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News>Issue Listing>March/April 2002 >Grounding Separately Derived Systems
| Grounding Separately Derived Systems |
New requirements ensure that whether a system is intentionally grounded or is grounded by choice, it must follow the requirements in 250.30. |
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Grounding separately derived systems is
required where the system meets the characteristics of 250.20.
This section gives the conditions under which electrical
systems are required to be grounded. A system that is grounded
has one conductor of the system intentionally grounded.
Whether the system is required to be grounded or is grounded
by choice, it must follow all the requirements for grounding
separately derived systems as covered in 250.30. A couple of
the rules that must be followed applicable to grounded
conductors includes identification requirements and
overcurrent protection. There have been a few changes included
in the 2002 Code relative to grounding separately
derived systems. This article explores the grounding and
bonding requirements for separately derived systems as covered
in the 2002 National Electrical Code.
Let’s first look at the definition of a
separately derived system.
Separately Derived System. A premises wiring system
whose power is derived from a battery, a solar photovoltaic
system, or from a generator, transformer, or converter
windings, and which has no direct electrical connection,
including a solidly connected grounded circuit conductor, to
supply conductors originating in another system.
To Ground or Not
to Ground
The choice of whether or not to ground a separately
derived system is often made for us by the Code where
the system is required to be grounded as specified by 250.20
(see figure 1).
Sometimes a system is not required to be grounded, but is
grounded to provide the benefits of grounding. Grounded
systems offer a few key advantages that ungrounded systems do
not. One advantage of the grounded system is the stabilization
of the voltage to ground during normal operation. Another
important benefit is the facilitation of overcurrent devices
to clear ground faults when any one of the ungrounded
conductors experiences a ground-fault event. For an ungrounded
system, the advantages differ slightly. The ungrounded system
becomes a grounded system accidentally upon the first ground
fault on any of the ungrounded (hot) conductors. Ungrounded
systems have the advantage of continuity of service as
overcurrent devices will not operate on the first ground
fault. Ground-fault detection indicating equipment is often
installed where ungrounded systems are employed. This article
looks at grounded systems and what is required to comply with
the Code.
The grounding rules for separately derived
systems are found in 250.30. The main components for
separately derived systems are the bonding jumper, the
grounding electrode conductor, grounding electrode, grounded
conductor, and equipment bonding jumper. The 2002 NEC was revised to include requirements for equipment bonding
jumpers and grounded conductors on the secondary side of the
system. These new sections include minimum sizing and routing
requirements. Also included for this cycle is an alternative
method for establishing a grounding electrode conductor
connection to the system. This alternative method parallels
the alternative methods for grounding electrode conductor taps
at services.
Bonding Jumper
The first component of grounding a separately derived
system is the bonding jumper and the rules for the bonding
jumper for separately derived systems are covered in
250.30(A)(1). This jumper connects the grounded conductor of
the system to the equipment grounding conductor at the source
or at any point up to and including the first system
overcurrent device. The bonding jumper functions just as the
main bonding jumper at the service does. It serves as the
vital link over which all the fault current flows in
ground-fault conditions. It must be sized in accordance with
250.28. The minimum size is taken from Table 250.66 based on
the circular mil area of the derived phase conductors
connected to the secondary of the system. The connection of
the bonding jumper must be made at the same location as the
grounding electrode conductor connection to the system.
Grounding
Electrode Conductor
Grounding electrode conductor is defined in Article 100.
The grounding electrode conductor is installed "to
connect the grounding electrode(s) to the equipment grounding
conductor, to the grounded conductor, or to both... at the
source of a separately derived system." The grounding
electrode conductor for a separately derived system is
required to be installed as specified by 250.30(A)(2)(a) or
(b). The grounding electrode conductor for a single separately
derived system is sized in accordance with Table 250.66 based
on the total area of the derived phase conductors. If a
grounding electrode of the concrete-encased, rod, pipe or
plate, or ground ring type is used as the sole electrode for
the system, the minimum size for those grounding electrode
conductors is permitted to be in accordance with 250.66(A),
(B), or (C).
Where multiple separately derived systems
are installed, the code permits a common grounding electrode
conductor to be installed and sized based on the total
circular mil area of the derived phase conductors of the
systems. This common grounding electrode conductor is not
required to be larger than the values given in Table 250.66.
The Code will then permit the connection of grounding
electrode conductor taps from each separately derived system
to connect to the common grounding electrode conductor (see figure
3). The size of each grounding electrode conductor tap is
based on the derived phase conductors of the respective
derived system to which the tap conductor is connected. Each
grounding electrode tap conductor shall connect the grounded
conductor of the derived system to the common grounding
electrode conductor (see NEC 250.30(A)(3) and figure
2).
The common grounding electrode conductor
and the grounding electrode conductor taps must be installed
in accordance with the requirements of 250.64(A), (B), (C),
and (E). The common grounding electrode conductor is required
to be installed without a splice or joint in accordance with
250.64(C) unless spliced by irreversible compression
connection or the exothermic welding process. The tap
conductors are required to be connected to the common
grounding electrode conductor by one of the following methods:
1. Exothermic welding process
2. Irreversible compression connections
3. Connection to a ¼ x 2 copper busbar
to which the unspliced common grounding electrode conductor
is connected. This connection shall be made in accordance
with 250.8 and 250.70.
