Transfer Equipment Used in Optional
Standby Systems for
Commercial Applications, Part I
transfer equipment used in optional standby systems
The Fundamentals and Key Interlocks
by Chad Kennedy
UTILITY SOURCE
CIRCUIT
BREAKER
A
(NORMALLY
CLOSED)
GENERATOR SOURCE
A1
INTERLOCK HOLDS KEY UNTIL
DEVICE IS IN THE OPEN (OFF)
POSITION AND LOCKED
LOCKED OPEN
L-0
CIRCUIT
BREAKER
B
(NORMALLY
OPEN)
A1
L-0
LOAD
Figure 1
Key Transfer Sequence:
Current Condition – Utility source feeding load – Breaker A Closed, Breaker B Open, Key A1 held in Breaker A.
Desired Condition – Generator source feeding load – Breaker B Closed, Breaker A Open.
4) Close Breaker B.
1) Open Breaker A and Rotate Key A1 to Lock Breaker Open – Key is now free.
5) Reverse procedure to restore service to breaker A.
2) Remove Key A1 from Breaker A and insert key into lock on Breaker B.
3) Rotate key A1 to unlock breaker B, Key A1 now held captive.
S
ignificant growth in optional standby systems for
commercial applications is being driven by the
demand for electrical power to be present that
will ensure continuity of business activities. Concerns
stem from weather related outages to other reliability
issues that can result in the loss of electric utility of a
building, communication center, or process. Loss of
power may impact food storage, or simply stop the cash
register at the local home store. Even though most of
those you ask will refer to the alternate power sources
as emergency power, it becomes clear that an alternate
source (generator) and connection with manual trans-
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transfer equipment used in optional standby systems
fer equipment will not serve as an emergency source of
power. Systems that are not tied to life or public safety
are optional stand-by systems. These systems are considered optional despite the fact that without this alternate
source, a bank, airline reservation system, or gas station
would no longer be able to serve its customer.
Transfer equipment can range from an extremely simple mechanical interlock to very complex multi-circuit
breaker transfer schema. This document is Part I of a series of articles that will explore various transfer equipment
configurations in an effort to assist the electrical industry in the understanding of these systems and to provide
guidance for the AHJ in reviewing the installation in order to grant approval of the transfer equipment.
Installations are expanding from the normal (utility)
main breaker and feeders systems to include an on-site
or portable roll-up generator. In fact, some installations
will have both an on-site generator and provisions for a
portable generator as a second level of back-up power. A
quick review of the scope of NEC Article 702 defines
optional standby systems as “those systems intended to
supply power to public or private facilities or property
where life safety does not depend on the performance
of the system. Optional standby systems are intended to
supply on-site generated power to selected loads either
automatically or manually. “FPN: Optional standby systems are typically installed to provide an alternate source of electric power
for such facilities as industrial and commercial buildings, farms, and residences and to serve loads such as
heating and refrigeration systems, data processing and
communications systems, and industrial processes that,
when stopped during any power outage, could cause
discomfort, serious interruption of the process, damage
to the product or process, or the like.” Fundamental Safety Application
Traditionally, optional standby systems were used in
industrial installations where the availability of qualified personnel and an on-site generator was more likely
to be present. In the event the normal utility source is
lost, some or all of the load could be manually or automatically transferred to the generator, based on the generator size and customer needs. A fundamental safety
concern for the transfer equipment is the prevention of
both electrical sources from connecting to the load at
the same time. If allowed, the generator could reverse
feed the utility system creating hazards for utility and
other personnel and also possibly damaging the generator. NEC 702.6 specifies that the transfer equipment
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shall be installed to prevent paralleling of the normal
(Utility) and alternate (Generator) sources, unless the
equipment is designed for parallel operation and meets
the requirements of Article 705. NEC 702.6 only contains one very restrictive exception to the transfer equipment requirement.
“Exception: Temporary connection of a portable generator without transfer equipment shall be permitted
where conditions of maintenance and supervision ensure that only qualified persons service the installation
and where the normal supply is physically isolated by
a lockable disconnecting means or by disconnection of
the normal supply conductors.”
In contrast to industrial installations, most commercial installations do not have qualified personnel or on-site
maintenance with supervision. It is important for these
installations that the transfer equipment and process be
simple, easy to understand, and provide all of the requirements specified in the NEC. Automatic transfer equipment
provides a robust and reliable solution but many times
these systems add more features, complexity, and cost than
the installation requires. Where manual transfer equipment is acceptable, there are several methods that can be
used. One such method of meeting the transfer equipment
requirements is the use of an interlock system.
The Key Interlock System
A key interlock system is one that employs one or more
key interlocks installed on associated equipment to permit operation of the equipment only in a pre-arranged
sequence. Key interlocking associated equipment differs
from ordinary locking methods by requiring that the
key be retained in the lock in either the locked or unlocked position depending upon the system design. The
retention of the key in the interlocked devices forces predefined equipment operation and does not depend upon
operator memory or skill. For an optional standby system,
key interlocks are installed on the normal source breaker (Utility) and the alternate source breaker (Generator)
which will allow the transfer of load from one source to
the other with a defined sequence of steps. For this system, the key is held in the normal source breaker until
the breaker is opened and locked. Once locked the normal
source breaker cannot close. The alternate source breaker
is normally in a locked open state and cannot be unlocked
without the key from the normal source interlock. Figure 1
provides each step of the transfer sequence.
