The Grounding Chain
The performance of the
grounding system is determined by the quality of the following five components
all of which Are of equal importance.
The
Grounding Electrode Conductor. Typically made from copper or copper-bonded
steel, the grounding electrode conductor must be large enough to withstand the
maximum available fault current over the maximum clearing time
The Grounding
Connections. Often overlooked, the grounding connections are used to tie the
elements of the electrode system together. Exothermically
welded connections provide a molecular bond that will never loosen or corrode.
Mechanical connectors, such as crimp, bolted, and wedge type, rely on physical
point to-point surface contact to maintain the integrity of the electrical
connection. IEEE® Standard 837 provides detailed information on the application
and testing of permanent grounding connections. ERICO can provide an
independent, third-party test report evaluating the performance of these
connectors in accordance with the testing procedures set forth in IEEE 837
Standard for Qualifying Permanent Substation Grounding Connections.
The Grounding
Electrode. The grounding electrode provides the physical connection to the
earth and is the instrument used to dissipate current into it. There are two
main types of electrodes. “Natural” electrodes are intrinsic to the facility
and include metal underground water pipe, the metal frame of the building (if
effectively grounded), and reinforcing bar in concrete foundations.
“Made” electrodes are
installed specifically to improve the performance of the ground system and
include wire meshes, metallic plates, buried copper conductor and rods or pipes
driven into the ground. The ground rod is
the most widely used
electrode.
Electrode to Soil
Resistance. Amount of rod surface and rod replacement are the controlling factors.
Doubling diameter reduces resistance by only 10% and is not cost effective.
Doubling rod length, however, theoretically reduces resistance by 40%. The most
common solution is proper placement of multiple rods that are driven to the required
depths.
The Soil. The soil resistivity, measured in
ohm-centimeters or ohm-meters, plays a significant role in determining the
overall performance of the grounding system and
must be known before a proper grounding system
can be engineered. Measuring soil resistivity allows the design engineer to
locate an area with the most conductive soil
and to determine the depth of the conductive soil so that electrodes can
be placed accordingly.
The grounding system will carry little or no
current for long periods of time until a fault occurs or a lightning strike or
other transient requires dissipation. At that point, the grounding system
components will be expected to perform like new while conducting large amounts
of current.
Most of the grounding system is concealed below
grade, making inspection of
the grounding components difficult or impossible. The underground environment
is a harsh one. The initial selection of the components used in the grounding
system is of critical importance to its long-term effectiveness.