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DOE Handbook
Electrical Safety
- Grounding -
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4.0 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
Engineering 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
the grounding conductors. Where used
for installation or construction purposes, these
specifications and drawings should also include detailed
installation instructions.
4.2 CIRCUIT AND SYSTEM GROUNDING
Circuit and system grounding consists of connecting the grounded conductor, the equipment
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 higher
voltage
lines.
2. To keep conductor enclosures and noncurrent-carrying metal enclosures and equipment at
zero potential to ground.
3. To facilitate the opening of overcurrent protection devices in case of
insulation failures
because of faults, short circuits, etc.
4.3 EQUIPMENT GROUNDING
Equipment
grounding systems, which consist of interconnected networks of equipment
grounding conductors, are used to
perform the following functions:
4-1
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
required to:
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.
4.4 BONDING
Caution shall be taken to ensure that the main bonding jumper and equipment
bonding jumper are
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.
4-2
NEC 250.4
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
their use. Electrical systems are
grounded to protect circuits, equipment, and conductor
enclosures from dangerous voltages and personnel from electrical shock.
4.5 GROUNDED OR UNGROUNDED SYSTEMS
Ungrounded systems may provide greater continuity of operations in the event of a fault.
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
still energized 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 and
raceways together with the
grounding conductors.
4-3
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 protection device.
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.
The 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 fault
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.
NEC 250.130
Figure 4-2. A grounded system is equipped with a grounded (neutral) conductor routed
between the supply transformer and the service equipment.
4-4
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,
equipment 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
reliability 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
conductor 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 and proper
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) conductor
routed between the supply transformer and the service equipment
because the supply
transformer is not earth grounded.
4-5
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 system. (See
Figure 4-4.)
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 supplying
phase-to-neutral loads shall also be solidly grounded (See Figure 4-5). The following electrical
systems are
required to be solidly grounded:
1. 240/120-V, single-phase, three-wire
2. 208Y/120-V,
three-phase, four-wire
3. 480Y/277-V, three-phase, four-wire
4. 240/120-V, three-phase, four-wire,
delta (midpoint of one phase used as a
grounded circuit conductor)
The following 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.
NEC 250.36
4-6
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
loads.
4.7 GROUNDING ELECTRODE CONDUCTOR (GEC)
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 path
2. The equipment grounding path
3. The
bonding path
NEC 250.20
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.
4-7
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 present
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 250.66
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.
Exception (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
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
components. 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
2. Equipment grounding conductors and grounding terminal
3. All metal enclosures enclosing
conductors and components.
If supplied, 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
1100 kcmil for
copper or 1750 kcmil for aluminum.
For example: What size main bonding jumper is required to ground the
metal enclosure of the
service equipment to the grounding terminal bar where the service entrance is made up of
one
#250 kcmil, THWN copper conductor per phase?
For example: What size main copper bonding jumper is required for a service entrance with a
makeup of
2400 kcmil copper conductors per phase?
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
Answer: The
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 kcmil
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
4.9 SYSTEM WITH GROUNDED CONDUCTOR
The main purpose of the grounded conductor is to carry
unbalanced neutral current or fault
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
equipment 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
switchboard.
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
follows:
1. Service equipment
2. Meter base
3. Current transformer (CT) can
4. Metal gutter or wire
way containing service entrance conductors.
See Figure 4-6 for the rules concerning the use of the
grounded conductor.
4-10
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
follows:
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
copper
Answer: The size
of the grounded conductor is at least #2 THWN copper.
NEC 250.24(b)
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.
4-11
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.
For 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
conductors in 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 equipment
grounding conductor shall also be adjusted in size.
5. The
equipment grounding conductor is never required to be larger than the circuit
conductors.
Step 1:
Service exceeding 1100 kcmil - NEC Table 250.66, 2400 kcmil x 0.125 = 300
kcmil
Answer: The grounded
conductor is required to be at least a #300 kcmil, THWN copper
conductor.
4-12
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 or are
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
source. 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.
4.11 UNGROUNDED SYSTEMS
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 in
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.
4-13
4.12 GROUNDING A SEPARATELY DERIVED SYSTEM
NEC 250.30 covers the rules for grounding separately
derived systems. The system grounding
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
pipe. 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
system disconnecting means or overcurrent protection device. The equipment grounding
conductor should
always be connected to the enclosure of the supply transformer, generator, or
converter.
The grounding
electrode conductor, the main bonding jumper, the grounded conductor, and the
equipment 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.
4-14
4.13 GROUNDING ELECTRODE SYSTEM
If 10 feet or more of metal water pipe is in the earth, the
water pipe is considered the grounding
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
of electrodes 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 concrete foundation
(within 2
inches), or ½-inch reinforcing steel or rods at least 20 feet in length (one
continuous length or spliced
together)
4. Bare #2 conductor encircling building at least 2½ feet in the ground (spliced together at
each end).
The grounding 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:
1. Rod
2.
Pipe
3. Plate
4. Building steel
5. Concrete-encased electrode.
(See Figure 4-8, which lists
some of the different types of grounding electrodes.)
4-15
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
See 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 are
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).
4.15 PERSONNEL PROTECTIVE GROUNDS
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
to the 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
equipment to create the grounding electrode system.
4-16
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
(AWG) copper.
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
b.
Cross-sectional area to carry maximum current without melting
c. Low resistance to keep voltage drop across the areas in which personnel are
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
clearing times.
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
workers while they service, repair, or are close to circuits that can be accidentally
reenergized.
4-17
4. For further information on the construction of personnel protective grounds, refer to
Section 7.5.
4.15.3 GROUNDING CLAMPS
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
grounded.
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 should become
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
Protective 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.
4-18
2. Hardhat (Class B) (See 2.12)
3. Appropriate electrical gloves and protectors (See 2.12).
4. Appropriate clothing (See 2.12).
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