Grounding and bonding
in commercial buildings
An understanding of the basic operations between a grounded and an
ungrounded electrical system is necessary for matching the appropriate
grounding topology to the electrical system performance.
BY SAM R. ALEXANDER, PE, LEED AP BD+C, exp, Maitland, Fla.
T
here are various benefits for
grounding and bonding ac transmission and distribution power
systems. The basis for selection of a
given grounding system type depends
on its ability to provide personnel safety and equipment protection. Primarily,
the electric power industry is concerned
with reducing shock and flash hazards
to personnel working with electrical
systems, limiting damages to the electrical system components due to transient overvoltages,
and minimizing
Generator
Transformer with delta
interruption to
with delta
primary windings and
output
grounded wye secondary
the commercial
windings
windings
or industrial proGround
cesses that the
system 1
G
electrical system
480 V, 3-pharse
supports.
Based on these
criteria, the preM
vailing groundT1
ing design philosophy is to
208 V, 3-phase
provide a grounded system over
an ungrounded
Ground system 4
Ground
one for satisfying
system 2
these objectives.
Ground
Nevertheless, an
system 3
120 V, 1-phase
understanding of
Figure 1: Boundary of the electrical systems is shown.
the basic operaCourtesy: Eaton Corp.
tion of each type
50
Consulting-Specifying Engineer • AUGUST 2012
of system is necessary for matching the
appropriate grounding topology to the
electrical system performance. Commercial buildings, with most of their equipment operating at 600 V and less, seem
to have standardized on a solid grounding and bonding approach. Proper application of this approach is done through
the lens of the National Electrical Code.
Reasons for grounded and
ungrounded systems
According to the NEC, there are two
main purposes for grounding the electrical ac system: one is to stabilize the
system voltage to earth during normal
operating conditions by providing an
earth’s reference frame for the system;
the other is to maintain within acceptable limits, excess voltages on the system due to lightning, line surges, and
incidental contact with higher voltages.
These two reasons allow the design
engineer to meet the two primary goals
of equipment protection and personnel
safety for the electrical system. A third
goal for grounding is to allow the processes supported by the electrical system
to continue in the presence of a faulted
condition. This is usually achieved by
either an ungrounded system or by application of a specialized form of grounding (high-resistance grounding).
Power systems in the 1950s tended to
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Explanation of terms
A
Figure 2: Commercial building transformer solid grounding is shown. Courtesy:
exp, with reference to NEC
be ungrounded, 3-phase, 3-wire, with
delta transformer and delta generator
configuration. The main benefit of this
grounding configuration is that it allows
a single bolted phase-to-ground fault to
operate indefinitely without damage at
the faulted location, and without tripping
of a protective overcurrent device. This
provides continuity of service while the
faulted conductor is located, albeit with
shock hazard risk to personnel. However, the majority of ground faults are not
the bolted type, but the low-level arcing (restriking) type. These restriking
ground faults, because of their relatively
low fault currents, can go undetected by
ground-fault monitoring equipment. The
danger here is that the restriking ground
faults produce escalating transient overvoltages on the conducting system insulation. If left unchecked, the voltage
stress on the system insulation can lead
to a double line-to-ground fault, which
would result in the unwanted tripping
of the protective overcurrent devices.
An even worse scenario would be the
destructive arc-flash hazard consequences. For this reason, ungrounded systems
are less likely to be constructed now,
and are more likely to be upgraded with
some type of an impedance grounded
system.
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There are various points on the electrical system available for grounding,
such as the midpoint of a single-phase
transformer, corner of the delta windings, or the center of the wye windings.
The points that are considered the neutral point of the system are most commonly used for grounding. The neutral
point affects, and is in turn affected by,
the other three phases identically on a
balanced 3-phase system. By its nature,
this point presents the best opportunity to realize the two main purposes
for grounding the electric power system. The grounding methods described
below involve connection to the neutral
point of a wye system (generator or
transformer). In general, where neutral
points for grounding are not available
on the generator or transformer windings as in a delta connection, grounding
transformers such as zigzag or wye-delta
transformers are used. These grounding
transformers effectively create a neutral
connection that can then be grounded.
