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- Grounding -
This section presents general rules for
the grounding and bonding of electrical installations. Qualified workers should clearly understand the concepts of grounding practices as required
by the NEC. They should also clearly understand the definition and intent of the following components of a grounding system that are explained
in this chapter:
1. Grounded conductor
2. Grounding conductor
3. Grounding electrode conductor
4. Bonding jumper
5. Grounding electrode
4.1 REGULATIONS, CODES, AND REFERENCES
4.1.1 ENGINEERING SPECIFICATIONS AND DRAWINGS
specifications and drawings should identify the requirements for all components
and clearly illustrate the grounding electrode system, the
grounding electrode conductor,
bonding points and bonding jumpers, and the connection point for the grounded conductor and
conductors. Where used for installation or construction purposes, these
specifications and drawings should also include detailed installation
4.2 CIRCUIT AND SYSTEM GROUNDING
Circuit and system grounding consists of connecting the grounded conductor, the
grounding conductor, the grounding bus bars, and all noncurrent-carrying metal parts to ground.
This is accomplished by connecting
a properly sized unspliced grounding electrode conductor
between the grounding bus bar and the grounding electrode system. There are three
fundamental purposes for grounding an electrical system:
1. To limit excessive voltage from lightning, line surges, and crossovers with
2. To keep conductor enclosures and noncurrent-carrying metal enclosures and equipment at
3. To facilitate the opening of overcurrent protection devices in case of insulation failures
because of faults, short
4.3 EQUIPMENT GROUNDING
Equipment grounding systems, which consist of interconnected networks of equipment
conductors, are used to perform the following functions:
1. Limit the hazard to personnel (shock voltage) from the noncurrent-carrying metal parts of
equipment raceways and other conductor
enclosures in case of ground faults, and
2. Safely conduct ground-fault current at sufficient magnitude for fast operation of the circuit
overcurrent protection devices.
To ensure the performance of the above functions, equipment grounding conductors are
1. Be permanent and continuous
2. Have ample capacity to safely conduct ground-fault current likely to be imposed on them; and
3. Have impedance sufficiently low to limit the voltage to ground to a safe magnitude and to
facilitate the operation of the circuit
overcurrent protection devices.
Caution shall be taken to ensure that the main bonding jumper and equipment bonding
sized and selected correctly. Bonding completes the grounding circuit so that it is continuous. If a
ground fault occurs, the
fault current will flow and open the overcurrent protection devices. The
means of bonding shall provide the following to ensure the grounding
system is intact:
1. Provide a permanent connection,
2. Provide a positive continuity at all times, and
3. Provide ampacity
to conduct fault current.
See Figure 4-1 on the proper grounding of electrical systems.
Figure 4-1. Circuit and system grounding consists of earth grounding the electrical system at the supply transformer and
the line side of the service equipment. Equipment grounding and bonding is accomplished by connecting all metal enclosures and raceways together
with the grounding conductors.
Electrical systems can be operated grounded or ungrounded, depending on the condition of
Electrical systems are grounded to protect circuits, equipment, and conductor
enclosures from dangerous voltages and personnel from electrical
4.5 GROUNDED OR UNGROUNDED SYSTEMS
Ungrounded systems may provide greater continuity of operations in the event of a
However, the second fault will most likely be more catastrophic than a grounded system fault.
Whenever ungrounded systems are used
in a facility, the maintenance personnel should
receive training in how to detect and troubleshoot the first fault on an ungrounded system.
"Grounded" means that the connection to ground between the service panel and earth has been
made. Ungrounded electrical systems are used
where the designer does not want the
overcurrent protection device to clear in the event of a ground fault.
Ground detectors can
be installed per NEC to sound an alarm or send a message to alert
personnel that a first fault has occurred on one of the phase conductors.
Ground detectors will
detect the presence of leakage current or developing fault current conditions while the system is
and operating. By warning of the need to take corrective action before a problem
occurs, safe conditions can usually be maintained until
an orderly shutdown is implemented.
Figure 4-1. Circuit and system grounding consists of earth grounding the electrical
system at the
supply transformer and the line side of the service equipment. Equipment
grounding and bonding is accomplished by connecting all metal enclosures
raceways together with the grounding conductors.
