Part III: Transfer Equipment
Applications and Considerations
transfer equipment used in optional standby systems
Transfer Equipment Used in Optional Standby
Systems for Commercial Applications
by Chad Kennedy
T
ransfer equipment installations can be extremely
complex, even for optional standby arrangements
that are critical to business operation. This is
the final installment in a series of articles examining
transfer equipment used in optional standby systems
for commercial applications. In Part I of this series, we
covered the fundamentals of transfer equipment used in
optional standby systems and the use of key interlocks
to provide a safe, simple, and reliable means of transfer.
Part II explored transfer equipment options along with
guidance, questions to ask, and considerations for each
system. This article will focus on transfer equipment for
3-phase, grounded systems. It will discuss requirements
for the selection of three- or four-pole transfer and the
impact to code-compliant installations. It will also discuss larger, more complex transfer applications found in
data centers and hospitals.
Three- or Four-Pole Transfer?
One of the most common questions that arises when discussing transfer equipment is “do we need to switch the
neutral?” The decision to use three- or four-pole transfer
equipment requires that we understand and evaluate the
potential for impact to other system design requirements
such as equipment ground-fault protection, system stability and safety, and code compliance. The general rule
of switching the neutral when ground-fault protection
is required or desired must be balanced with how the alternate source is connected to the electrical system. The
NEC permits the standby generator to be connected as
a separately or non-separately derived system and this
should be the first question asked when reviewing a system design plan. A separately derived system is defined in
Article 100 as “A premises wiring system whose power is
derived from a source of electrical energy or equipment
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fault and protect the system. Failure to use four pole transfer equipment for a separately
derived system would adversely affect the equipment groundfault protection and ability to
recognize load unbalance. A more complicated and costly modified differential ground fault
system would have to be designed and installed.
transfer equipment used in optional standby systems
equipment in order to provide
the normal and alternate sources
with mutually exclusive grounded
circuit conductors (neutral), see
figure 1. This is an important requirement since for separately
derived systems the generator is
required to be grounded per NEC
250.30. The use of four-pole transfer equipment and resulting separation of the systems allow the
overcurrent protective device to
recognize load unbalance current
and avoid nuisance ground-fault
tripping.
Figure 2 illustrates that
Figure
1
Figure
1. Four-pole
transfer equipment used for separately derived system connection
this
is
accomplished
by preventing
Four pole transfer equipment used for separately derived system connection.
the neutral current from having a
parallel path through the equipment grounding conductor (EGC).
Another benefit of using four-pole
transfer equipment in a separately
derived system is demonstrated in
figure 3. When a real ground fault
occurs the current is not allowed
to return to the source using the
grounded conductor (neutral).
This allows the overcurrent protective device to accurately recognize
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the fault and protect the system.
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Failure to use four-pole transfer
equipment for a separately derived
system would adversely affect the
equipment ground-fault protecFigure
2 imbalance current path with four-pole transfer equipment. Note that having the service neutral dis- tion and ability to recognize load
Figure
2. Load
Load
imbalance
current path with four pole transfer equipment. Note that having the service
connected from the load prevents neutral current from returning through the main bonding jumper (MBJ) and equip- unbalance. A more complicated and
neutral disconnected from the load prevents neutral current from returning through the main
ment ground to the alternate source.
costly modified differential groundbonding jumper (MBJ) and equipment ground to the alternate source.
other than a service. Such systems have no direct electri- fault system would have to be designed and installed.
cal connection, including a solidly connected grounded
circuit conductor, to supply conductors originating in Four-Pole Transfer
another system.” Section 702.10 specifies that portable System Considerations –
generators can also be connected as a separately or non- System Stability and Safety
separately derived system. Understanding if the system NEC 250.30(A)(1) requires separately derived systems
was designed as separately or non-separately derived to have a system bonding jumper similar to the main
connection is essential for transfer applications. Plan- bonding jumper required for services. A concern and
ning for one type of system and getting the other can potential issue arises if the system bonding jumper is not
installed or has been removed. When the load is translead to serious system performance issues.
