Most properties today,
whether residential, commercial, or industrial, include
at least one building or structure on that property.
Often there are multiple buildings on a single property.
Some include buildings that are each supplied by its own
utility service and others have an electrical service at
one point and deliver electrical power to the other
buildings or structures by feeder(s) or by branch
circuit(s). This article takes a closer look at the
grounding and bonding requirements and methods for
separate buildings or structures supplied from other
than a service.
When a building is constructed it
requires a solid foundation that connects the building
to the earth. Grounding for electrical systems and
equipment serves as the foundation for the electrical
system or service. Whether a building or structure on a
property is supplied by a service or by a feeder, a
grounding electrode is generally always required. The
electrode provides two important electrical functions.
First, where electrical equipment and systems are
connected to the earth through the grounding electrode
or grounding electrode system, differences of potential
between the earth and those conductive elements are
minimized. This connection to the grounding electrode
reduces shock hazards and maintains the same potential
during normal operation. During abnormal events, such as
ground faults and short circuits, the connection to
ground works to minimize potential differences during
the duration of the overcurrent device clearing time.
The second important benefit of the grounding electrode
at the separate building or structure is limiting
voltages imposed by lightning, line surges, or
unintentional contact with higher voltages.
Grounding electrical systems also
provides benefits of stabilizing voltage to ground
during normal system operation. It should be noted that
the grounding electrode and grounding electrode
conductor have little or no effect in the operation of
overcurrent protective devices. The earth or grounding
does not provide an effective fault current path.
There are two important definitions
one must fully understand to grasp the performance
concepts and criteria for grounding and bonding. Let’s
take a look at these two definitions and establish the
difference between the two terms.
Grounded. Connected to earth or
to some conducting body that serves in place of the
earth.
Bonding (Bonded). The permanent
joining of metallic parts to form an electrically
conductive path that ensures electrical continuity and
the capacity to conduct safely any current likely to be
imposed.1
Note: There are many derivatives of
these two terms used in a variety of ways throughout the NEC. Perhaps this is one reason for some
misunderstandings and misapplication of the grounding
and bonding rules. It is important to have a complete
understanding of the performance required from each of
the two terms grounding and bonding.
Clearly two separate performance
functions are described in the definitions of these
terms. To simplify their meaning, think of ground as
earth. When an object or conductor is grounded, it, by
definition, is connected to the earth. When objects are
bonded together, the joining must be done in a manner
that creates a path between them with the capacity to
conduct safely any current (fault current or normal
current) likely to be imposed (see figure
1). The tripping of overcurrent devices is directly
related to the bonding, and not the grounding, function
as stated by the definition and in the performance
language of 250.4(A) and 250.4(B) which describes the
purpose of each function.
Grounding for services and separately
derived systems is required, and the grounding electrode
connection (grounding point) is generally located at the
service or source of a separately derived system as
required by the NEC. The non-current-carrying
conductive materials enclosing electrical conductors or
equipment, or forming part of such equipment, are
required to be connected together (bonded) and connected
to the supply source grounding point in a manner that
establishes an effective fault current path [NEC 250.4(A)(3)]. Generally as these non-current-carrying
parts are bonded together, they are also simultaneously
grounded because of the connection to earth at the
grounding point for the service or separately derived
system. Basically grounding and bonding are two separate
performance functions that generally happen
simultaneously, and both must be ensured for safety. It
is important to understand the performance required for
each of these functions to accurately apply the rules of
the NEC to installations and systems in the
field.
Grounding
Electrode Required
When applying the grounding and bonding rules for
multiple buildings, one must verify that both
performance functions are met and are in concert with
the requirements of the NEC. As the feeder
arrives at the separate building or structure, several NEC rules must be applied. Rules for separate structure
disconnecting means and their locations must be applied.
Generally this is also where the initial or first
grounding and bonding connections will be made. The NEC requires that a building or structure supplied by a
feeder or branch circuit must have a grounding
connection through the available grounding electrode or
grounding electrode system at that building or
structure. Basically this required grounding electrode
is the same for this building or structure whether a
service or a feeder supplies it. It must have a
grounding electrode(s). The grounding electrode system
must be made up of the specified electrodes in Part III
of Article 250.
