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GENERAL SECTIONS
Rule 094
See
Photo
Fig. 093-8. Example of aluminum grounding conductor transitioning to copper for burial (Rule 093E5).
example of one common grounding conductor for the circuit and equipment is
shown in Fig. 093-9.
094. GROUNDING ELECTRODES
Grounding electrodes can be existing electrodes or made electrodes. Existing
electrodes are existing conductive items buried in the earth for a purpose other
than grounding but can also serve as a grounding electrode. Most utilities use
made electrodes, which are purposely constructed and buried to serve as
grounding electrodes. Requirements for existing electrodes are outlined in Figs.
094-1 through 094-3.
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Rule 094
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Fig. 093-9. Example of common grounding conductor for neutral
and equipment (Rule 093F).
Made electrodes must penetrate the moisture level and be below the frost
line. They must be metal or combined metals that do not corrode and they must
not be painted, enameled, or covered in any way with an insulating material.
The driven ground rod is the most commonly used made electrode. Requirements for made electrodes are outlined in Figs. 094-4 through 094-11.
Fig. 094-1. Existing electrode—metallic water piping system (Rule 094A1).
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GENERAL SECTIONS
Rule 094
Fig. 094-2. Existing electrode—local (water piping) system (Rule 094A2).
Fig. 094-3. Existing electrode—steel reinforcing bars in concrete foundations and footings (Rule 094A3).
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Rule 095
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Fig. 094-4. Made electrodes—driven ground rods (Rule 094B2).
095. METHOD OF CONNECTION TO ELECTRODE
The connection to the grounding electrode must be permanent (except for
removal due to inspection or maintenance) and be mechanically sound, corrosion-resistant, and have the required ampacity for the fault current to
which it will be subjected. Suitable connection methods are shown in
Fig. 095-1.
The Code also has specific rules for connecting to steel framed and non-steelframed structures. The connection to water piping systems is also outlined.
When water piping is used as the grounding electrode, bonds must be made
around meters or other removable fittings.
The Code (in Sec. 094, “Grounding Electrodes”) does not list gas piping as an
acceptable electrode. Made electrodes or grounded structures should be separated
from high-pressure (150 lb/in2 or greater) pipelines containing flammable liquids
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GENERAL SECTIONS
Fig. 094-5. Made electrodes—buried wire (counterpoise) (Rule 094B3a).
Fig. 094-6. Made electrodes—buried strips (Rule 094B3b).
Rule 095
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Fig. 094-7. Made electrodes—buried plates or sheets (Rule 094B3c).
Fig. 094-8. Made electrodes—butt plates and wire wraps (Rule 094B4).
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GENERAL SECTIONS
Rule 095
Fig. 094-9. Made electrodes—butt plates and wire wraps at transformer locations (Rule 094B4a).
Fig. 094-10. Made electrodes—direct-buried concentric neutral cable (Rule 094B5).
or gases by a distance of 10 ft or more. No distances are specified for separating
grounding electrodes from low-pressure gas lines. High-pressure pipelines are
used as transmission facilities. Low-pressure pipelines are most commonly used
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Rule 096
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Fig. 094-11. Made electrodes—concrete-encased electrodes (Rule 094B6).
to supply natural gas to homes. The requirements for separating grounding electrodes from high-pressure pipelines are shown in Fig. 095-2.
Rule 095C requires that the connection to the grounding electrode be free
from rust, enamel, or scale. This can be done by cleaning or using fittings that
penetrate such coatings.
096. GROUND RESISTANCE REQUIREMENTS
The main intent of Rule 96 is to assure a grounding resistance low enough to
permit prompt operation of circuit protective devices (e.g., fuses, reclosers,
relay-controlled circuit breakers).
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GENERAL SECTIONS
Rule 096A
See
Photo
Fig. 095-1. Connection of grounding conductor to grounding electrode (Rule 095A).
Supply stations normally require extensive grounding
systems consisting of a ground grid or mat combined with grounding electrodes. They are designed to limit touch, step, mesh, and transferred potentials.
The Code notes IEEE Standard 80 as a reference for substation grounding.
096B. Single-Grounded (Unigrounded or Delta) Systems. Single-grounded systems, typically grounded wye transmission systems that do not carry a neutral
and are grounded only at the source transformer, must have a ground resistance
not exceeding 25 Ω. This rule states that if a single electrode exceeds 25 Ω, then
two electrodes in parallel must be used. The Code does not specifically comment
on what happens if the second electrode does not bring the ground resistance
below 25 Ω; however, the main idea of this rule is to have a ground resistance low
enough to permit prompt operation of circuit protective devices.
096C. Multigrounded Systems. Multigrounded systems are the most common
type of distribution system. A typical 12.47/7.2-kV, three-phase, four-wire
grounded-wye distribution system is multigrounded. For a system to be multigrounded, the following must occur:
096A. Supply Stations.
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Fig. 095-2. Grounding electrode separation from high-pressure pipelines (Rule 095B2).
• The circuit must have a neutral of sufficient size and ampacity.
• The neutral must be connected to a grounding electrode at each transformer location.
• The neutral must be connected to a grounding electrode not less than four
times in each mile of the entire line. The grounds at transformers can be
counted in the four grounds in each mile, but the grounds at individual services (i.e., meters) cannot be counted.