4.5.1 GROUNDED SYSTEMS
Grounded systems are equipped with a grounded conductor that is required to be run to each service
disconnecting means. The grounded conductor can be used as a current-carrying conductor to accommodate all neutral related loads. It can also
be used as an equipment grounding conductor to clear ground faults ahead of the service disconnecting means. A network of equipment grounding
conductors is routed from the service equipment enclosure to all metal enclosures throughout the electrical system. The equipment grounding
conductor carries fault currents from the point of the fault to the grounded bus in the service equipment where it is transferred to the grounded
conductor. The grounded conductor carries the fault current back to the source and returns over the faulted phase and trips open the overcurrent
Note: A system is considered grounded if the supplying source, such as a transformer or generator is grounded in
addition to the grounding means on the supply side of the service equipment disconnecting device for separately derived systems.
neutral of any grounded system serves two main purposes: (1) it permits the utilization of
line-to-neutral voltage and thus will serve as
a current-carrying conductor to carry any neutral
current, and (2) it plays a vital role in providing a low-impedance path for the flow of
currents to facilitate the operation of the overcurrent devices in the circuit. (See Figure 4-2.)
Consideration should be given
to the sizing of the neutral conductor for certain loads due to the
presence of harmonic currents.
Figure 4-2. A grounded system is equipped with a grounded (neutral) conductor routed between the supply transformer
and the service equipment.
4.5.2 UNGROUNDED SYSTEMS
Ungrounded systems operate without a grounded conductor. In other words, none of the circuit
conductors of the electrical system are intentionally grounded to an earth ground such as a
metal water pipe, or building steel. The same
network of equipment grounding conductors is
provided for ungrounded systems as for solidly grounded electrical systems. However,
grounding conductors (EGCs) are used only to locate phase-to-ground faults and
sound some type of alarm. Therefore, a single sustained line-to-ground
fault does not result in
an automatic trip of the overcurrent protection device. This is a major benefit if electrical system
is required or if it would result in the shutdown of a continuous process. However, if an
accidental ground fault occurs and is allowed to
flow for a substantial time, overvoltages can
develop in the associated phase conductors. Such an overvoltage situation can lead to
insulation damage, and while a ground fault remains on one phase of an ungrounded
system, personnel contacting one of the other phases and
ground are subjected to 1.732 times
the voltage they would experience on a solidly neutral grounded system. (See Figure 4-3.)
Figure 4-3. An ungrounded system does not have a grounded (neutral) conductor routed between the supply transformer and the service
equipment because the supply transformer is not earth grounded.
Note: All ungrounded systems should be equipped with ground detectors
maintenance applied to avoid, to the extent practical, the overcurrent of a sustained ground fault
on ungrounded systems. If
appropriate maintenance is not provided for ungrounded systems, a
grounded system should be installed to ensure that ground faults will be
cleared and circuits,
equipment, and personnel are safe.
4.5.3 HIGH-IMPEDANCE GROUNDING
Electrical systems containing three-phase,
three-wire loads, as compared to grounded neutral
circuit conductor loads, can be equipped with a high-impedance grounded system. High impedance
grounded systems shall not be used unless they are provided with ground fault
Figure 4-3. An ungrounded system does not have a grounded (neutral)
routed between the supply transformer and the service equipment because the supply
transformer is not earth grounded.
indicators or alarms, or both, and qualified personnel are available to quickly locate and eliminate
such ground faults. Ground
faults must be promptly removed or the service reliability will be
reduced. See NEC for requirements on installing a high-impedance grounding
Figure 4-4. A high-impedance grounding system has a high-impedance unit, installed between the grounded (neutral) conductor and the grounding
electrode conductor, which is used to regulate fault current.
4.6 GROUNDING REQUIREMENTS
Alternating current systems of
less than 50 volts shall be grounded as required in NEC.
