Abstract-This paper is based on the new grounding chapter
in the recently published third edition of On-Site Power
Generation: A Reference Book. [1] It applies to low
voltage power systems rated 600 volts and less, and explains
types of grounding, fault protection, code requirements, and
appropriate solutions. The interrelationship of grounding,
ground-fault sensing and transfer switching is discussed. The
importance of safety to personnel, protection of equipment and
continuity of power to electrical loads is stressed.
For optimum safety of personnel, protection of equipment
and continuity of power to electrical loads, proper grounding
of on-site power systems is essential. If the on-site power is
the sole source of power, then its grounding is relatively
straight forward and follows the National Electrical Code Article 250. However, if the on-site power system serves an
alternate source of power (i.e., emergency or standby power),
then grounding of the system becomes more complex.
Furthermore, portable and mobile generators have special
grounding requirements.
Special consideration should be given to location of ground
connections, means of minimizing circulating ground currents
and providing proper ground-fault sensing. This paper
discusses recommended practices for both system and equipment
grounding without detracting from the ability of alternate
power sources to provide reliable emergency power. It also
provides some design guidelines in selecting transfer
switching equipment, as necessary to meet code requirements
and assure proper system grounding.
There are unique circumstances relating to emergency power
systems that require special grounding considerations. In
large building complexes, consideration should be given to
possible power disruption within the facility and the need for
area protection. At the same time, due consideration should be
given to effects of multiple ground connections, proper
sensing of ground faults and conformance to current code
requirements.
Electrical power systems require two types of grounding;
namely, system grounding and equipment grounding. As
will be shown, each has its own function.
II. SYSTEM GROUNDING
System grounding pertains to the nature and location of an
intentional interconnection between the electric system
conductors and grounding electrode systems which provide an
effective connection to ground (earth). In other words, it
relates to how and where the grounded conductor is connected
to earth.
Purpose of System Grounding: According to Section
250-2 of the National Electrical Code (NEC), system and
circuit conductors are grounded to limit voltages due to
lightning, line surges, or unintentional contact with higher
voltage lines, and to stabilize the voltage to ground during
normal operation. Systems are solidly grounded to facilitate
overcurrent device operation in case of ground faults.
Power Systems: The following emergency power
systems are commonly used to supply phase-to-neutral loads and
should be solidly grounded:
120/240 volt, single-phase, three-wire system
208Y/120-volt, three-phase, four-wire wye system
480Y/277 volt, three-phase, four-wire wye system
600/347 volt, three-phase, four-wire wye system
240/120 volt, three-phase, four-wire delta system
Solidly Grounded Systems: To facilitate overcurrent
device operation the equipment grounding conductors for
solidly grounded systems are required to be bonded to the
system grounded conductor at the service equipment and at the
source of a separately derived system (standby or emergency
power). This bond completes the ground-fault current return
path from the equipment grounding conductors to the system
grounded conductor. System grounding conductors and grounding
electrodes are not intended to conduct the ground-fault
current that is due to a ground-fault in equipment, raceways,
and other enclosures. In solidly grounded systems, the
ground-fault current flows through the equipment grounding
path.
Solidly grounded systems exercise the greatest control of
overvoltages but result in the highest magnitudes of
ground-fault current. However, the inherent regulation of
alternate source generators generally limits the available
fault current. If line-to-neutral loads must be served, high
resistance grounding of the generator neutral is not permitted
because the neutral should never be used as a circuit
conductor in systems that are grounded through a high
resistance.
High Resistance Grounding: Where line-to-neutral
loads need not be served, there is a growing trend toward
high-resistance grounding of the neutral conductor. This is
particularly true in industrial applications and where medium
voltage generators are used for emergency power. High
resistance grounding combines the advantages of ungrounded
systems and solidly grounded systems. Fault current is limited
to a value that will permit continued operation of the
emergency power system until the normal power is restored.
System overvoltages are held to acceptable levels and faults
are easier to locate than on an ungrounded system. For these
reasons, high resistance grounding is often used in process
industries where an unscheduled outage can have serious
consequences.
