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Section 5.3.1:
RECOMMENDED GROUNDING GUIDELINES
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)
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