~ National Lightning Safety Institute ~
Definition of Terms Used in Grounding
In order to promote a uniform understanding of grounding issues, the following glossary is presented by the National Lightning Safety Institute.
Concrete Encased Electrodes: The rebar in concrete can be an effective part of the grounding electrode subsystem. Since concrete is alkaline and hydroscopic (absorbent) in nature, this type ionizing and moist medium can create a large and effective earth sink by using the foundation ground of any AFS. It is critical, however, that the rebar be connected to the primary ground electrode, buried ring electrode and/or other ground points in keeping with the concept of a wholly-uniform and integrated single point ground for the entire facility. Concrete encased electrodes are recognized as a beneficial component of the earth electrode system.
Current Magnitudes: Typical lightning current magnitudes peak in the 20-30kA range. However, magnitudes over 400kA have been recorded. Approximately 3% of magnitudes measure above 100kA. IEEE recommends that lightning protection engineers use 40kA as a design threshold for lightning protection systems.
Deep Wells: Due the typical high cost of deep wells, other alternatives first should be explored. These include: additional ground rods; connection of perimeter security fences to augment the ground grid; radial buried ground wires or ground straps configured away from building corners; treatment or augmentation of soils with artificial backfills; and low-cost drip irrigation systems.
Driven Rods: Copper plated steel rods are driven below grade and connected to ground wires.
Earth Electrode Subsystem: A network of electrically interconnected rods, plates, mats, or grids installed for the purpose of establishing a low resistance contact to earth.
Equipotential Plane: A grid, sheet, mass, or masses of conducting material which, when bonded together, offers a negligible impedance to current flow.
Facility Ground System: The electrically interconnected systems of conductors and conductive elements that provide current paths to earth. The facility ground system includes the earth electrode subsystem, lightning protection subsystem, signal reference subsystem, fault protection subsystem, as well as the building structure, equipment racks, cabinets, conduits, junction boxes, raceways, duct work, pipes, towers, other antenna supports and other normally non-current carrying metallic elements.
Frequency and Skin Effect: Lightning is a high frequency, high current pulse. At high frequencies and high currents, energy is transmitted along conductors with high skin effect. Skin effect limits current flow to the extreme outer surfaces of conductors.
Ground: Usually meaning the same as dirt or soil or earth.
Ground Conductor Connections: Exothermic connections provide the lowest inductance and the highest reliability of all connection alternatives. Even a low inductance path in a lightning circuit can invite large voltage gradients, which in turn may facilitate arcing to alternative paths. Gradients over 50kV/m are common in both air and earth situations. Such arcing, known as "side flash," may be the result of tight bends in above-grade wire conductors.
Ground Electrode: A conductor (usually buried) for the purpose of providing an electrical connection to ground.
Ground Ring: A ground wire of No. 2 size encircling or surrounding a building, tower or other above-ground structure. Usually the ground ring should be installed to a minimum depth of 2.5 ft. and should consist of at least 20 ft. of bare copper conductor. It should be installed beyond the building drip line.
Halo Grounded Ring: A grounded No. 2 wire, installed around all four walls inside a small building, at an elevation of approx. six inches below the ceiling. There are drops installed from the halo to the equipment cabinets and to waveguide ports, interior cable trays etc. Halo rings serve as connector points to achieve ground references of interior metallic objects. These, in turn, are connected to the main ground bus bar.
Inductance and Voltage Potentials: Lightning will follow the path of lowest inductance. The higher the frequency, the higher the inductive reactance value in calculating the total impedance of the circuit. Resistive values can be eliminated for all practical purposes in high frequency lightning conductor calculations for distances approximately 2000 feet or less.
Impedance: The impedances of typical grounding electrode conductor wires linearly increase as a function of frequency.
Resistance of Electrode: Recommended IEEE practice is to provide a resistance of less than 25 ohms for any made earth ground electrode. Local conditions will vary this target figure. Figures of 10 ohms or less are standard practice in commercial codes and US Government tactical and long haul communications systems standards. Lower values, in the 1 to 5 ohm range, are useful only for electrical safety at dc and 50/60 Hertz. Lightning is an RF event, with typical HF characteristic impedances.
Shield: A housing, screen, or cover which substantially reduces the coupling of electric and electro-magnetic fields into or out of circuits or prevents the accidental contacts of objects or persons with parts or components operating at hazardous voltage levels.
Spark Gap: A short air space (dielectric) between two conductors.
Types of Connectors: a) Mechanical, as in a threaded clamp; b) Pressure, as in a compression clamp; c) Thermal, as in CADWELDÒ , which results in a exothermic or molecular connection. Thermal connectors are said to be bonded, or electrically-joined.