~ National Lightning Safety Institute ~

Section 5.3.1

Prominent lightning engineers and major technical codes and standards agree as to proper grounding guidelines. We present summaries of those generally accepted designs.

1. From Golde, Lightning, Academic Press, NY, 1977, vol. 2, chapter 19 by H. Baatz, Stuttgart, Germany, p. 611:

"Equalization of potentials should be effected for all metallic installations. For lightning protection of a structure it is of greater importance than the earthing resistance...

The best way for equalization of potentials utilizes a suitable earthing system in the form of a ring or foundation earth. The downconductors are bonded to such a ring earth; additional earth electrodes may be unnecessary…"

2. From Sunde, Earth Conduction Effects in Transmission Systems, Van Nostrand, NY, 1949, p. 66:

"Adequate grounding generally requires that the resistance of the ground , at the frequency in question, be small compared to the impedance of the circuit in which it is connected. By this criterion, it may be permissible in some instances to have a ground of high resistance, several thousand ohms, as in the case of "electrostatic" apparatus ground, the impedance to ground of insulated apparatus cases being ordinarily quite high. In other [situations], however, a resistance of only a few ohms may be required for effective grounding."

3. From Horvath, Computation of Lightning Protection, Research Studies Press, London, 1991, p. 20:

"The earthing of the lightning protection system distributes the lightning current in the soil without causing dangerous potential differences. For this purpose the most effective earthing encloses the object to be protected. The potential increases on the earthing and on all earthed metal parts of the object relative to the zero potential at a distant point. It may reach a very high value but it does not cause any danger if the potential differences inside the object to be protected are limited. Potential equalization is realized by the bonding of all extended metal objects."

4. From Hasse, Overvoltage Protection of Low Voltage Systems, Peter Peregrinus Press, London, 1992, p. 56.

''Complete lightning protection potential equalization is the fundamental basis for the realization of internal lightning protection; that is the lightning overvoltage protection for the electrical and also the electronic data transmission facilities and devices in buildings. In the event of a lightning stroke, the potential of all installations in the affected building (including live conductors in the electrical systems with arrestors) will be increased to a value equivalent to that arising in the earthing system -- no dangerous overvoltages will be generated in the system…

Nowadays lightning protection potential equalization is considered indispensable. It ensures the connection of all metal supply lines entering a building, including power and communication cables, to the lightning protection and earthing system by direct junctions across disconnection spark gaps, or arrestors in the case of live conductors."

5. From IEEE Emerald Book, Powering and Grounding Sensitive Electronic Equipment, IEEE Std 1100-1992, IEEE, NY, 1995, p. 216:

"It is important to ensure that low-impedance grounding and bonding connections exist among the telephone and data equipment, the ac power system's electrical safety-grounding system, and the building grounding electrode system. This recommendation is in addition to any made grounding electrodes, such as the lightning ground ring. Failure to observe any part of this grounding requirement may result in hazardous potential being developed between the telephone (data) equipment and other grounded items that personnel may be near or might simultaneously contact."

6. From International Standard IEC 1024-1, Protection of Structures Against Lightning, International ElectroTechnical Commission, Geneva, 1991, p. 23:

"In order to disperse the lightning current into the earth without causing dangerous overvoltages, the shape and dimensions of the earth-termination system are more important than a specific value of the resistance of the earth electrode. However, in general, a low earth resistance is recommended.

From the viewpoint of lightning protection, a single integrated structure earth termination is preferable and is suitable for all purposes (i.e. lightning protection, low voltage power systems, telecommunication systems).

Earth termination systems which must be separated for other reasons should be connected to the integrated one by equipotential bonding…"

7. From FAA-STD-019b, Lightning Protection, Grounding, Bonding, and Shielding Requirements for Facilities, Federal Aviation Administration, Washington DC, 1990, p. 20:

"The protection of electronic equipment against potential differences and static charge build up shall be provided by interconnecting all non-current carrying metal objects to an electronic multi-point ground system that is effectively connected to the earth electrode system."

8. From MIL-STD-188-124B, Grounding, Bonding and Shielding, Department of Defense, Washington DC, 1992, p. 6 and p. 8:

"The facility ground system forms a direct path of known low voltage impedance between earth and the various power and communications equipments. This effectively minimizes voltage differentials on the ground plane which exceed a value that will produce noise or interference to communications circuits." (p.6)

"The resistance to earth of the earth electrode subsystem should not exceed 10 ohms at fixed permanent facilities." (p. 8)

9. From MIL-STD-1542B (USAF), Electromagnetic Compatibility and Grounding Requirements for Space Systems Facilities, Department of Defense, Washington DC, 1991, p. 19:

"This Standard, MIL-HDBK-419, and MIL-STD-188-124 do not recommend the use of deep wells for the achievement of lower impedance to earth. Deep wells achieve low dc resistance, but have very small benefit in reducing ac impedance. The objective of the earth electrode subsystem is to reduce ac and dc potentials between and within equipment. If deep wells are utilized as a part of the earth electrode subsystem grounding net, the other portion of the facility ground network shall be connected to them."

10. From National Electrical Code, NEC-70-1996, National Fire Protection Association, Quincy MA, 1996, Article 250 - Grounding, p. 120 & p. 144:

"Systems and circuit conductors are grounded to limit voltages due to lightning, line surges, or unintentional contact with high voltage lines, and to stabilize the voltage to ground during normal operation. Equipment grounding conductors are bonded to the system grounded conductor to provide a low impedance path for fault current that will facilitate the operation of overcurrent devices under ground-fault conditions." (p. 120)

"Metal Underground Water Pipe. A metal underground water pipe in direct contact with the earth for 10 ft. (3.05 m) or more (including any metal well casing effectively bonded to the pipe) and electrically continuous (or made electrically continuous by bonding around insulating joints or sections or insulating pipe) to the points of connection of the grounding electrode conductor and the bonding conductors. Continuity of the grounding path or the bonding connection to interior piping shall not rely on water meters or filtering devices and similar equipment. A metal underground water pipe shall be supplemented by an additional electrode of a type specified in Section 250-81 or in Section 250-83. The supplemental electrode shall be permitted to be bonded to the grounding electrode conductor, the grounded service-entrance conductor, the grounded service raceway, or any grounded service enclosure." (p. 145)

11. From MIL-HDBK-419A, Grounding, Bonding, and Shielding for Electronic Equipments and Facilities, Department of Defense, Washington DC, 1987, p. 1-2, p. 1-6, p.1-102 and p. 1-173:

"The value of 10 ohms earth electrode resistance recommended in Section 1.2.3.1a represents a carefully considered compromise between overall fault and lightning protection requirements and the estimated relative cost of achieving the resistance in typical situations." (p. 1-2)

"At fixed C-E facilities, the earth electrode subsystem should exhibit a resistance to earth of 10 ohms or less." (p.1-6)

"All metallic pipes and tubes (and conduits) and their supports should be electrically continuous and are to be bonded to the facility ground system at least at one point." (p. 1-102)

"Water pipes and conduit should be connected to the earth electrode subsystem to prevent ground currents from entering the structure." (p. 1-173)