ferred to the alternate source the electrical system no
longer has a ground reference, see figure 4. This situation
Four-Pole Transfer System Considerations –
can lead to system stability issues with abnormal phaseGround Fault
Separately
Figure 3 derived systems require four-pole transfer to-ground voltages which potentially damage electronic
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Systems for Commercial Applications Part III 052609.doc
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79
Figure 2
Load imbalance current path with four pole transfer equipment. Note that having the service
transfer
equipment
used
in optional
standby
systems
neutral disconnected
from the
load prevents
neutral current
from returning
through the main
bonding jumper (MBJ) and equipment ground to the alternate source.
with sensitive electronic components
and should be considered in the system
design. For this very reason, there are
industries and engineers that will not
consider switching the neutral due to the
risk of loss to expensive electronics that
are running or communicating with vital operations.
Four-Pole Transfer System
Considerations – Code Compliance
It may be challenging to determine if code
requirements have been met for a separately derived system when the alternate
source is not on-site. This is often the case
Figure
3. Example
Figure
3 of possible ground-fault current flow with four-pole transfer equipment. A ground fault
on the load of the transfer equipment cannot return to the alternate source using the grounded circuit when a portable generator is intended to
conductor (neutral) but instead must return through the equipment grounding conductor or bus.
be the alternate source. Will the portable
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Kennedy
generator
have
the system bonding jumper installed?
devices and controls. Stability can also be affected if the
The
lack
of
a
proper
system bonding jumper can result
system has significant unbalanced line-to-neutral loads
causing imbalanced line-to-neutral voltages. Since the in an unsafe system condition by allowing the system to
alternate source is not grounded and four-pole trans- lose its reference to ground (ungrounded system) and by
fer equipment was used, detection and locating system preventing the installed equipment ground-fault protecground faults become complicated. Failure to locate and tion from performing as designed.
repair single phase-to-ground faults can result is serious
power system damage if the fault escalates to a multiple Three-Pole Transfer System Considerations
phase-to-ground fault.
The most common transfer system solutions are threeFour-pole transfer equipment solutions often have pole. Non-separately derived systems will require
an option to include an overlap or break before making three-pole transfer equipment because the alternate
contact for the (neutral) grounded circuit conductor. source generator is connected as a feeder. This conLine-to-neutral voltage disturbances caused by switch- nection uses the normal source main bonding jumper
ing the grounded circuit conductor can cause problems which connects the grounded conductor (neutral) to
grounding electrode conductor
as the grounding means for the
alternate source grounded conductor. NEC-2005 250.24(A)(5)
specifies that “A grounding connection shall not be made to any
grounded conductor on the load
side of the service disconnecting means except as otherwise
permitted in this article.” However, NEC-2008 indicates the
following: “A grounded conductor shall not be connected to normally non-current-carrying metal
parts of equipment, to equipment
grounding conductor(s), or be reconnected to ground on the load
side of the service disconnecting
Figure 4
means except as otherwise permitFigure 4. Four-pole transfer equipment where the alternate source generator does not have a system bonding jumper
Four pole transfer equipment where the alternate source generator does not have a system
bonding jumper.
80 IAEI NEWS September.October 2009
Four Pole Transfer System Considerations – Code Compliance
It may be challenging to determine if code requirements have been met for a separately
derived system when the alternate source is not on-site. This is often the case when a portable
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generator system bonding jumper condition. It is still important to understand the generator
bonding because if three pole transfer is used and the generator is bonded there will be a
parallel path for neutral current flow along the equipment grounding conductors between the
service and generator.
Potential ungrounded system stability and safety concerns which can arise in four pole
transfer systems do not occur in three pole transfer systems.
