Quantcast Resistance Requirements

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MIL-HDBK-419A
2.2 RESISTANCE REQUIREMENTS.
2.2.1 General. The basic measure of effectiveness of an earth electrode is the value in ohms of the resistance
to earth at its input connection. Because of the distributed nature of the earth volume into which electrical
energy flows, the resistance to earth is defined as the resistance between the point of connection and a very
distant point on the earth (see Section 2.4). Ideally, the earth electrode subsystem provides a zero resistance
between the earth and the point of connection. Any physically realizable configuration, however, will exhibit a
finite resistance to earth. The economics of the design of the earth electrode subsystem involve a trade-off
between the expense necessary to achieve a very low resistance and the satisfaction of minimum system
requirements. This subsystem shall also interconnect all driven electrodes and underground metal objects of the
facilities including the emergency power plant. Underground metallic pipes entering the facility shall also be
bonded to the earth electrode subsystem.
2.2.2 Resistance to Earth. Metal underground water pipes typically exhibit a resistance to earth of less than
three ohms. Other metal elements in contact with the soil such as the metal frame of the building, underground
gas piping systems, well casings, other piping and/or buried tanks, and concrete-encased steel reinforcing bars
or rods in underground footings or foundations generally exhibit a resistance substantially lower than 25 ohms.
2.2.2.1 National Electrical Code Requirements. For the fault protection subsystem, the NEC (2-2) states in
Article 250 that a single electrode consisting of a rod, pipe or plate which does not have a resistance to ground
of 25 ohms or- less shall be augmented by one additional made electrode. Although the language of the NEC
clearly implies that electrodes with resistances as high as 25 ohms are to be used only as a last resort, this 25
ohm limit has tended to set the norm for grounding resistance regardless of the specific system needs. The 25
ohm limit is reasonable or adequate for application to private homes and other lower powered type facilities.
2.2.2.2 Department of Defense Communications Electronics Requirements. The above criteria however, is not
acceptable for C-E facilities when consideration is given to the large investments in personnel and equipment.
A compromise of cost versus protection against lightning, power faults, or EMP has led to establishment of a
design goal of 10 ohms for the earth electrode subsystem (EES) in MIL-STD-188-124A. The EES designed in
MIL-STD-188-124A specifies a ring ground around the periphery of the facility to be protected. With proper
design and installation of the EES, the design goal of 10 ohms should be attained at reasonable cost. At
locations where the 10 ohms has not been attained due to high soil resistivity, rock formations, or other terrain
features, alternate methods listed in Paragraph 2.9 shall be considered for reducing the resistance to earth.
2.2.3 Lightning Requirements. For lightning protection, it also is difficult to establish a definite grounding
resistance necessary to protect personnel. The current which flows in a direct lightning stroke may vary from
several hundred amperes to as much as 300 thousand amperes. Such currents through even one ohm of
resistance can theoretically produce hazardous potentials. It is impractical to attempt to reduce the resistance
of a facility to earth to a value low enough to absolutely prevent the development of these potentials.
Techniques other than simply achieving an extremely low resistance to ground must therefore be employed to
protect personnel and equipment inside a structure from the hazards produced by a direct stroke. Experience
has shown that a grounding resistance of ten ohms gives fairly reliable lightning protection to buildings,
transformers, transmission lines, towers, and other exposed structures. At some sites, resistances as low as one
ohm or less can be achieved economically. The lower the resistance, the greater the protection; therefore,
attempts should be made to reduce the resistance to the lowest practical value.
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