III. EQUIPMENT GROUNDING
It is important that all exposed metallic parts of
electrical equipment be bonded to a grounding electrode. This
includes metallic parts such as generator frames and engines
of engine generator sets, mounting bases, electrical conduit
and enclosures.
Purpose of Equipment Grounding: The purpose of
equipment grounding is to assure the following functions:
1. To maintain a low potential difference between nearby
metallic members and thereby protect people in the area from
electric shock.
2. To facilitate the operation of circuit protective
devices by providing low impedance conducting paths for
ground-fault currents.
3. To provide an effective electrical conductor system over
which ground-fault currents can flow without creating a fire
or explosive hazard.
Equipment Grounding Paths: Only by providing
adequate equipment grounding paths for metallic enclosures,
frame of an engine generator set, etc., in a manner which
assures adequate current-carrying capability, and an
adequately low-value of ground-fault circuit impedance, can
both electric shock hazard and fire hazard be avoided.
Technical investigations have pointed out that it is important
o have good electric junctions between sections of conduit or
metal raceways that are used as equipment grounding paths, and
to assure adequate cross-sectional area and conductivity of
these grounding paths. Where systems are solidly grounded,
equipment grounding conductors are bonded to the system
grounded conductor and the grounding electrode conductor at
the service equipment and at the source of a separately
derived system in accordance with the requirements of Sections
250-30, 250-64 and 250-130 of the National Electrical Code
(NEC).
The same network of equipment grounding paths is required
for systems that are solidly grounded, high resistance
grounded, or ungrounded. The equipment grounding paths in
high-resistance grounded or ungrounded systems provide shock
protection and a conducting path for phase-to-phase fault
current if two ground faults occur simultaneously on different
phase conductors.
Engine Generator Frame: The engine generator frame,
automatic transfer switch enclosure, conduits and other
exposed components of an emergency power systems should be
permanently bonded together and connected to the grounded
conductor of the service equipment providing normal power
and/or the grounded conductor of the engine generator set. By
means of grounding electrode conductors, these grounded
conductors are connected to grounding electrodes at their
respective locations.
The National Electrical Code permits the grounding
electrode to be a metal underground water pipe which may be
part of a municipal water piping system or a well on the
premises. However, buried portion of a water pipe system less
than 10 feet in length or that may include plastic or cement
materials should not be used. For small emergency power
systems where the ground currents are of relatively low
magnitude, existing buried water pipe systems are usually
preferred as electrodes because they are economical in first
cost. Nevertheless, before reliance can be placed on any
existing electrodes, it is essential that their resistance to
earth be measured to insure that some unforeseen discontinuity
has not seriously affected its suitability. Also, care should
be exercised to insure that all parts which might become
disconnected are effectively bonded with flexible bonding
jumpers. Larger emergency power systems in industrial and
commercial buildings often use the effectively/grounded metal
frame of the building and concrete-encased metal below ground
level as a grounding electrode. The concrete encasement of
steel, in addition to contributing to low grounding
resistance, serves to provide some measure of corrosion
protection compared to direct contact with the earth. Due
consideration must also be given to the ampacity of the
equipment grounding path in order to handle the higher
available ground-fault current.
Another alternative is to use “made” electrodes such as
driven electrodes, buried cables and plates. The type selected
will depend upon the type of soil encountered and the
available depth. Although the National Electrical Code permits
up to 25 ohms resistance to ground for “made” electrodes,
grounding electrodes for engine generator sets in industrial
and commercial installations should be kept to a minimum and
preferably below five ohms. Where the building steel is bonded
to the ground grid system as well as most pipelines, it is not
unusual to obtain an impedance as low as 0.1 to 0.2 ohms.
Although the National Electrical Code requires only one
electrode, sometimes more than one is necessary.
Another reason for positive grounding is that the radio
frequency noise suppressors in some generators are connected
to the engine generator frame. Inadequate frame grounding can
result in interference to communication or audio equipment and
improper operation of electronic governors. If the engine
generator set is mounted on flexible or resilient pads, a
flexible bonding jumper must be used.
