ERICO Practical Guide to Electrical Grounding - Manual (Page 57)

INTRODUCTION
In addition to electrical service grounding and supple-
mental building grounding, designers and installers of
electrical systems face critical grounding and bonding
decisions throughout the entire building. The purpose of
this chapter is to focus on the equipment grounding and
bonding requirements set forth in the National Electrical
Code (NEC). Keep in mind that the purpose of bonding is
different from that of grounding. Metallic components of
electrical systems are bonded to ensure electrical continuity
of the components. The purpose of bonding is to create an
equipotential plane that ensures that all metallic
components are at the same potential to ground. Grounding,
on the other hand, is an intentional connection to the earth
or some other conducting body that serves in place of the
earth. The purpose of grounding conductive materials, such
as metal raceways and equipment enclosures, is to limit and
stabilize the voltage to ground on such enclosures.
Unintentional contact with higher voltage lines or lightning
strikes results in increased voltages on the electrical
equipment. The most important reason however, for
grounding such enclosures is to provide a low impedance
path for ground-fault current. The low impedance path
ensures that the overcurrent device which is protecting the
conductors will operate. Several specific bonding
requirements are included in the NEC, covering topics such
as: service bonding, enclosure bonding, bonding over 250
volts, bonding of piping systems and exposed structural
steel, and swimming pools and fountain bonding.
Grounding requirements include: general equipment
grounding provisions, specific equipment grounding
provisions, grounding cord-and-plug connected equipment,
and receptacle grounding.
BONDING
Service Equipment Bonding. Section 250-71 of the NEC
contains the general provisions for bonding of service
equipment. Service equipment is any equipment necessary
for the main control and means of cutoff of the supply of
electricity to a building or structure. Specifically, the
following service equipment must be effectively bonded
together: service raceways, cable trays, service cable
armor/sheath, cablebus framework, service equipment
enclosures and any metallic raceways which contain a
grounding electrode conductor. Keep in mind that it is
critical that these components be effectively bonded
together to ensure the fastest possible clearing of faults.
This is because for most service entrance conductors the
only overcurrent protection provided is on the line side of
the utility transformer. In most cases the rating or setting of
these primary devices will not be adequate to protect the
service equipment if large magnitude fault currents are not
cleared promptly.
Installers should also be aware that Section 250-71 (b)
contains a frequently overlooked provision regarding the
interconnection of other systems which may be present in
the building or structure. This section requires that an
accessible means be left at the service equipment, in an
external location, which can be used for connecting
bonding and grounding conductors of other systems. Recall
that Section 250-54 requires a common grounding
electrode system to be installed and prohibits separate
grounding system installations. Installers of the service
equipment must provide a means for interconnecting the
grounding systems of communication circuits, radio and
television equipment and CATV circuits. Section 250-71
(b) lists three permissible methods to facilitate the intercon-
nection of these systems. The first option is to use the
exposed metallic service raceways. The second option is to
connect to the exposed GEC. The last option is to bond a
copper or other corrosion-resistant conductor of at least a
No. 6 AWG copper, to the service raceway or equipment.
ERICO has a complete line of CADWELD connections
and mechanical connectors that can be used to meet the
requirements of Sections 250-71, 800-40, 810-21 and
820-40.
Section 250-72 lists the permissible methods which can be
used to bond together the service equipment listed above.
Five basic methods are listed, any one of which can be used
to bond the service equipment together. The first method is
to use the grounded service conductor. On the line side of
the service equipment there is no separate equipment
grounding conductor. The grounded conductor assumes this
role on the line side of the service. Section 250-113 lists the
permissible means for any connection made to the
grounded conductor. These include CADWELD
exothermic welded connections, listed pressure connectors
(wirenuts), listed clamps, and other listed means. The
second method is to use threaded connections. This
includes threaded couplings or bosses. It is important that
these connections be made wrenchtight to ensure a low
impedance connection. The third method is to use
threadless couplings or connectors. These fittings are
available for rigid metal conduit, intermediate metal
conduit and EMT. Once again it is important that the
connections be made up wrenchtight to ensure the low
impedance ground path. Installers should note that the NEC
specifically prohibits the use of standard locknuts or
bushings, even if a double arrangement is used, (one on the
inside and one on the outside) to achieve the bonding
required by this section. The fourth method is to use
bonding jumpers. Bonding jumpers ensure electrical
continuity by providing a low impedance path across
concentric or eccentric knockouts that may be part of the
service equipment. The last method is to use other approved
49 Chapter 3: Building Interior Bonding & Grounding
Grounding Book 4/14/99 10/5/99 6:01 PM Page 49 (Black plate)