Systems of 50 to 1,000 V should be solidly grounded as required by NEC. Systems
phase-to-neutral loads shall also be solidly grounded (See Figure 4-5). The following electrical
systems are required to be
1. 240/120-V, single-phase, three-wire
2. 208Y/120-V, three-phase, four-wire
3. 480Y/277-V, three-phase,
4. 240/120-V, three-phase, four-wire, delta (midpoint of one phase used as a
grounded circuit conductor)
systems are not required to be solidly grounded:
Figure 4-4. A high-impedance grounding system has a high-impedance unit, installed
between the grounded (neutral) conductor and the grounding electrode conductor,
which is used to regulate fault current.
1. 240-V, three-phase, three-wire delta
2. 480-V, three-phase, three-wire
3. 600-V, three-phase, three-wire.
These electrical systems do not supply phase-to-neutral loads. They supply only phase-tophase
4.7 GROUNDING ELECTRODE
The main purpose of the grounding electrode conductor (GEC) is to connect the electrical
system to earth ground.
The GEC actually provides three grounding paths to the grounding
electrode system. They are as follows:
1. The grounded conductor
2. The equipment grounding path
3. The bonding path
Figure 4-5. Systems of 50 to 1,000 V AC that operate grounded are required to have the grounded conductor connected to
earth ground at the supplying transformer and service equipment.
In grounded systems, the GEC connects to the neutral bar in the service equipment enclosure.
In ungrounded systems, the GEC connects
to the grounding terminal bar. It grounds the
following items to the grounding electrode system:
1. The grounded conductor, if present
2. The equipment grounding conductor, if present
3. The metal of conduits, if present
4. The metal of enclosures, if
5. The bonding jumpers bonding together metal enclosures and conduits
6. The metal enclosure of the service equipment
4.7.1 SIZING THE GROUNDING ELECTRODE CONDUCTOR
NEC 250.66 requires the grounding electrode conductor to be sized by the circular
mils rating of
the largest service entrance conductor or conductors and selected from NEC Table 250.66
based on these values.
For example, the size of the service entrance conductors from a delta, three-phase, four-wire
midpoint tap is #250 kcmil, THWN copper for
phases A and C, #2/0 for phase B, and #1/0 for
the neutral. What size copper GEC is required to ground this system to a metal water pipe?
Note: NEC Table 250.66 is used to size the grounding electrode conductor for both grounded
and ungrounded systems. The table is used where
the grounding electrode conductor is
connected to a metal water pipe or the metal frame of building steel.
4.7.2 EXCEPTIONS TO NEC
There is an exception to the main rule. It has three parts and pertains to specific types of
grounding electrodes. The exception
applies to grounded and ungrounded systems.
Exception (A) applies to made electrodes only, such as rod, pipe, or plate electrodes. The
grounding electrode conductor is not required to be larger than #6 copper or #4 aluminum.
Exception (B) to NEC 250.66 requires at least a
#4 copper conductor to be used as a grounding
electrode conductor to ground the electrical system to a concrete-encased electrode.
(C) requires at least a #2 copper conductor to be used as a grounding electrode
conductor to ground the electrical system to a ground ring.
Step 1: Finding the largest phase-NEC 250.66 #250 kcmil is the largest phase
Step 2: Finding the size GEC-NEC Table 250.66 #250
kcmil requires #2 cu
Answer: The size of grounding electrode conductor (GEC) is at least #2 copper.
4.8 MAIN BONDING JUMPER
The primary function of the main bonding jumper is to connect the grounded circuit conductors
and the equipment grounding conductors at the service equipment. The main bonding jumper
serves as the main link between the system grounded
conductors and the grounding electrode
system where metal equipment enclosures and raceways are utilized to enclose conductors and
If the main bonding jumper is left out, there is no complete circuit for fault current,
which poses a potentially dangerous situation.
The main bonding jumper shall connect together the following items:
1. Grounded conductors and grounded terminal
Equipment grounding conductors and grounding terminal
3. All metal enclosures enclosing conductors and components.
the manufacturer's main bonding jumper is the preferred conductor to be used as
the main bonding jumper. NEC requires the main bonding jumper
to be a (1) wire, (2) screw, (3)
bus bar, or (4) a similar suitable conductor.
