ERICO Practical Guide to Electrical Grounding - Manual (Page 11)

3
BUILDING GROUNDING —
AN OVERVIEW
Despite the electrical designers’ best efforts, electrical
ground faults, short circuits, lightning and other transients
can and often do occur in building electrical distribution
systems. ERICO believes that, besides attempting to
minimize the occurrence of these faults, designers and
installers of electrical grounding systems should design
these systems to clear these faults in the quickest possible
manner. This requires that the grounding system be
constructed to achieve the lowest practical impedance.
Many factors determine the overall impedance of the
grounding system. Building components, such as structural
steel and interior piping systems, can be used to create an
effective grounding system. The manner in which these
components are installed and interconnected can have a
dramatic effect on the overall effectiveness of the grounding
system. One of the primary factors that can increase the
impedance of the grounding system is the type and manner
in which the electrical connections to the grounding system
are made. ERICO has a complete line of connectors which
can be used to make grounding connections without
affecting the integrity of the grounding system. Contractors
and others who install these systems cannot underestimate
the importance of ensuring that each grounding connection
is made in a manner that is efficient and effective.
Interconnected electronic equipment, such as telecommuni-
cation systems and computer systems, also require a low-
impedance grounding system. Specific bonding and
grounding techniques are available and are covered in
Chapter 4, which will help to enhance the operation of this
sensitive electronic equipment.
Designers and installers of these systems will do well to
include all aspects of facilities protection in the initial
design. The figure below includes the major subsystems of
facilities grounding. Any omission of these subsystems by
design personnel is risky at best. Later additions and/or
modifications to the system can be very costly.
With these thoughts in mind, let’s look at the components
of the building grounding system and see how these
individual components impact the overall effectiveness of
the grounding system.
GROUND RESISTANCE
While many factors come into play in determining the
overall effectiveness of the grounding system, the resistance
of the earth itself (earth resistivity) can significantly impact
the overall impedance of the grounding system. Several
factors, such as moisture content, mineral content, soil type,
soil contaminants, etc., determine the overall resistivity of
the earth. In general, the higher the soil moisture content,
the lower the soil’s resistivity. Systems designed for areas
which typically have very dry soil and arid climates may
need to use enhancement materials or other means to
achieve lower soil resistivity. ERICO has products available
which help to reduce earth resistivity and maintain a low
system impedance. See the discussion on GEM™ on page
14.
Ground resistance is usually measured using an instrument
often called an earth resistance tester. This instrument
includes a voltage source, an ohmmeter to measure
resistance directly and switches to change the instrument’s
resistance range. Installers of grounding systems may be
required to measure or otherwise determine the ground
resistance of the system they have installed. The National
Electric Code (NEC), Section 250-84, requires that a single
electrode consisting of rod, pipe, or plate that does not have
a resistance to ground of 25 ohms or less shall be
augmented by one additional electrode of the type listed in
Section 250-81 or 250-83. Multiple electrodes should
always be installed so that they are more than six feet (1.8
m) apart. Spacing greater than six feet will increase the rod
efficiency. Proper spacing of the electrodes ensures that the
maximum amount of fault current can be safely discharged
into the earth.
To properly design a grounding system, the earth resistivity
must be measured. Several methods can be used to measure
earth resistivity: the four-point method, the variation
in-depth method (three-point method) and the two-point
method. The most accurate method and the one that ERICO
recommends is the four-point method. The details of
making these measurements and the set-up for the
measurements are included with the testing equipment.
Chapter 1: Building and Service Entrance Grounding
Fault Protection Subsystem
Lightning Protection Subsystem
Signal Reference Subsystem
Earth Electrode Subsystem
Grounding Book 4/14/99 10/5/99 6:01 PM Page 3 (Black plate)