Supplemental Equipment Bonding: Possibility of
electrical shock can be reduced by additional supplemental
equipment bonding between the conductive enclosures for
conductors and equipment and adjacent conductive materials.
This additional equipment bonding helps minimize any potential
between exposed noncurrent-carrying metal parts of the
electric system and adjacent grounded building steel when
ground faults occur. The inductive reactance of the
ground-fault circuit will normally prevent a significant
amount of ground-fault current from flowing through the
supplemental bonding connections.
Ground-fault current will flow through the path that
provides the lowest ground-fault circuit impedance. The
ground-fault current path that minimizes the inductive
reactance of the ground-fault circuit is through the equipment
grounding conductors that are required to run with or enclose
the circuit conductors. Therefore, practically all of the
ground-fault current will flow through the equipment grounding
conductors, and the ground-fault current through the
supplemental bonding connections will be no more than required
to equalize the potential at the bonding locations.
IV. GROUND-FAULT SENSING AND PROTECTION
Providing reliable ground-fault protection in electrical
systems having alternate emergency or standby power can be
difficult unless adequate equipment is selected and properly
coordinated. Multiple neutral-to-ground connections may
prevent adequate sensing of ground fault currents and may also
cause nuisance tripping of circuit breakers. To assure proper
ground-fault protection, understanding the evolution of code
requirements can be helpful.
National Electrical Code: Prior to 1971, most
automatic transfer switches for three-phase four-wire systems
had three poles with no provisions for switching the neutral
conductor. The neutral conductor was grounded at both sources
of power. When ground-fault protection of equipment became a
requirement in 1971 (NEC Section 230-95), application problems
began to appear in new emergency and standby power system
installations. The problems dealt primarily with obtaining
proper sensing of ground-fault currents because of multiple
neutral-to-ground connections. Under some conditions, the
ground-fault sensors would fail to detect fault currents.
Under other conditions, unbalanced loads would cause
ground-fault sensors to trip breakers, even though a ground
fault or short-circuit did not exist.
Becoming aware of the above problems, the National Fire
Protection Association (NFPA) decided to form a technical
committee (including coauthors Rene Castenschiold and Gordon
Johnson) to recommend changes to the NEC. At that time, the
1975 NEC included the following:
1. Section 230-95 stated, “Ground-fault protection of
equipment shall be provided for solidly-grounded wye
electrical services of more than 150 volts to ground, but not
exceeding 600 volts phase-to-phase for each service
disconnecting means rated 1000 amperes or more.”
2. Section 250-5(d) stated, “A premises wiring system
whose power is derived from generator, transformer, or
converter windings and has no direct electrical connection to
supply conductors originating in another supply system.”
3. Section 445-1 stated, “Generators and their associated
wiring and equipment shall be considered to be separately
derived systems and shall comply with the applicable
provisions of Articles 230, 250, 700, and 750.
The technical committee made recommendations (not
unanimously, however) which resulted in the following two
changes in the 1978 NEC:
1. The definition for separately derived systems per
Section 250-5(d) was changed by adding the phrase “including
a solidly-grounded circuit conductor” and to read as
follows: “A premises wiring system whose power is derived
from generator, transformer, or converter windings and has no
direct electrical connection, including a solidly grounded
circuit conductor, to supply conductors originating in another
supply system…”
(Note-this definition was removed from Article 250 in the
1999 NEC, but retained in Article 100.)
2. Section 445-1 was changed by deleting the phrase
requiring generators to be considered as separately derived
systems. The section now reads as follows: “Generators and
their associated wiring and equipment shall comply with the
applicable provisions of Articles 230, 250, 700 and 750.”
Effects of the Above Changes: The net effect of the
1978 code changes was that an alternate power source, such as
engine generator set, could no longer be interpreted as a
separately derived system if its neutral conductor is directly
connected with the neutral of the normal source of power.