There is a continuous,
directused in optional standby systems
transfer
equipment
connection to the grounded circuit conductor and service Main Bonding Jumper.
and system ground faults. This is a
benefit for installations intending
to use a portable generator. Unlike the four-pole separately derived situation, three-pole transfer
equipment solutions for this case
will already provide a modified differential ground-fault system and
thus provide equipment groundfault protection regardless of the
portable generator system bonding
jumper condition. It is still important to understand the generator bonding because if three-pole
transfer is used and the generator is
bonded there will be a parallel path
for neutral current flow along the
equipment grounding conductors
Figure
5. Three-pole
transfer equipment used in non-separately derived system
Figure
5
between the service and generator.
pole transfer equipment used in non-separately derived system.
tedThree
in this
article.” Since this is a non-separately derived
Potential ungrounded system stability and safety consystem, load side grounding is only permitted for sepa- cerns which can arise in four-pole transfer systems do not
Advanced
Applications
rate
building
or structure applications where an equip- occur in three-pole transfer systems. There is a continument grounding conductor is not run with the building ous, direct connection to the grounded circuit conductor
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Kennedy An example of a non-separately derived system
supply.
and service main bonding jumper.
using three-pole transfer equipment is shown in figure 5.
Since the normal and alternate sources share a direct con- Advanced Applications
nection to the grounded circuit conductor, installations There are power transfer applications more complex than
where equipment ground-fault protection is required for the traditional two source transfer discussed up to this
the alternate source will require a modified differential point. In particular, the systems seen in data centers and
ground-fault system to properly discern load imbalances hospitals are not only more complex but also much larger
than most transfer systems. The
following sections will provide
basic insight into the systems typically supplied in hospitals and data
centers and how optional standby
systems are utilized.
Data Centers
Figure
6 example of the power distribution one-line diagram in a data center
Figure
6. Typical
Typical example of the power distribution one line diagram in a data center.
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Few installations implement the
redundancy found in the data center environment. The power distribution and transfer equipment
is often seen as a key competitive
advantage and decision point for
potential customers. This reflects
current business reliance on the
internet and data access and storage capabilities as part of day-today operation and revenue generation. It is also possible that a
September.October 2009 IAEI NEWS
81
transfer equipment used in optional standby systems
Photo 1. Part of the main switchgear line-up in a data center
data center may be deemed critical to communication
such as a telephone exchange or voice-over-Internet
protocol (VoIP) where government regulation drives
such a facility to be an emergency system. It is also
recognized that portable systems are often used to
provide back-up while an emergency system is undergoing maintenance. Figure 6 illustrates the basic
data center power distribution structure and redundancy. The distribution of power to the data center
power distribution units (PDUs) is the primary objective of these systems. Starting at the PDU there
are typically two sources of power from redundant
uninterruptible power supply (UPS) switchboards.
A double pole, double throw switch is one method
used to select which source feeds the PDU. The UPS
switchboards are also each redundantly supplied
from one or multiple UPS modules and a combination source supplied by utility or generator sources.
These sources are usually key interlocked with the
UPS module providing power to the UPS switchboard under normal conditions. The UPS module
is supplied by the main switchgear which usually
contains feeds from utility transformers and on-site
generators. The utility main breakers, tie breakers,
and generator breakers are normally controlled by
a programmable logic controller (PLC) based automatic throw-over system to transfer power as needed
to supply the PDUs. Although these systems are classified as optional standby systems, the performance of the
data center power transfer system exceeds what would
normally be supplied for emergency or legally required
systems. Photo 1 is an example of just one of the equipment line-ups making up the main switchgear in a
data center.