NEC requires the main bonding jumper to be at least
the same size as the grounding electrode
conductor where the circular mils rating of the service entrance conductors does not exceed
kcmil for copper or 1750 kcmil for aluminum.
For example: What size main bonding jumper is required to ground the metal enclosure of
service equipment to the grounding terminal bar where the service entrance is made up of one
#250 kcmil, THWN copper conductor per
For example: What size main copper bonding jumper is required for a service entrance with a
makeup of 2400 kcmil copper conductors
Note: In this case the main bonding jumper is greater in size than the grounding electrode
conductor, which is only required
to be #3/0 copper per NEC Table 250.66 based upon the 2400
kcmil copper conductors.
Step 1: Finding the largest phase — NEC 250.28
#250 kcmil is the largest phase
Step 2: Finding the bonding jumper — Table 250.66 #250 kcmil requires #2 copper
size of the main bonding jumper (GEC) is at least #2 copper.
Step 1: Finding the largest phase — NEC 250.28, 2400 kcmil x 0.125 = 300
Step 2: Finding the main bonding jumper — NEC Table 250.66, requires 300 kcmil
Answer: The main bonding jumper is required
to be at least 300 kcmil copper.
4.9 SYSTEM WITH GROUNDED CONDUCTOR
The main purpose of the grounded conductor is to carry unbalanced neutral current or
current in the event that one phase should go to ground.
Note: The grounded conductor does not always have to be a neutral
conductor. It can be a
phase conductor, as when used in a corner grounded delta system.
In solidly grounded service-supplied systems,
the equipment grounding conductors shall be
bonded to the system-grounded conductor and the grounding electrode conductor at the service
equipment. The grounded conductor may be used to ground the noncurrent-carrying metal parts
of equipment on the supply side of the service
disconnecting means per NEC 250.142. The
grounded conductor can also serve as the ground-fault current return path from the service
to the transformer that supplies the service.
The grounded conductor shall not be used to ground the metal parts of enclosures enclosing
conductors and components on the load side of the service per NEC 250.142. See NEC
250.182, 250.130 and 250.140 for exceptions to this basic
rule. NEC 250.24 requires the
grounded conductor to be connected as follows:
1. The grounded conductor shall be connected to the
grounded (neutral) service conductor.
2. The connection shall be at an accessible point.
3. That accessible point can be anywhere
from the load end of the service drop or service
lateral to and including the neutral bar in the service disconnecting means or service
The NEC allows the grounded conductor to be terminated and connected to ground at a
multitude of locations on the supply
side of the service equipment. These locations are as
1. Service equipment
2. Meter base
3. Current transformer
4. Metal gutter or wire way containing service entrance conductors.
See Figure 4-6 for the rules concerning the use
of the grounded conductor.
Figure 4-6. The grounded (neutral) conductor is used to carry normal neutral current or ground fault current in case a ground fault
should develop on one of the ungrounded (hot) phase conductors.
NEC 250.24 lists the rules for sizing the grounded conductor where it
is not used as a grounded
neutral circuit. NEC gives the rules for calculating and sizing the grounded conductor when it is
used as a
circuit conductor. The minimum size for the grounded conductor is computed as
1. The basic rule is to select the size directly
from NEC Table 250.66 when the size of the
service-entrance conductors is not larger than 1100 kcmil copper or 1750 kcmil aluminum.
2. When the service entrance conductors are larger than 1100 kcmil copper or 1750 kcmil
aluminum, the grounded conductor shall be 12½
percent of the largest phase conductor.
3. Where the service phase conductors are paralleled, the size of the grounded conductor
shall be based on the total cross-sectional area of the phase conductors.
For example: What size THWN copper grounded conductor is required
for a service having a
total kcmil rating of 250 per phase? (All phase conductors are THWN copper)
Step 1: Service less than 1100
kcmil - NEC Table 250.66, 250 kcmil requires #2
Answer: The size of the grounded conductor is at least #2 THWN copper.
Figure 4-6. The grounded (neutral) conductor is used to carry normal neutral current
or ground fault current in case
a ground fault should develop on one of the
ungrounded (hot) phase conductors.