An emergency source could now only be considered to be a
separately derived system and have its neutral termination
separately grounded when the associated transfer switch
provided switching of the neutral conductor. In order to
comply with the new definition for separately derived systems,
grounding of the engine generator’s neutral conductor could
therefore only be done by one of two methods.
1. Permanently ground the neutral conductor of the
emergency source generator at only the grounding electrode of
the incoming service for the normal utility source. There
would be no switching of the neutral conductor, and the
emergency source would not therefore be considered a
separately derived system. It later became apparent that this
method did not resolve all ground-fault sensing problems.
2. Permanently ground the neutral conductor of the
emergency source generator at its location. This would require
transfer switching of the neutral conductor, and the emergency
source would be defined as a separately derived system.
Additional Code Changes: For emergency power
systems the 1984 NEC added Section 700-7(e) requiring sensing
and indication of ground-fault currents under certain
conditions when the engine generator set(s) is feeding the
load. In subsequent NEC editions this requirement is
identified as 700-7(d). To accomplish this properly it is
necessary for the emergency source to be a separately derived
system. The 1993 NEC added Section 700-8 requiring a sign to
identify all sources connected to a grounding electrode when
it is remote from the engine generator set(s). Article 250 in
the 1999 NEC was completely rewritten.
V. TRANSFER SWITCHING
There are three approaches to consider in meeting the
current requirements of the National Electrical Code. For a
three-phase 4-wire grounded system, they include (1) using a
3-pole transfer switch with the engine generator set
considered as a non-separately derived source, (2) using a
4-pole transfer switch with the engine generator set
considered as separately derived source, and (3) using a
3-pole transfer switch with overlapping neutral contacts with
the engine generator set considered as a separately derived
source. However, just meeting the minimal requirements of the
code does not necessarily provide the degree of reliability
needed for a good emergency power system. The system designer
should consider the following:
An engine generator set is often remotely located from the
grounded utility service entrance and the ground potentials of
the two locations may not be the same. Good engineering
practice requires the automatic transfer switch to be located
as close to the load as possible to provide maximum protection
against power failure due to cable or equipment failures
within the facility (area protection). Thus, the distance of
cable between incoming service and the transfer switch and
then to the engine generator set may be substantial. Consider
what would happen if a cable failure occurred, as in Figure
1,
with the engine generator not grounded to its own grounding
electrode. The load would be automatically transferred to an
ungrounded emergency power system. This, in turn, could
jeopardize emergency service continuity and possibly lead to
additional failures. Concurrent failure of equipment or cable
failure (break-down between line and equipment ground) after
transfer to emergency may not be detected. Thus the generator
frame may approach line potential, causing a substantial
voltage difference between the generator frame and the
grounded conductor (neutral).
Some local codes require ground-fault protection while the
engine generators is operating. This may present a sensing
problem if the neutral conductor of the generator is not
connected to a grounding electrode at the generator site and
proper isolation of neutrals is not provided. Furthermore,
some local and state codes (Pennsylvania and Massachusetts)
require area protection which may crate additional problems
unless the neutral conductor of the generator is isolated and
grounded at the generator location.
When the transfer switch is in the emergency position,
other problems may occur if the engine generator set is not
properly grounded. For example, a ground-fault condition could
cause nuisance tripping of a normal source ground-fault
circuit breaker even though load current is not flowing
through the breaker. This would prevent automatic retransfer
to the normal source and isolation of the fault condition.
This would not be in accord with Section 230-95 of the National Electrical Code which since 1978 states, “Where
ground-fault protection is provided for the service
disconnecting means and interconnection is made with another
supply system by a transfer device, means or devices may be
needed to assure proper ground-fault sensing by the
ground-fault protection equipment.” Furthermore, both the
normal neutral conductor and the emergency neutral conductor
would be simultaneously vulnerable to the same ground-fault
current. Thus a single fault could jeopardize power to
critical loads even though both utility and emergency power
are available. Such a condition may be in violation of codes
requiring independent wiring and separate emergency feeders.