Types of grounding
High-resistance grounding (HRG),
with its application in the voltage range
of 480 V to 13.8 kV, provides a means
for limiting the problems with transient
overvoltages associated with ungrounded
grounded electrical system is one in which
at least one conductor from the system, or
point on the conductive system, is connected to
either earth or some other conducting body that
serves in place of the earth. This connection can be
with or without an intermediary impedance device.
With an extremely low-impedance device, the
system is said to be solidly or effectively grounded.
With an impedance device, the system can be
either resistively or reactively grounded.
A bonded electrical system is one in which
the non-current-carrying conductive materials of
the electrical system are connected together in
such a way that they present a low-impedance
path for ground-fault currents. This bonded connection permits phase-to-ground-fault currents on
the grounded system to flow back to the electrical
source for subsequent safety actions by the system.
Because of the interconnectivity of a grounded and
bonded system, a bonded system also aids the
objective of a grounded system.
An ungrounded electrical system has no direct
connection between the system conductors and
earth or ground, except through the very high naturally occurring reactance due to the line-to-ground
capacitive coupling. Regardless of the name’s implication, the NEC still requires the conductive equipment enclosures of an ungrounded system to be
grounded for the same reason a grounded system
is required to be grounded. This code also requires
that an ungrounded system be bonded similarly to
a grounded system to provide a low-impedance
path for phase-to-phase fault currents to circulate
back to the source.
Ground-fault currents are unwanted flow of
electrical currents on the electrical system due to
unintentional connection between an ungrounded
conductor of an electric circuit and earth. Ground
faults on average make up 95% of all faults on the
electrical systems, with the most common type
of ground faults being the arcing type. All forms
of grounding and bonding attempt to minimize
or eliminate ground faults. Therefore, the various
grounding methods mentioned will be in the context
of treating ground-fault currents.
systems while still providing the benefits of service continuity. The ideal voltage range is 5 kV and less. In general,
increasing the ground-fault current flow
improves overvoltage control but elevates
the point of fault damage. Conversely,
decreasing the ground-fault current elevates overvoltage but decreases pointof-fault damage. Correct application of
HRG in the medium-voltage (MV) range
Consulting-Specifying Engineer • AUGUST 2012
51
Detection of the fault is provided by a coordinated set
of ground-fault relays.
of 2.4 to 13.8 kV would require a maximum limit on the single line-to-ground,
point-of-fault ground-fault current to a
value below 7 amp. In addition, the inherent line-to-ground capacitive charging
current must be less than or equal to the
current through the grounding resistor.
range of ground-fault currents allowed
for LRG systems, the capacitive charging
current to ground has very little impact on
sizing the grounding resistor. This resistance is then simply the line-to-neutral
voltage across the grounding resistor
divided by the ground-fault current.
Figure 3: This shows commercial building generator solid grounding (SDS).
Courtesy: exp, with reference to NEC
Mathematically, the ground-fault current is the vectorial sum of the grounding
resistor current and the capacitive charging current. The capacitive charging current is a function of the electrical system
that must be initially estimated. With
these quantities and conditions satisfied,
the range of HRG ground-fault currents
can be calculated.
Low-resistance grounding (LRG)
schemes are designed to limit groundfault currents in the range of 100 to 400
amps on systems with voltage ranges
of 480 V to 15 kV. With this increase
ground-fault current magnitude, the LRG
aim is to eliminate overvoltage transients
at the expense of increasing the pointof-fault, ground-fault damages. In order
to minimize these damages, however, a
system of protective devices is formed as
part of the LRG scheme. Ideally, the fault
is isolated while the rest of the electrical
system continues to function. With the
52
Reactance grounding (RG) is
another alternative used on MV systems
in the range of 2.4 to 15 kV. With this
grounding scheme, an inductor is used
to limit the flow of ground-fault currents. It has been shown that reactance
grounded systems produce transient
overvoltages at much higher groundfault currents than resistive grounded
systems. In order to limit the transient
overvoltages to acceptable limits, the
resulting ground-fault current could be
as much as 60% of the 3-phase bolted
fault. Since this is much higher than the
400-amp limit for LRG at the same voltage range, reactance is not as commonly
used in the electrical industry, except for
tuned reactance grounding.
