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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.
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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.
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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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Webmaster:
Kirt Blattenberger, BSEE, UVM 1989
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