Three Pole Transfer Switching: Using three pole
transfer switches is the least expensive approach and requires
less space. However, ground-fault currents may trip the normal
source breaker when the load is connected to the engine
generator set. See diagram in figure
2. Furthermore, use of
3-pole transfer switches does not provide area protection
within the facility and this could lead to an ungrounded power
system in the event of cable failure. See diagram in figure
1.
Finally, with the neutral conductor connected to the incoming
normal service, it is difficult to provide ground-fault
sensing on the emergency side as required to meet Section
700-7(d) of the National Electrical Code.
Four Pole Transfer Switching: A better solution is
to use a four pole transfer switch which provides complete
isolation of service and generator neutral conductors. This
eliminates possible improper ground-fault sensing and nuisance
tripping caused by multiple neutral-to-ground connections.
When this is done the generator will comply with the current
National Electrical Code definition of a separately derived
system. With the neutrals thus isolated, ground-fault
protection with conventional sensors can be added to the
generator output.
Four pole transfer switches have been satisfactorily
applied on applications where the loads are passive and
relatively balanced. However, unbalanced loads may cause
abnormal voltages for as long as 10 to 15 milliseconds when
the neutral conductor is momentarily opened during transfer of
the load. Transfer switches are frequently called upon to
operate during total load unbalance caused by a single phasing
condition. Inductive loads may cause additional high transient
voltages in the microsecond range. Furthermore, it should be
remembered that the contacts of the fourth switch pole do
interrupt neutral currents and are, therefore, subject to
arcing and contact erosion. However, contact erosion of the
fourth pole can be minimized if the fourth pole is designed to
open last (after the three other poles interrupt the load
current) and make first without contact bounce. In any event,
a good maintenance program must reaffirm at intervals the
integrity of the fourth pole as a current-carrying member with
sufficiently low impedance.
Both the National Electrical Code and Underwriters
Laboratories, Inc., recognize the use of four pole transfer
switches for 3-phase, 4-wire systems. In some European
countries, including Germany and France, it is mandatory to
use 4-pole switching devices.
Overlapping Neutral Contacts: Another method for
isolating the normal and emergency source neutrals is for a
3-pole automatic transfer switch to include overlapping
neutral transfer contacts. This feature provides the necessary
isolation between neutrals and at the same time minimizes
abnormal switching voltages. By means of overlapping contacts,
the only time the neutrals of the normal and emergency power
sources are connected together is during transfer and
retransfer. With a sole-switch, this duration can be less than
the operating time of the ground-fault sensor which is usually
set anywhere from 6 to 24 cycles (100 to 400 milliseconds).
Figure 3 shows a typical system utilizing a three-pole
transfer switch with overlapping neutral transfer contacts for
isolating the neutral conductors. Note that there is no
possible flow of fault current through the neutral conductor
that would detract from or effectively reduce ground-fault
detection. Furthermore, there is no possible flow of
unbalanced current through the generator neutral to alter the
response level of the ground-fault sensor and possibly cause
nuisance breaker tripping. As with four pole transfer
switches, conventional ground-fault sensing can be readily
added to the emergency side. As shown in figure
3,
ground-fault sensing on the emergency side is often used for
actuating an alarm circuit rather than tripping a breaker,
such as required per Section 700-7(d) of the National
Electrical Code.
With overlapping neutral transfer contacts, the load
neutral is always connected to one source of power. In that
there is no momentary opening of the neutral conductor when
the transfer switch operates, abnormal and transient voltages
are kept to a minimum. Another advantage is that, there is no
erosion of the overlapping contacts due to arcing, thus
assuring current-carrying integrity and no increase in
impedance of the neutral circuit. It should be noted that a
3-pole transfer switch with over lapping neutral contacts
should not be specified as a 4-pole switch, according to
Underwriters Laboratories Standard UL-1008 and the National
Electrical Code. [4] [2]
Selecting Transfer Switching Equipment: For
three-phase, four-wire systems involving one transfer switch
and one engine generator set, response to the questions in
Table 1 serve as a guide in selecting the type of transfer
switch needed for a particular application.