Ground-fault neutralizer (GFN)
is another form of reactance grounding known as tuned reactance grounding. As the name implies, the inductive
reactance is tuned to the ungrounded
Consulting-Specifying Engineer • AUGUST 2012
phase natural capacitive charging current to ground. This tuning effect by
the inductive reactance essentially cancels (neutralizes) the current contribution from the capacitive charging current. This leaves a small portion of the
ground-fault current that is essentially
resistive in nature. This resistive neutralto-ground current is in phase with the
neutral to ground voltage. The benefit
of this phase unison is that an arcing
fault to ground is less likely to be sustained by the voltage when the ac current and voltage reach their zero value
simultaneously. The GFN application is
similar to the HRG application, in that
the ground fault is allowed to persist so
that the electrical service is continued.
Detection of the fault is provided by a
coordinated set of ground-fault relays.
GFN drawback is similar to RG in that
reactance grounding in general tends to
increase transient overvoltages. Plus, the
grounding circuitry has to be re-tuned
after any switching arrangement is made
to the electrical system.
Solid grounding (SG) was usually the
solution more than 60 years ago when
engineers were looking for an alternative to address the problem of transient
overvoltages due to arcing ground faults
on ungrounded systems. Even though its
application was not as successful in the
2.4 to 13.8 kV range due to high pointof-fault energy, SG is consistently applied
at voltages below 600 V even today. A
solidly grounded neutral system will
produce the maximum fault current for a
given faulted condition. Therefore, it provides the best opportunity for early detection of arc-flash hazards on the electrical
systems. The overcurrent device coordination which is an essential part of the
SG system ensures that only the faulted
circuit is isolated while the rest of the
system continues to function.
Boundary (grounding zone) of the
electrical system
The ground-fault effects of the various grounding schemes outlined above
are confined within specific areas of the
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electrical systems known as grounding zones or grounding systems. The
boundaries of these grounding systems
are created by demarcations such as the
primary delta windings of transformers, or the dc point of ac/dc inverters
and converters. These systems that are
magnetically coupled together, or electrically isolated, except through some
form of equipment bonding are considered separate systems.
In Figure 1, the 480 V, 3-phase system
includes the primary delta windings of
Systems 2 and 4, the ungrounded wye
connected motor, the solidly grounded
wye-wye transformer, the ungrounded
delta winding source generator, and
grounded wye secondary of the source
transformer. System 2 has an ungrounded delta transformer secondary, and an
ungrounded single-phase transformer
primary. System 3 has an ungrounded
single-phase transformer secondary, and
System 4 has a grounded wye transformer
secondary.
When separate systems develop their
own bonding and grounding connections,
they are called separately derived systems
(SDSs). Power sources such as transformers and generators are usually configured
as SDSs. However, when they are electrically connected to another system, they
become part of that system and are classified as non-SDSs. Transformer T1 and
generator G in Ground System 1, Figure
1 are considered non-SDS.
transformer (see connection C at 208 V
panel in Figure 2). This second grounding and bonding configuration is identical to what is required for commercial
building service entrance equipment that
is served by a utility transformer. In this
case, the neutral-to-ground connection
is called the main bonding jumper. A
third bonding connection B is also indicated. The three A, B, C connections
cannot be used simultaneously as this
would establish a parallel path for the
grounded conductor. However, any two
of the three A, B, C connections will be
a code compliant installation based on
NEC 250.30(A)(1). To ensure this code
compliancy, the grounding electrode
conductor tap between neutral bus and
external ground bus has to be connected
accordingly. With A and B connections,
the tap is made at the transformer. With
B and C connections, the tap is made
at the panel. With A and C connections,
the tap is made at either the transformer
or the panel.