Grounding
Electrode
The choice of which grounding electrode to use for a
separately derived system very much depends on the building
construction and which grounding electrodes are available. The Code requires the grounding electrode for a separately
derived system to be as near as practicable and preferably in
the same area as the grounding electrode conductor connection
to the system. The Code goes a step further and
requires that the electrode be the nearest one of the
following:
1. Effectively grounded structural metal
member of the building.
2. Effectively grounded metal water pipe
within 1.5 m (5 ft) of the entrance of the water pipe to the
building.
3. Other electrodes specified in 250.52
where the electrodes above are not available.
Clearly the Code is specific as to
which electrode must be used and sets up a priority list of
sorts. The key requirement is in the wording
"available," "as near as practicable and
preferably in the same area," and most important,
"the nearest one of the following."
This is an important factor in determining
the grounding electrode for the separately derived system. If
there is effectively grounded building steel, or an
effectively grounded metal water pipe, the system is required
to be grounded to either of the two, but must utilize the
nearest of the two in that order of priority. So the
alternative method for using a common grounding electrode
conductor to which taps may be connected can be limited to the
availability and proximity of other grounding electrodes
inherent to the building construction.
Equipment Bonding
Jumper
Where an equipment bonding jumper is installed between
the secondary of the derived system and the first system
overcurrent device, it is required to be sized in accordance
with 250.28 and Table 250.66. This equipment bonding jumper is
sized very similar to the equipment bonding jumper on the
supply side of the service disconnecting means. Equipment
bonding jumpers are required when the wiring method enclosing
the secondary conductors is not suitable as a bonding means.
Examples of this are when the secondary from a transformer to
the panelboard or equipment is installed in rigid nonmetallic
conduit or flexible metal conduit. An equipment bonding jumper
is needed to bond the enclosures together and to ensure an
effective path for ground-fault current between the panelboard
enclosure and the transformer enclosure. If a metallic wiring
that qualifies as an effective bonding means is installed as
the wiring method between the transformer and the panelboard,
and bonding is assured in accordance with Part VI and
specifically 250.96 of the NEC, a bonding jumper would
not be required. The equipment bonding jumper size is based on
the total circular mil area of the secondary derived phase
conductors of the system. Where the total circular mil area
exceeds the values in Table 250.66, the equipment bonding
jumper shall not be less than 12 ½ percent of the derived
phase conductors (see figure
4).
Grounded Conductor
The 2002 NEC includes requirements for sizing
and routing of grounded conductors of separately derived
systems. These new requirements are very similar to the
requirements for routing and sizing the grounded conductor for
a service. The grounded conductor must be sized in accordance
with 220.22 and in no case shall it be smaller than the values
in Table 250.66. Grounded conductors of separately derived
systems must follow the identification requirements in Article
200 and overcurrent devices are generally not permitted to be
inserted in the grounded conductor of the system. Two
exceptions exist that modify this general requirement.
Bonding of Water
Piping and Structural Steel
The water piping or structural steel that is
interconnected to form a building frame that exists in the
area served by the separately derived system is required to be
bonded to the separately derived system grounding electrode
conductor in accordance with 250.104. This bonding jumper is
required to be sized based on the total circular mil area of
the derived phase conductors of the system. This bonding
jumper provides an effective path for ground fault current
that is close to the separately derived system, and also
serves to eliminate any differences in potential to these
metallic parts from the newly derived voltage of the system.
Generators
When a generator is installed for emergency, optional
standby, or legally required standby systems in accordance
with Chapter 7 of the Code, the system may or may not
be required to be grounded in accordance with the grounding
rules for separately derived systems in 250.30. The transfer
equipment is the determining factor. If there is a switching
action in the grounded conductor between the normal source or
service and the generator, then the system produced by the
generator is a separately derived system and must be grounded
in accordance with 250.30. See figure
4 and figure 5.
If there is no switching action in the
grounded conductor in the transfer equipment, then the
grounded conductor remains grounded through its connection to
the electrode system at the service enclosure. A careful look
and understanding of transfer switches and equipment used with
generators is needed in making this determination (see photo
1, photo 2,
and photo 3).
Uninterruptible
Power Supplies
Some uninterruptible power supplies (UPS) systems
are also separately derived and must also be grounded in
accordance with 250.30. This information is usually provided
by the manufacturer of the UPS system in the form of equipment
labeling of installation instructions. Section 645.15 includes
specific requirements for grounding systems and equipment for
information technology rooms where UPS systems are often
employed.
Summary
The grounding rules for separately derived systems
have been revised and expanded for the 2002 NEC. New
requirements include information on equipment bonding jumper
sizes, grounding conductor sizes and installation, and
alternative methods for grounding electrode conductor
connections to these systems. The bonding requirements for
water piping and structural steel in the area served by the
separately derived system have also been expanded. Obtain your
copy of the 2002 NEC and the IAEI/NFPA Analysis of
Changes to become familiar with these revisions and how
they will impact installations of separately derived systems.
As always, there may be local amendments to the requirements
of the NEC. Always check with the local authority
having jurisdiction for any local requirements.
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 a member of the
IBEW. Additionally, he holds all IAEI certifications. He
also holds ICC Electrical Inspections Certification. He
is a member of the UL Electrical Council.
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