Table 1
Questions
Yes
No
a. Is the application for emergency power per Section 700
of the National Electrical Code?
b. Is the incoming service of the normal source rated 1000
amperes or more?
c. Is the service a solidly-grounded wye that is rated more
than 150 volts to ground?
d. If the engine generator set is located remote from the
normal incoming service, is area protection required?
e. Is sensing and alarm indication of a ground fault on the
emergency or standby source required? |
If the response to the first three questions (a, b, and c)
are all “yes,” it is necessary to use a 4-pole transfer
switch or a 3-pole transfer switch with overlapping neutral
contacts in order to satisfy the requirements of the 1999 National
Electrical Code. If the answer is “no” to any one of
these questions, then proceed with questions d and e.
If the response is “yes” to either question d or e, use
a 4-pole transfer switch or a 3-pole transfer switch with
overlapping neutral contacts in order to satisfy the
requirements of the National Electrical Code. If the
answer is “no” to both questions, a less expensive 3-pole
transfer switch without overlapping neutral contacts can be
used. Then the neutral terminal of the generator should be
grounded at the incoming service of the normal source.
Although the above questions apply to three phase four wire
systems, the same approach can be taken for single phase three
wire systems to determine the transfer switching equipment.
However, most single phase systems are 400 amperes or less,
and therefore can be satisfied with 2 pole transfer switches.
It should be kept in mind that the above selection approach
only applies to applications with one engine generator set and
one transfer switch.
VI. OTHER CONSIDERATIONS
Because of the variety of emergency and standby power
systems, it is not always feasible to provide fixed
recommended practices. Each installation should be evaluated
and designed to satisfy specific load, customer and code
requirements. Examples of other special conditions are as
follows:
Multiple Transfer Switches: Multiple transfer
switches, located close to the loads, are often used rather
than one transfer switch for the entire load. In such cases,
consideration should be given to the possibility of cable or
equipment failure possibly causing an emergency or standby
power system to become ungrounded. This is particularly
important if a solidly interconnected neutral conductor is
grounded at the service equipment only.
Under such conditions, there is a possibility of tripping
the ground-fault circuits when on ground fault exists. Figure
4 illustrates a typical system with multiple three-pole
transfer switches. When power is supplied from the normal
source of the load through a transfer switch the neutral
current will divide between two neutral conductors. Some of
the current will return by the normal return path. The
remainder of the current will flow to the neutral terminal of
the engine generator and through the neutral conductor of the
other transfer switch to the normal source. This can cause
tripping of either ground-fault circuit breaker even when no
ground-fault exists. The problem can be corrected by the use
of the three-pole transfer switches with overlapping neutral
or four-pole transfer switches.
Multiple Engine-Generator Sets: When multiple
engine-generator sets are connected in parallel and serve as a
common source of power, each generator neutral is usually
connected to a common neutral bus within the paralleling
switchgear which, in turn, is grounded. The associated
switchgear containing the neutral bus should be located in the
vicinity of the generator sets. A single system-grounding
conductor between the neutral bus and ground simplifies the
addition of ground-fault sensing equipment.
It may be argued when individual grounding resistors are
used, circulation of harmonic currents between paralleled
generators is not a problem, since the resistance limits the
circulating current to negligible values. However, if third
harmonics are suppressed in the engine-generator sets,
circulating currents are usually not a problem.
Engine generator sets which are physically separated and
used for isolated loads may necessitate additional
neutral-to-ground connections. By using multiple four-pole
transfer switches or three-pole switches with overlapping
neutral contacts, proper isolation and ground-fault sensing
can be obtained.
Lightning Protection: Protection from power
failures caused by lightning storms is a prime purpose for
providing emergency power systems. It, therefore, seems only
logical that the emergency power system be properly protected
against lightning. Particular concern should be given to
buildings housing emergency power equipment that are
constructed of minimal nonconducting materials. When engine
generator sets are installed on roof tops, additional
consideration may have to be given to the installation of
surge arrestors and grounded metallic masts.