For example: What size THWN copper grounded conductor is required for a parallel service
having a total kcmil rating of 2400
per phase? (All conductors are THWN copper)
Note: NEC Table 250.66 is used only if the service conductors are rated less than 1100 kcmil
for copper or 1750 kcmil for aluminum.
4.10 EQUIPMENT GROUNDING CONDUCTOR
Equipment grounding conductors for ac systems, where
used, should be run with the
conductors of each circuit per NEC 250.119, and 250.134.
Earth and the structural metal frame of a building
may be used for supplemental equipment
bonding, but they shall not be used as the sole equipment grounding conductor for ac systems.
circuits having paralleled conductors in multiple metal raceways, an equipment grounding
conductor shall be run in each raceway. Each paralleled
equipment grounding conductor must
be full size based on the circuit overcurrent protection. (See NEC 250.122)
4.10.1 SIZING THE
EQUIPMENT GROUNDING CONDUCTOR
NEC 250.122 lists the requirements for calculating the size of the equipment grounding
an electrical circuit. There are basically five steps to be applied in sizing,
selecting, and routing the equipment grounding conductors:
This method is used where the service entrance conductors are over 1100 kcmil copper or
1750 kcmil aluminum. NEC Table 250.66 cannot
be used for sizing the grounded conductor.
The grounded conductor is required to be not less than 12½ percent of the cross-sectional area
of the largest phase conductor.
1. NEC Table 250.122 shall be used to size the equipment grounding conductor.
2. When conductors
are run in parallel in more than one raceway, the equipment grounding
conductor is also run in parallel.
3. Where more than one circuit
is installed in a single raceway, one equipment grounding
conductor may be installed in the raceway. However, it must be sized for the largest
overcurrent device protecting conductors in the raceway.
4. When conductors are adjusted in size to compensate for voltage drop, the
grounding conductor shall also be adjusted in size.
5. The equipment grounding conductor is never required to be larger
than the circuit
Step 1: Service exceeding 1100 kcmil - NEC Table 250.66, 2400 kcmil x 0.125 = 300
The grounded conductor is required to be at least a #300 kcmil, THWN copper
For example: What size THWN copper equipment grounding conductor is required to be run in a
raceway with a 70 A overcurrent protection
device protecting the circuit?
4.10.2 SEPARATE EQUIPMENT GROUNDING CONDUCTORS
The possibility of worker exposure to electric
shock can be reduced by the use of separate
equipment grounding conductors within raceways.
The separate equipment grounding conductors
contribute to equalizing the potential between
exposed noncurrent-carrying metal parts of the electrical system and adjacent grounded
building steel when ground faults occur. The resistance (inductive reactance) of the ground fault
circuit normally prevents a significant
amount of ground fault current from flowing through the
separate equipment grounding conductors.
Ground fault current flows through
the path that provides the lowest ground fault circuit
impedance. Fittings and raceway systems have been found that are not tightly connected
corroded which prevents good continuity. Therefore, the equipment grounding conductor shall
be the path for the fault current to
travel over and clear the overcurrent protection device
protecting the circuit.
NEC 250.134(B) requires the equipment grounding conductors
to be routed in the same
raceway, cable, cord, etc., as the circuit conductors. All raceway systems should be
supplemented with separate
equipment grounding conductors.
Note: The equipment grounding conductor shall be routed with supply conductors back to the
Additional equipment grounding may be made to nearby grounded structural members
or to grounding grids, but this shall not take the place
of the co-routed equipment grounding
conductors. Raceway systems should not be used as the sole grounding conductor.
Three-phase, three-wire, ungrounded systems (delta), which are extensively used in industrial
establishments, do not require
the use of grounded conductors as circuit conductors.
The same network of equipment grounding conductors shall be provided for ungrounded
systems as for grounded systems. Equipment grounding conductors are required in ungrounded
systems to provide shock protection and to present
a low-impedance path for phase-to-phase
fault currents in case the first ground fault is not located and cleared before another ground fault
occurs on a different phase in the system.