Advanced Applications – Hospitals
Article 517 details the requirements of power transfer in
health care facilities including the division of loads into
essential and nonessential systems. A typical hospital
power flow diagram is shown in figure 7. The loads classified as emergency are segregated from normal distribution equipment and fed from unique automatic transfer
switches. However, there are loads classified as nonessential which must meet Article 702 optional standby
system requirements. These loads can be supplied from
the same utility and generator sources as the emergency
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transfer equipment used in optional standby systems
Plan Review and Inspection
Transfer equipment applications
can range from simplistic installations to very complex optional
standby systems for data centers
and hospitals. The considerations
for three- or four-pole transfer,
ground-fault protection of equipment, system stability and safety, as well as code-compliance
challenges must be integrated to
work as a system. Clearly these
considerations have dependencies
and therefore must be considered
as part of an overall system design to ensure that the optional
standby system provides the
desired benefits and does not
negatively impact normal sysFigure 8
Figure 7. Typical power system transfer structure for hospitals. Neutral connections to ATSs were removed for tem performance. In particular,
Typical power system transfer structure for hospitals. Neutral connections to ATSs were
clarity.
understanding the design intent
removed for clarity.
loads as long as they originate from separate vertical as separately or non-separately derived and attenA Final note on Ground Fault Protection
switchboard
sections and the wiring is kept entirely sep- tion
to four-pole
transfer systems using portable
Optional standby systems continue to grow in size to not only power a few
business
critical
arate
frombutthe
700.5
generators
is required to provide a safe, code-comfunctions
areemergency
being sized tosystem
maintainwiring.
continuitySection
of business
for an entire
facility. NEC
702 doesthe
not exempt
a system
from
ground
accordance
with NEC
215.10.
requires
generator
to be
rated
forfault
allprotection
possibleinloads
pliant
installation.
So here are a few items you can
The risk of losing your electrical system for an extended period due to not protecting the
orsystem
have with
some
type
of
load
management
with
priority
review:
ground fault far outweighs the possible short-term condition of taking the system
given
loads. system due to a fault. Even ground fault protection
downto
andemergency
saving the distribution
on anquestion is always – Is this an NEC 700,
1. The first
emergency system is permitted. The only disconnect that exists with regard
fault
701 toorground
702 system?
protection is in NEC 517.17 where it is prohibited between the generator and the essential
system on
and Ground-Fault
on the load side of theProtection
transfer switch of the essential 2.
electrical
system.
Is the
system configured as separately or nonA electrical
Final Note
Remember
that
ground
fault
protection
protects
your
electrical
system
to
ensure
it
will
Optional standby systems continue to grow in size separately derived?
continue to operate after appropriate maintenance. Removal of ground fault simply provides
toanot
only power
a few business
critical functions
butis potentially
few moments
of operations
while the switchboard
and or system
destroyed
3. Does
theby
transfer switch (3- or 4-pole) match the
fault taking
arethebeing
sizedplace.
to maintain continuity of business for configuration of the system’s source?
an entire facility. NEC 702 does not exempt a system
4. Is the system grounding system appropriately infrom ground-fault protection in accordance with NEC
stalled and bonded?
215.10. The risk of losing your electrical system for an
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5. Is this system large enough for ground-fault proextended
period due to not protecting the system with
Kennedy
tection
and is it installed?
ground fault far outweighs the possible short-term
6. Do we have the appropriate disconnects and overcondition of taking the system down and saving the
distribution system due to a fault. Even ground-fault current protection located and sized appropriately?
protection on an emergency system is permitted. The
7. Is the alternate source a portable generator? If so,
only disconnect that exists with regard to ground-fault how is the system configuration and necessary generator
protection is in NEC 517.17 where it is prohibited be- bonding condition communicated?
tween the generator and the essential electrical system
and on the load side of the transfer switch of the esChad Kennedy is the manager, Industry Standards for Power
sential electrical system. Remember that ground-fault Equipment and is a registered professional engineer in the state of
protection protects your electrical system to ensure it South Carolina. He has been with Square D for over nineteen years
will continue to operate after appropriate maintenance. and has served as the engineering manager for custom switchgear and
switchboard design supporting the design of systems for power transfer
Removal of ground fault simply provides a few mo- and distribution in industrial, commercial, and retail areas. He is a
ments of operation while the switchboard and/or sys- member of NFPA, serving on NEC Panel 13, and is an active particitem is potentially destroyed by the fault taking place. pant in IEEE, UL, NEMA and IAEI.
www.iaei.org
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