Since lightning currents (direct or through arresters) may
flow through various equipment ground components, the
connection to earth is a most important part of the equipment
ground system. Arrester grounds which are normally secured by
driven ground rods near the arrester should not exceed 5 ohms.
A grounding reactor is sometimes preferred in medium
voltage applications where the feeders from the generators are
exposed to lightning surges (overhead power line). For the low
frequencies (50 and 60 Hz), the reactor forms a low impedance
to ground. For the lightning and switching surges, which are
high frequency, the reactor forms a higher impedance thus
limiting the current without allowing the high overvoltages
associated with an ungrounded system. However, it should be
kept in mind that the neutral cannot be used as a circuit
conductor where the system is grounded through an impedance.
Damage to structures and equipment due to surge effects is a
subject in itself, and protection against this type of damage
is not within the scope of this paper. For additional
information and references, see Chapter 3 of IEEE Standard
142-1991 (Green Book). [3]
Mobile Engine Generator Sets: Though somewhat more
difficult, proper grounding of mobile engine generator sets is
extremely important. The mobility of such equipment often
precludes the installation of a predesigned ground mat such as
might be employed as a fixed installation. The portable
equipment must be designed and grounded to permit personnel to
approach (and touch) the apparatus without risk of dangerous
electric shock.
Providing sensitive fast operating ground-fault protection
with mobile engine generator sets provides additional
protection. Switching devices or connectors should not be
installed in the grounding conductor which may be subject to
loss of conductivity. Grounding of the engine generator frame
and vehicle chassis should be positive and thoroughly tested
prior to engine starting.
Plate electrodes are sometimes used for grounding mobile
engine generator sets. In addition to the minimum requirements
of Section 250-52(d) of the 1999 National Electrical Code, the
following conditions are recommended: [5]
1. 1.5 square feet of the plate surface should be below the
surface and a linear distance of at least one (1) foot is
common to earth and air above the earth.
2. The electrode should be located within a distance of no
more than three (3) feet from the exterior surface of the
generator set.
3. The electrode should be exposed to rain or moisture at
least to the same degree as is the housing or exterior metal
surfaces of the generator set.
Connection to the plate electrode would be in addition to a
solid connection to the equipment grounding conductor of the
facility or building to which the mobile power is being
served. There is no restriction on the number of grounding
electrodes conductors or electrodes that may be used. The more
the better from a safety standpoint.
Small Portable Generator Sets: Grounding small
portable generator sets presents a different problem. It is
often impractical at construction and similar sites to ground
the generator set. Section 305.6(a) Exception 1 of the National
Electrical Code permits leaving such sets ungrounded as
long as the only loads are connected to receptacles mounted on
the generator set. The code does require that such sets have
GFCI (ground-fault circuit interrupters) for personnel
protection either on the receptacles or the extension cord,
and as with all electrical equipment an equipment ground
system be used. An exception (Section 305.6(a) Exception No.
1) to the GFCI requirement is allowed for 120 volt two wire
sets, not over 5 kW, that has both its electrical connectors
insulated from ground.
Generator Sets Mounted in Vehicles: Section
250-34(b) of the National Electrical Code does permit
the frame of a vehicle to serve as the grounding electrode for
a system supplied by a generator located on a vehicle,
provided that:
1. The frame of the generator is bonded to the vehicle
frame, and
2. The generator supplies only equipment located on the
vehicle and/or cord-and-plug-connected equipment through
receptacles mounted on the generator, and
3. The noncurrent-carrying metal parts of equipment and the
equipment grounding conductor terminals of the receptacles are
bonded to the generator frame, and
4. The system complies with all other provisions of Article
250 of the National Electrical Code.
Using the frame of a vehicle as the grounding electrode is
practical when the generator is the sole source of power,
particularly if other grounding means are not readily
available. However, if the generator serves as an alternate
source of power (i.e., emergency or standby power), then
grounding as explained above for mobile engine generator sets
may be more appropriate. This would assure solid grounding of
the generator frame to equipment grounding conductors of the
facility or building to which the mobile power is being
served. Furthermore, ground-fault sensing and whether or not
the generator set is to be considered as a separately derived
source are factors to consider. In either case, it is
important that the frame of the generator is bonded to the
vehicle frame.