Grounding electrode conductors and bonding jumpers shall be computed, sized, and installed
the same manner as if the system were a grounded system. Apply all the requirements listed in
Sections 4.6 through 4.8 for sizing the
elements of an ungrounded system.
Step 1: Finding EGC - NEC Table 250.122, 70 A OCPD requires #8 copper
Answer: The equipment
grounding conductor is required to be at least #8 THWN copper.
4.12 GROUNDING A SEPARATELY DERIVED SYSTEM
NEC 250.30 covers the rules for grounding separately derived systems. The system
conductor for a separately derived system shall be grounded at only one point. That single
system grounding point is at the
source of the separately derived system and ahead of any
system disconnecting means or overcurrent devices. Where the main system disconnecting
means is adjacent to the generator, converter, or transformer supplying a separately derived
system, the grounding connection to the system
grounded conductor can be made at or ahead
of the system disconnecting means.
The preferred grounding electrode for a separately
derived system is the nearest effectively
grounded structural metal member of the building or the nearest effectively grounded water
If neither is available, concrete-encased electrodes or made electrodes are permitted.
In a grounded, separately derived system, the equipment
grounding conductors shall be bonded
to the system-grounded conductor and to the grounding electrode at or ahead of the main
means or overcurrent protection device. The equipment grounding
conductor should always be connected to the enclosure of the supply transformer,
The grounding electrode conductor, the main bonding jumper, the grounded conductor, and the
grounding conductor are calculated, sized, and selected by the rules listed in
Sections 4.7 through 4.10. (See Figure 4-7.)
Figure 4-7. The grounded (neutral) conductor can be used to carry both normal neutral current and abnormal ground fault current.
4.13 GROUNDING ELECTRODE SYSTEM
If 10 feet or more of metal water pipe is in the earth, the water pipe is considered the
electrode, but it shall be supplemented by an additional electrode. NEC 250.50 lists four types
of electrodes. If one or all
are available, they shall be bonded together to make up the grounding
electrode system. The bonding jumper that connects these electrodes
shall be at least as large
as the grounding electrode conductor of the system sized by NEC Table 250.66. The four types
are as follows:
1. Metal water pipe in contact with the earth for 10 feet or more. Interior metal water pipe
beyond 5 feet from the
water entrance shall not be used as a part of the grounding
electrode system or as a conductor to interconnect those electrodes.
2. Metal frame of the building, where effectively grounded
3. Bare #4 conductor at least 20 feet in length and near the bottom of the
(within 2 inches), or ½-inch reinforcing steel or rods at least 20 feet in length (one
continuous length or spliced
4. Bare #2 conductor encircling building at least 2½ feet in the ground (spliced together at
electrode conductor at the service equipment can be connected to any
convenient interbonded electrodes that provide a solid, effective connection.
Metal water pipe
shall be supplemented by an additional electrode, which can be any of the following electrodes:
4. Building steel
5. Concrete-encased electrode.
(See Figure 4-8, which lists some of the different
types of grounding electrodes.)
Figure 4-8. If the building steel, metal water pipe, concrete-encased electrode, and ground ring are available, they must be grounded
and bonded to the service equipment to create the grounding electrode system.
4.14 GROUND-FAULT PROTECTION OF EQUIPMENT
Section 2.7 for GFCIs for personnel protection. An increased degree of protection in solidly
grounded systems can be achieved in providing
ground-fault protection that will shunt trip circuit
protective devices when user-selected levels of ground fault or leakage current flow
detected in electrical circuits. This is required to be installed on all solidly grounded wye
services of more than 150 V to ground
but not exceeding 600 V phase-to-phase where the
service disconnecting means is rated at 1,000 A or more (See Figure 3-1).
Personnel working on or close to deenergized lines or conductors in electrical equipment should
be protected against
shock hazard and flash burns that could occur if the circuit were
inadvertently reenergized. Properly installed equipotential protective
grounds can aid in
lessening such hazards by providing additional protection to personnel while they service,
repair, and work on such
systems. (See Section 7.5).