Where there are no electrodes as
specifically described in 250.52(A)(1) through (A)(6),
one of the electrodes described in 250.52(A)(4) through
(A)(7) is required to be installed. One exception
relaxes the requirement for the grounding electrode at a
separate structure; the electrode is not required where
the structure is supplied by one branch circuit that
includes an equipment grounding conductor for grounding
the conductive non-current-carrying metal parts of all
equipment in the structure. So, as can be seen, the
grounding electrode at a separate building is generally
required [NEC 250.32(A)].
Two methods of grounding and bonding
connections at separate buildings or structures are
provided in the NEC. One method is required where
the feeder or branch circuit includes or provides an
equipment grounding conductor in addition to any
grounded system conductor as part of the feeder or
branch circuit. The second (alternative) method is where
the feeder or branch circuit does not provide an
equipment grounding conductor where it supplies the
separate building or structure. Keep in mind the
grounding electrode is required generally at the
separate building regardless of whether or not an
equipment grounding conductor is provided, unless
qualifying under the provision of the exception
discussed previously.
Figure
2
Where
Equipment Grounding Conductor Is Provided
Feeders are generally required to include an equipment
grounding conductor [NEC 215.6]. It is more
common in wiring installations today to distribute
feeders that include equipment grounding conductors. In
recent NEC cycles there has been an increased
emphasis on migrating away from using the grounded
conductors for grounding purposes on the load side of a
service or separately derived system. There are general
rules in the NEC that prohibit grounding
connections to the grounded (often neutral) conductors
on the load side of the service or separately derived
system grounding point [NEC 250.24(A)(5);
250.142(B); 250.32(B)(1)]. The primary reason is to keep
normal (neutral) currents from flowing over other
electrically conductive metallic paths back to the
source. Keep in mind that current (normal or fault
current) will attempt to follow any and all paths to
return to the source and will divide over those paths
inversely proportional to the impedance of each path.
Where the feeder supplies a separate
building or structure and includes an equipment
grounding conductor, the equipment grounding conductor
is required to be connected (bonded) to the structure
disconnecting means. The size of the equipment grounding
conductor (of the wire type) must meet the minimum sizes
specified in 250.122 based on the overcurrent protective
device size for the feeder or branch circuit supplying
the building or structure (see figure
3).
The required grounding electrode
conductor must also be connected to the disconnecting
means. The minimum size of the grounding electrode
conductor must be in accordance with 250.66 [250.32(E)].
The grounded (often the neutral) conductor must
terminate on a grounded conductor terminal bar in the
disconnect enclosure that is insulated (isolated) from
the enclosure. This keeps any neutral conductor current
from contact with metal parts at that point on the
system. Neutral current should return to the service
equipment over one path, the neutral in this case, which
is the performance goal. In fault conditions, the
equipment grounding conductor serves as the protective
bonding circuit to provide an effective bonding
connection back to the source that will facilitate the
operation of the overcurrent device and open the faulted
condition. The grounding electrode under these fault
conditions serves to keep the metal equipment and other
conductive materials at the same (earth) potential
during the time duration of the event.
Where
Equipment Grounding Conductor Is Not Provided
The second method for grounding and bonding at a
separate building or structure is allowed where the
feeder does not provide an equipment grounding
conductor, but does include a system grounded (often a
neutral) conductor. This second method is a bit more
difficult to utilize because there are more specific
restrictions that must be considered and adhered to.
Three conditions must exist before one may use the
grounded conductor for grounding purposes at a separate
building or structure.
The first condition is that an
equipment grounding conductor, of any form specified in
250.118, is not provided or run with the supply to the
structure. This means that only the phase conductor(s)
and the system grounded conductor either as direct
burial, in nonmetallic conduit underground, or as
overhead conductors are included. The key is that no
equipment grounding conductor is included.
The second condition is that no
continuous metallic paths exist or are otherwise
present, and that are bonded to the grounding system in
both buildings. Examples of continuous metallic paths
could be metal water piping, building steel, metallic
conduit, cable shields, metal ducts, and so forth.