Upgrading Transfer Switching Equipment: Upgrading
older transfer switch equipment to present codes and standards
can be a real challenge, particularly for larger and more
complex industrial and commercial facilities. Awareness that
three-pole transfer switches may not always provide complete
isolation and proper ground-fault sensing has not been too
apparent in the past. With more emergency and standby power
systems being updated, the concern regarding safety and
product liability should not be treated too lightly. This
concern provides another reason for upgraded installations to
include adequate grounding and preferably new transfer
switches that provide switching of the neutral conductor.
Regardless of whether or not ground-fault sensing is required,
proper switching of the neutral conductor is good engineering
practice because of the system isolation, improved grounding
and the extra safety it can provide. [6]
There are a number of factors that should be considered
when grounding on-site power systems. Simply conforming to
minimum code requirements will not necessarily assure the
degree of reliability required for such systems. Thorough
consideration should be given to protecting against power
disruption within the building or facility, and providing
adequate ground-fault protection. Techniques for both
equipment grounding and system grounding should be such as to
provide optimum safety and assure maximum continuity of power
to essential loads. This includes proper grounding and
ground-fault sensing when the transfer switch is in the
emergency position as well as the normal position. Grounding
instructions provided by the manufacturer of the equipment
should be followed.
As a general rule, most applications requiring ground-fault
sensing, area protection, multiple transfer switches, or
multiple engine generator sets, the on-site power should be
considered as a separately derived system. In such
cases the neutral of the engine generator set is grounded at
its location.
It is not intended for the scope of this paper to cover in
detail all aspects of grounding on-site power systems. Those
requiring more detailed information may find the following
list of references helpful.
REFERENCES
[1] EGSA On-Site Power Generation: A Reference Book.
Third Edition. Electrical Generating Systems Association,
Coral Springs, FL (edited by coauthor Gordon Johnson)
[2] National Electrical Code, NFPA No. 70-1999.
National Fire Protection Association, Quincy, MA
[3] IEEE Recommended Practice for Grounding of
Industrial and Commercial Power Systems (Green Book), Standard
142-1991, Institute of Electrical and Electronic Engineers,
New York, NY (chaired by coauthor Gordon Johnson)
[4] Standard for Automatic Transfer Switches, UL
1008, Underwriters Laboratories, Melville, NY
[5] Grounding of Alternate Power Sources, by R.
Castenschiold. IEEE Conference Record. Industry Applications
Society Annual Meeting, October 1997. Institute of Electrical
and Electronic Engineers, New York, NY
[6] Factors to Consider When Upgrading Transfer
Switching to Present Codes and Standards, by R.
Castenschiold and I. F. Hogrebe. IEEE Transactions Industry
Applications, Vol. 29, No. 3, May/June 1993; Institute of
Electrical and Electronic Engineers, New York, NY
BIBLIOGRAPHY
[B1] IEEE Recommended Practice for Emergency and Standby
Power Systems (Orange Book), Standard 446-1995, Institute
of Electrical and Electronic Engineers, New York, NY
[B2] IAEI Soares Book on Grounding, 6th Edition, by
J.P. Simmons, International Association of Electrical
Inspectors, Richardson, TX
[B3] More About Standby Generator Grounding, GFP, and
Currents that Go Bump in the Night by Hugh O. Nash, Jr.,
IEEE Transactions Industry Applications, Vol. 33, No. 3.,
May/June 1997, Institute of Electrical and Electronics
Engineers, New York, NY
[B4] National Electrical Grounding Research Project, by
T. Lindsey, IAEI News, May/June 1998, International
Association of Electrical Inspectors, Richardson, TX
Rene Castenschiold, P.E., Life Fellow, IEEE
LCR Consulting Engineers, P A
Green Village, NJ 07935
Gordon S. Johnson, P.E., Life Fellow, IEEE
306 7th Street S
Dundee, FL 3383
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