4.15.1 PURPOSE OF PERSONNEL PROTECTIVE GROUNDS
Personnel protective grounds are applied to deenergized
circuits to provide a low-impedance
path to ground should the circuits become reenergized while personnel are working on or close
circuit. In addition, the personnel protective grounds provide a means of draining off static
and induced voltage from other sources while
work is being performed on a circuit (Figure 4-9
illustrates an example of a personnel protective ground).
Figure 4-8. If the building
steel, metal water pipe, concrete-encased electrode, and
ground ring are available, they must be grounded and bonded to the service
to create the grounding electrode system.
Figure 4-9. Equipotential personnel protective grounds are used to protect electrical workers
while they service, repair, or are
close to circuits that can be accidentally reenergized.
4.15.2 CRITERIA FOR PERSONNEL PROTECTIVE GROUNDS
Before personnel protective
grounds are selected, the following criteria shall be met for their
use, size, and application.
1. A grounding cable shall have a
minimum conductance equal to #2 American Wire Gage
2. Grounding cables shall be sized large enough to carry fault current
long enough for the
protective devices to sense and the circuit breaker to clear the fault without damage to
cable insulation. An example
would be a 4/0 Neoprene-insulated welding cable that will
pass 30,000 A for 0.5 sec without melting its insulation.
3. The following
are factors that contribute to adequate capacity:
a. Terminal strength depends on the ferrules installed on the cable ends
Cross-sectional area to carry maximum current without melting
c. Low resistance to keep voltage drop across the areas in which personnel
working at a safe level during any period to prevent reenergization. The voltage drop
should not exceed 100 volts for 15-cycle clearing
times or 75 volts for 30-cycle
d. Verify that the grounding cable and clamp assembly is tested periodically by using
the millivolt drop, micro-ohm meter, AC resistance, or DC resistance test methods.
For example, if it is desired to maintain a maximum of
100 volts across a worker
whose body resistance is 1000 ohms, during a fault of 1000 amperes, a personnel
protective ground resistance
of 10 milliohms or less is required.
Figure 4-9. Equipotential personnel protective grounds are used to protect electrical
while they service, repair, or are close to circuits that can be accidentally
4. For further information on the construction of personnel protective grounds, refer to
Grounding clamps used in personnel protective grounds are manufactured specifically for this
use. The size of grounding clamps
shall match the size of conductor or switchgear bus being
The ground clamp also shall be rated to handle the full capacity
of the available fault currents.
Fault currents can typically range in magnitude up to over 200,000 A.
4.15.4 SCREW-TIGHTENING DEVICES
Approved screw-tightening devices designed for the purpose of pressure metal-to-metal contact
are required for connections to an
adequate system ground.
4.15.5 GROUNDING CABLE LENGTH
Grounding cables should be no longer than is necessary, both to minimize
voltage drop and to
prevent violent movement under fault conditions. For example, as a general rule, grounding
cables should not exceed
30 feet for a transmission line and 40 feet for substation use.
4.15.6 GROUNDING CABLE CONNECTION
Grounding cables shall be
connected between phases to the grounded structure and to the
system neutral to minimize the voltage drop across the work area if the circuit
inadvertently reenergized. Workers shall install the ground end clamp of a grounding cable first
and remove it last.
4.15.7 CONNECTING GROUNDING CABLES IN SEQUENCE
Grounding cables shall be connected to the ground bus, structure, or conductor first,
then to the
individual phase conductors. The first connection of the grounding cables to the circuit phase
conductors shall be to the
closest phase of the system and then to each succeeding phase in
the order of closeness.
4.15.8 REMOVING PROTECTIVE GROUNDS
When removing personnel protective grounds, reverse the order they were applied to the
phases. The grounding cable conductors attached
to the ground bus, structure, or conductors
shall always be removed last.
4.15.9 PROTECTIVE APPAREL AND EQUIPMENT
apparel shall be worn when applying or removing grounds. An insulating tool (hot
stick) shall be used to install and remove grounding cables.
Protective apparel (PPE) should include at least the following:
1. Safety glasses and, if necessary, a face shield appropriate
for existing fault currents.
2. Hardhat (Class B) (See 2.12)
3. Appropriate electrical gloves and protectors (See 2.12).
4. Appropriate clothing