ERICO Practical Guide to Electrical Grounding - Manual (Page 12)

BUILDING GROUNDING
Electrical design and installation professionals need to
consider several different building grounding systems for
any building or structure on which they may work. Building
grounding components can be broken down into several
subdivisions:
• The building exterior grounds
• The electrical service grounding
• The building interior bonding
• Equipment grounding and bonding
• Lightning protection
This chapter will look at the first two items. Lightning
protection will be covered in Chapter 2, interior bonding
and grounding will be covered in Chapter 3 and equipment
grounding and bonding in Chapter 4.
BUILDING EXTERIOR GROUNDS
It is important to keep in mind that the requirements
contained in the NEC constitute minimum electrical instal-
lation requirements. For many types of installations, the
requirements listed in Article 250 of the NEC do not go far
enough. These minimum requirements cannot ensure that
the equipment operated in these buildings will perform in a
satisfactory manner. For these reasons electrical design
personnel often will require additional grounding
components. One of the most common of these consists of
a copper conductor that is directly buried in the earth and
installed around the perimeter of the building. The steel
building columns are bonded to this conductor to complete
the grounding system.
The columns around the perimeter of the building are
excellent grounding electrodes and provide a good path into
the earth for any fault currents that may be imposed on the
system. The electrical designer, based on the size and usage
of the building, will determine whether every column or
just some of the columns are bonded. ERICO recommends
that at least one column every 50 feet shall be connected to
the above described ground ring. (Fig. 1-1)
When grounding large buildings, and all multiple building
facilities, perimeter grounding provides an equipotential
ground for all the buildings and equipment within the
building that are bonded to the perimeter ground. The
purpose of this perimeter grounding is to ensure that an
equipotential plane is created for all components that are
connected to the perimeter ground system. The size of the
ground ring will depend upon the size of the electrical
service but is seldom less than 1/0 AWG copper. In some
cases, an electrical design requires ground rods to be
installed in addition to the perimeter ground ring. The use
of ground rods helps to minimize the effects of dry or
frozen soil on the overall impedance of the perimeter
ground system. This is because the ground rods can reach
deeper into the earth where the soil moisture content may
be higher or the soil may not have frozen. ERICO offers a
complete line of ground rods from 1/2 inch to 1 inch in
diameter to meet the needs of the designer and installer. It
is recommended that the ground ring and ground rods be
copper or copperbonded steel and installed at least 24 inch
from the foundation footer and 18 inch outside the roof drip
line. This location will allow for the greatest use of the
water coming off of the roof to maintain a good soil
moisture content.
Although less common than in the past, “triad” ground rod
arrangements (rods placed in a triangular configuration) are
sometimes specified, usually at the corners of the building
or structure. Figure 1-2 shows possible conductor/ground
rod configurations. Triad arrangements are not
recommended unless the spacing between the ground rods
is equal to or greater than the individual ground rod length.
Three rods in a straight line spaced at least equal to the
length of the individual ground rods are more efficient and
result in a lower overall system impedance.
Installers of these perimeter ground systems need to
provide a “water stop” for each grounding conductor that
passes through a foundation wall. This is especially
important when the grounding conductor passes through
the foundation wall at a point that is below the water table.
The water stop ensures that moisture will not enter the
building by following the conductor strands and seeping
into the building. A CADWELD Type SS (splice) in the
unspliced conductor and imbedded into the concrete wall
provides the required water stop (Fig. 1-3).
4 Practical Guide to Electrical Grounding
3'-0"
Grade
Typical Installation
Weld At Column Base.
First Floor
2'-0"
Typical Down Conductor
Fig. 1-1
Grounding Book 4/14/99 10/5/99 6:01 PM Page 4 (Black plate)