from an outdoor antenna is not properly earth grounded in accordance with the NEC, the receiver can be destroyed by voltage surges caused by nearby lightning strikes.
CATV [Article 820]. The metallic sheath of CATV cable entering a building or structure must be grounded to the earth as close as practicable to the point of entrance to the building or structure [820.33]. The grounding conductor shall be insulated and shall not be smaller than 14 copper AWG and its length shall be as short as practicable run in as straight a line as practicable [800.40].
In one- and two-family dwellings, the grounding conductor shall not exceed 20 ft in length. Where it is not practicable to limit the grounding conductor to 20 ft, the coaxial cable shall be grounded to a separate 8 ft ground rod [250.52(A)(5)] that must be bonded to the power grounding electrode system with a conductor not smaller than 6 AWG [820.40(D)].
Network-powered Broadband Communications Systems. The metallic sheath of network-powered broadband communications systems cable entering a building or structure must be grounded to the earth as close as practicable to the point of entrance to the building or structure [830.33]. Selecting a grounding location to achieve the shortest practicable grounding conductor helps in limiting potential differences between the network-powered broadband communications circuits and other metallic systems [830.30 FPN].
The grounding conductor shall be insulated and shall not be smaller than 14 AWG and shall have a current-carrying capacity approximately equal to the grounded metallic member and protected conductor of the network-powered broadband communications cable. The grounding conductor shall not be required to exceed 6 AWG [830.40].
In one- and two-family dwellings, the grounding conductor shall not exceed 20 ft in length. Where it is not practicable to limit the grounding conductor to 20 ft, the grounded conductor shall terminate to a separate 5 ft communications ground rod [830.40(B)(2)(2)] that must be bonded to the power grounding electrode system with a conductor not small than 6 AWG [830.40(D)].
Other important communications grounding standards that should be considered includes:
Failure to properly ground communications systems to the building or structure grounding electrode system as required by the NEC can result in electric shock, the destruction of electrical components, and property damage. According to insurance industry data, improper grounding of communications systems has led to $500 million dollars per year of property or equipment damage to lightning.
Mike Holt's Comment: Please let me know if you have any suggestions on how to make this article better, Mike@MikeHolt.com
Copyright © 2003 Mike Holt Enterprises,Inc.