The intent of a multigrounded system is to always carry a neutral and to have
four grounds in each mile of the entire line. To check the four grounds in each
mile, a “one-mile window” can be used. Examples are shown in Fig. 096-1.
The Code does not specify a ground resistance for multigrounded systems.
The Code notes that multigrounded systems are dependent on the multiplicity
of grounding electrodes, not the ground resistance of any individual electrode.
For underground installations where the supply cable has an insulating
jacket over the concentric neutral or the supply cable is in conduit, the cable
must be terminated and grounded four times in every mile. If an express
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GENERAL SECTIONS
Rule 096C
Fig. 096-1. Example of checking “four grounds in each mile” (Rule 096C).
direct-buried underground feeder is constructed with an insulating jacket but
without frequent termination points, the cable jacket must be stripped back
and a suitable grounding electrode must be connected four times in each
mile. If a supply cable has a semiconducting jacket, the cable can be treated
similar to a bare concentric neutral cable and the jacket does not need to
be stripped back for grounding. The semiconducting jacket must not exceed
100 mΩ radial resistivity. Use of semiconducting jacketed cable is not very
common due to the fact that these cables are higher in cost than insulated
jacketed cable.
Rule 096 provides an exception to the four grounds in every mile for underwater crossings. Grounding on each side of the underwater crossing should be
given special attention to make up for any lack of grounding in the underwater
portion of the cable.
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Rule 097
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097. SEPARATION OF GROUNDING
CONDUCTORS
Rule 097A requires that separate grounding conductors be run to separate
grounding electrodes for primary surge arresters over 750 V, secondary circuits under 750 V, and shield wires. But Rule 097B allows a single grounding conductor and single grounding electrode if a ground connection exists
at each surge arrester location and the primary neutral or shield wire and
secondary neutral are connected together. When the primary and secondary
neutrals are connected, Rule 097C requires the common neutral to be multigrounded (see Rule 096C). Rule 097A is typically applied in conjunction
with Rule 097D1. An example of this application is a delta-delta transformer
bank fed from an ungrounded primary system as shown in Fig. 097-1.
Rules 097B and 097C are typically applied to grounded-wye–roundedwye three-phase systems and grounded-wye single-phase systems fed from
a multigrounded primary system as shown in Fig. 097-2.
On multigrounded systems the primary and secondary neutrals should be
interconnected. The NESC uses the word “should” in this case, not “shall,”
as there are times when separation of primary and secondary neutrals on a
multigrounded system is applicable. The most common reason for separating primary and secondary neutrals on a multigrounded system is to minimize
Fig. 097-1. Example of separate primary and secondary grounding (Rules 097A and 097D1)
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GENERAL SECTIONS
Rule 097
Fig. 097-2. Example of a common neutral with single grounding (Rules 097B and 097C).
stray voltage on the secondary neutral imposed by the primary neutral. The
requirements separating primary and secondary neutrals for stray voltage or
other valid reasons are outlined in Fig. 097-3.
If a made electrode is used to ground surge arresters on an ungrounded
system exceeding 15 kV phase to phase, the NESC requires that the ground
rod(s) be at least 20 ft from buried communication cables.
Rule 097G focuses on grounding requirements for joint-use poles. If separate
grounding conductors (pole grounds) are run to the supply neutral and the
communications messenger, a bond between the pole grounds should be
added. If a single grounding conductor (pole ground) is used on the joint-use
pole, it should be connected to both the supply neutral and the communications messenger. Most utilities use a single-pole ground for grounding both
power and communications. The single-pole ground method will require a
review for special cases like the delta-delta transformation or for stray voltage
applications discussed in this rule.
098. NUMBER 098 NOT USED IN THIS EDITION.
099. ADDITIONAL REQUIREMENTS FOR
COMMUNICATION APPARATUS
This rule outlines how to ground communication apparatus when grounding
is required in other parts of the Code. This rule references Note 2 of Rule
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Rule 099
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Fig. 097-3. Separating primary and secondary neutrals for stray voltage (Rule 097D2).
097D2, which discusses cooperation between supply and communications
employees to isolate primary and secondary neutrals (typically for resolving
stray-voltage problems).
A communications grounding conductor shall preferably be made of copper
or other material that will not corrode and shall not be less than AWG No. 14.
The communications grounding conductor must be connected as shown in
Fig. 099-1.
A separate communications ground rod is not required per Rule 099A. If a
communications ground rod is used because a supply service does not exist,
the communications ground rod may be smaller in diameter and length per the
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Rule 099
Fig. 099-1. Additional requirements for communications grounding (Rules 099A and 099B).
exception to Rule 099A3. However, if a supply service does exist and a communications ground rod is used to supplement the supply grounding system,
the exception to Rule 099A3 permitting smaller rods does not apply. Rule
099A does not prohibit a supplemental communications ground rod, but only
if the supply service does not exist can the smaller communications-size
ground rod be used. If a standard-size ground rod (per Rule 094B2) is used for
communications grounding to supplement the supply ground rod, an AWG
No. 6 copper or equivalent jumper must bond the two ground rods together as
shown in Fig. 099-2.
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Rule 099
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Fig. 099-2. Bonding of communications and supply electrodes (Rule 099C).
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