Prevent Shocks in Your Communications
Understanding how the NEC relies on ground resistance theory leads to a successful application of communications grounding.
Grounding in communications systems can be confusing, but if you understand some basics, you can replace that confusion with the certainty you've done the job right. Grounding this equipment to earth serves two purposes:
The reduction of excessive current that enters a building or structure via metal raceways or cables [250.4(A)(2)].
The establishment of a zero voltage reference point on the system.
An effectively grounded system is connected to earth through ground connections of low enough impedance and sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazards to connected equipment or people . In grounding design, always start with impedance. An understanding of how it works can prevent considerable frustration.
What do we mean by “ground resistance?” In an AC system, this is the impedance the grounding system presents to the flow of current into the soil. Have you ever noticed nobody conducts ground resistance tests with a 480V generator? Instead, they use battery-powered equipment that takes DC measurements — resistance rather than impedance. These measurements provide meaningful results that can be applied to an AC system.
Ground resistance determines how well a system can shunt surges to the earth. Of all the factors that determine ground resistance, soil resistivity plays by far the largest role. Soil resistivity is largely a function of the electrolytes (minerals and dissolved salts) that exchange ions with surrounding materials. Particle size also plays a role. Moisture content assists in electrolyte exchanges to such an extent that you can think of it as a sort of control valve. Electrolyte levels generally take a long time to change, but moisture levels can change rapidly — even during a single day — and the effect of moisture level can be dramatic. Correct for this problem by digging deeper — moisture levels are more stable at greater depths, and grounding systems appear to be more reliable if the electrode reaches the water table. Installing the electrode below the frost line reduces deviation in system resistance.
To achieve the desired resistance level, you can add more electrodes (at the proper spacing) or you can lower the soil/electrode interface resistance. For example, suppose you need a very low resistance but the site is on rocky soil only a few feet above bedrock. Suppose soil testing reveals that driving standard rods at the minimum spacing would require more space than the site has available. You could install longer rods, but driving them into that soil would be problematic, if not impossible. Various electrode designs use extensive horizontal space — but require excavation and backfill. You could use a chemical ground rod, which is essentially a well that holds a rod surrounded by added electrolyte material.
Where do you make this earth connection? Any of the following are suitable as a ground for communications equipment [800.40(B)(1), 810.21(F), 820.40(B)(1), and 830.40(B)(1)] (Fig. 1 on page 72).
The building or structure grounding electrode system (250.50).
The grounded interior metal water piping system within 5 ft of the point of entrance into the building (250.52).
The power service accessible means external to enclosures (250.94).
The metallic power service raceway.
The service equipment enclosure.
The building or structure grounding electrode conductor or the grounding electrode conductor metal enclosures.
Bonding and termination.
A grounding electrode for communications systems must be bonded to the building or structure grounding electrode system with a 6 AWG copper or larger conductor [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 reduces the voltage potential differences between systems. This becomes crucial where these different systems are integrated (Fig. 2 above).
Termination fittings encased in concrete or buried in the earth must be listed for direct burial and marked “DB” [800.40(C), 820.40(C), and 830.40(C)]. You must terminate ground conductors to the grounding electrode with exothermic welding, or a listed lug, pressure connector, or clamp.
Two principles apply to all communications equipment: ground as close as practical to the main electrode, and use a short, insulated grounding conductor. The Code requirements are almost identical for both types:
“…must be grounded to an electrode as close as practicable to the point of entrance of the cable to the building or structure.” This applies to:
Metallic members of the telecommunications cable sheath and primary protectors (800.33).
Metallic sheath of CATV cable (820.33).
Metallic sheath of network-powered broadband communications systems (830.33).
Network-powered broadband communications systems, in an unspecified location (830.30 FPN).
“The grounding conductor shall…not be smaller than 14 AWG copper and its length shall be as short as practicable and run in as straight a line as practicable.” This applies to:
Metallic members of the telecommunications cable sheath and primary protectors (800.40); CATV (800.40).
Network-powered broadband communications systems. The grounding conductor needs not exceed 6 AWG copper (830.40).
Radio and television antennas, but these require 10 AWG copper (810.21).
Radio and television equipment [Art. 810] has specific requirements, as follows. The antenna mast that supports radio, HAM, television, and satellite receiving antennas must be grounded [810.15]. Each conductor (coaxial, control, and signal conductors) of a lead-in from an outdoor antenna must be equipped with a listed antenna discharge unit or grounding block. The antenna discharge unit must be grounded, and must be located outside or inside as near as practicable to the entrance of the conductors to the building. It may not be located near combustible material [810.20].
Failure to properly ground communications systems can result in electric shock and/or property damage. According to insurance industry data, improper grounding of communications systems has led to $500 million per year of property or equipment damage due to lightning. Make sure the systems you design and install don't become a part of this statistic.
Are you still confused by the Code? For additional information on Code-related topics please visit www.mikeholt.com or send an e-mail directly to the author at [email protected].vv