In general, the building single transformer grounding and bonding installation can be expanded to multiple
transformer arrangements where there
are several transformers per floor of
a multistory building. This is done by
extending the common grounding electrode conductor either vertically through
floors, or horizontally within each floor.
Generator solid grounding
Commercial building generators
grounding and bonding connections can
be done as either an SDS or a non-SDS.
The choice of which configuration to
use is determined by the choice of the
transfer equipment that will transfer
power connections from the utility
to building generator(s) upon loss of
utility power. If the transfer equipment (switch) allows switching of its
neutral connections (i.e., 4-pole), then
the generator connected to the transfer switch has to be connected as an
SDS. This arrangement will ensure
conformity to the safety performance
requirements of NEC 250.6(B) (see
Figure 3). If the transfer switch does
not allow switching of its neutral connections (i.e., 3-pole), then the generator has to be connected as a non-SDS,
to again comply with NEC 250.6(B)
(see Figure 4). Even though there is no
neutral-to-ground connection at generator G2, the generator is not considered
ungrounded. This is because the neutral
connection of the generator, while not
Transformer solid grounding
Commercial building transformers are
usually connected as SDSs. The main
characteristic of the SDS is the bonding
of the grounded neutral conductor to the
bonded equipment enclosure or to the
bonded ground bus. For transformers,
there are two configurations for making
this neutral-to-ground solid connection.
The first configuration has this connection at the transformer itself (see connection A at transformer in Figure 2).
The second configuration has this
neutral-to-ground connection at the first
disconnecting means downstream of the
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Figure 4: A commercial building generator is shown with solid grounding (nonSDS). Courtesy: exp, with reference to NEC
Consulting-Specifying Engineer • AUGUST 2012
53
Multiple generators serving
a commercial building tend
to be connected as separately
derived systems.
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connected to ground at the generator itself, is connected to
the ground at the service entrance equipment MDP via the
transfer switch. Also, the frame of the generator is grounded
with an auxiliary grounding electrode in compliance with
NEC 250.54. This grounding electrode provides the same
benefit at the generator that grounding an electrical system
will provide.
Multiple generators serving a commercial building tend
to be connected as SDSs. This is so because of the requirements for ground-fault devices at facilities large enough to
require multiple generators. For instance, proper functioning
of these ground-fault devices necessitate that the generators be connected as SDSs. Generators connected in parallel
pose special problems in the form of grounding methods
and equipment protection. Suffice to say here that matching the electrical parameters of these paralleled generators
minimizes circulating third harmonic currents that can affect
ground-fault overcurrent devices.
Paralleled generators grounding can be implemented with
a common neutral bus connected to a single ground bus, or
with individual neutral buses connected to their respective
ground buses. In order to use the parallel lineup with the
common neutral bus, the switchboard with generator overcurrent devices has to be adjacent to the generators themselves.
This is because the neutral-to-ground connection on the SDS
has to be at the generators or at the first disconnecting means
downstream of the generators (NEC 250.30(A)(1)). By this
code requirement, if the generator switchboard were to be
located remotely from the generators themselves, then the
neutral-to-ground bond would have to be at the integral overcurrent device of each generator. It must be stressed here that
this application of solid grounding for generators described
above is not common practice for generators with voltages
above 600 V. This is because the single line-to-ground faults
under solid grounding at these higher voltages tend to be
greater than the 3-phase bolted faults that generator manufacturers design their generators to handle.
Regardless of whether generators or transformers are grounded as SDSs or non-SDSs, if they serve a specific commercial
facility, then all grounding electrodes are required (NEC 250.50)
to be bonded together to form a grounding electrode system.
This increases the integrity of the building grounding system
while not violating the requirements for different grounding
zones because the current carrying conductors are not interconnected between grounding zones.
Alexander is a senior electrical engineer with exp. His expertise
is in electrical engineering for building systems, and he works
primarily in commercial and governmental buildings.
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