The last condition that must be met
prior to utilizing this method is that no equipment
ground-fault protection is installed on the supply
service or feeder, as neutral-to-ground connections on
the load side of this equipment can nullify or
desensitize the equipment protection.
If all of these conditions are met,
then the grounded conductor of the feeder or branch
circuit is permitted to be used for grounding and
bonding the electrical equipment. It must be connected
to the structure disconnecting means enclosure to which
the required grounding electrode conductor is also
connected. The minimum size of the grounded conductor of
the feeder or branch– circuit must satisfy two minimum
sizing requirements. First, it must be adequate to carry
the maximum load on the grounded (often a neutral)
conductor as specified in 220.22. Second, it also must
not be smaller than the required equipment grounding
conductor for the feeder or branch circuit using
250.122, based on the size of fuse or circuit breaker
ahead of it.
In addition to being a normal
current-carrying conductor, the grounded conductor keeps
the electrical equipment and other conductive materials
at ground (earth) potential. During both normal and
fault conditions, the grounded conductor serves as the
required effective ground-fault current path to
facilitate the operation of the feeder or branch-circuit
overcurrent devices.
Figure
4
Separate
Building or Structure Disconnecting Means
The NEC includes requirements for a disconnecting
means where a feeder(s) or branch circuit(s) supplies a
separate building [NEC 225.31]. This
disconnecting means must be located either outside or
inside the building served and must be readily
accessible nearest the point of entrance of the
conductors, or be located outside the building. These
requirements, for the most part, parallel the
requirements for locations of service disconnects as
specified in Article 230.
The required disconnect for the
feeder or branch circuit supplying the separate building
or structure generally must also be suitable for use as
service equipment [NEC 225.36]. Disconnecting
means and equipment identified as "suitable for use
as service equipment" are constructed to include
the features and provisions for grounding and bonding.
Generally, this means that a "main bonding
jumper" in the form of a screw, strap, bus, or
conductor is provided for connection between the
grounded conductor and the enclosure. This equipment
allows for proper grounding and bonding connections at
the separate building or structure for either of the two
grounding and bonding methods provided in 250.32(B)(1)
or (2) [see figure 5].
Keep in mind that this disconnecting
means is required generally, and it must be suitable for
use as service equipment. The grounding electrode is
required, and it must be connected to the disconnecting
means enclosure just as it is at the service equipment
on the upstream side of the separate building or
structure. The difference is how the connection of the
grounded (often a neutral) conductor at this
disconnecting means is made and is directly related to
the two alternate methods provided in 250.32(B).
Remember, it is important to keep current on its
intended path during normal operation.
Summary
It is important to understand the different performance
criteria between grounding and bonding. Specific
performance requirements are now included as rules in
Article 250 and help in the understanding of the purpose
of the prescriptive requirements. Three definitions of
performance-based terms are provided in 250.2 and assist
users in increasing the understandability of the reasons
and purposes of the functions of grounding compared to
bonding. A properly grounded and bonded electrical
system and installation provide both functions as a
seamless and simultaneous protection system. Generally
if objects are effectively bonded, they are usually also
grounded effectively. If objects are only grounded by
definition, they may not be effectively bonded to
facilitate overcurrent device operation. Remember the
earth is not permitted as an equipment grounding
conductor and must not be used as an effective
ground-fault current path. Visit the definitions in
Article 100 and establish a solid understanding of the
meaning of the grounding and bonding terms and words. It
is important for accurate and proper application of
rules to installations and systems. This article is
based on the requirements contained in the 2002 edition
of the NEC. Always consult the authority having
jurisdiction for any local electrical requirements or
ordinances in your area.
1 National
Electrical Code, 2002, Article 100, Definitions.
National Fire Protection Association, Inc., Quincy, MA
02269.
Michael J. Johnston is IAEI’s
director of education, technical editor and an IAEI
principal member on CMP-5. Johnston was formerly
employed as an electrical field inspections supervisor
for the city of Phoenix, Arizona. He is fully certified
in many areas–. He is a member of the IBEW. He
achieved both journeyman E-2 and master electrician E-1
licenses in the state of Connecticut. Additionally, he
holds all IAEI certifications. He also holds ICBO
Electrical Inspections Certification. He is a member of
the UL Electrical Council
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