This solution is desirable because of the ability to mechanically lock the disconnecting device to prevent paralleling of sources, because it is a simple transfer operaMay.June 2009 IAEI NEWS
73
transfer equipment used in optional standby systems
Figure 2. Key interlocks installed on devices located in different compartments or enclosures
tion which forces a correct sequence of operation, and
because of its relatively low cost. Another benefit of this
approach is the adaptability of key interlock systems to
other power system configurations such as Main-TieMain and Main-Tie-Generator. This flexibility means
that the devices used to make up the transfer equipment
do not have to be physically in the same compartment
or enclosure. Examples of those applications are demonstrated in figures 2 – 4.
It is also possible that where existing equipment is
installed and generator provisions are being added, key
interlocks are usually easily field installable. However,
that leads us down the path of understanding how to
determine if the installation is acceptable for approval
by the plan reviewer or electrical inspector.
a very broad term used to permit the use of switchboards,
double throw switches, panelboards and other equipment
configurations to transfer power. The term transfer switch
is very specific and although not distinct in difference
in Article 702, Article 700 establishes a solid distinction
where 700.6 in general discusses transfer equipment and
a subsection beneath, 700.6(C), establishes requirements
specific for a transfer switch. It is important to understand this distinction as the 2008 NEC requires the transfer switch, rated 600 V or less, to be listed.
There are at least three methods to assist in the approval process. The first is product listing. Listing agencies evaluate transfer equipment using a number of different standards, such as UL 67 Panelboards, UL 98
Enclosed Switches (double-throw switches), UL 891
Switchboards, or UL 1008 Transfer Switch Equipment.
Equipment Approval
The electrical equipment will have a listing mark that
NEC 702.4 requires that transfer equipment be approved designates the primary use of the equipment. Since the
for the intended use. In order to guide this discussion, we panelboard standard includes special testing requireneed to sort out the terminology with regard to trans- ments, we would not list a panelboard as a transfer
fer equipment and transfer switch. Transfer equipment is switch. This testing would be missed if the panelboard
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transfer equipment used in optional standby systems
Figure 3. Example of key interlock mounting location in a circuit breaker device
was listed to a transfer switch standard. However,
equipment may come marked “suitable for use in
an NEC 702 application” such that the installer
and inspector understand that it can be used in
that application.
The transfer equipment, such as key interlocks,
may be installed in the field. The language in
NEC 702 is very specific to require approval and
not listing in order to permit the modification of
equipment in the field. However, if the inspector
is concerned about a field installed assembly that
is not identified for use with the equipment, it may
require a third party to perform a field evaluation
of the installation to ensure compliance with basic
standard’s requirements.
Finally, the inspector can choose to approve the
system with or without a listing mark. Consider a
facility that has multiple sources and equipment
that is located in various locations of the building,
or maybe key interlocks on two different manufacturer’s switches under a trough. It is reasonable to
Figure 4. Key interlocks installed on devices located in the same compartment or enclosure
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75
transfer equipment used in optional standby systems
expect that an electrical inspector could approve such an
installation without third party evaluation.
fer system. Understand if additional keys are provided
from the manufacturer of the equipment or the key
interlock provider. Are additional keys going to exist
Plan Review and Inspection
on the premises of the installation? If so, ask to underOptional standby system installations require transfer stand if the extra key is secured. If it cannot be demequipment. Key interlocks play a significant role to ensure onstrated that additional keys can be secured, then
electrical safety across the country and will insure proper destruction of the extra keys may be necessary to grant
transfer operations. Transfer equipment utilizing key in- or obtain approval.
terlocks provide positive locking operation, are easy to in5. If there are any questions about the transfer operastall and operate, and forcibly direct a correct sequence of tion, walk through the operation of the key interlocks
operation. However, there are a few key items to ensure during inspection.
compliance and reach approval of the installation.
The safe transfer of electrical power is the objective;
1. Is the equipment listed or marked to identify it for avoiding the inadvertent interconnection between sources
use in an NEC 702 transfer application?
which could back-feed a system and injure personnel. Once
2. If existing equipment is involved and a key interlock is again, key interlocks have been the cornerstone of a safe,
being added, understand the retro fit activity so the parties simple, and reliable means of transfer for decades. There
involved will understand if a field evaluation may be neces- are other options and Part II will explore more equipment
sary. Is it a kit provided by the equipment manufacturer? Is and provide additional guidance and questions to ask in
this simply a set of interlocks being installed in the field but addition to considerations for those systems.
not endorsed by the manufacturer of the equipment?
Chad Kennedy is the manager, industry standards for power equip3. What type of system is being considered? Is it a di- ment and is a registered professional engineer in the state of South Carorect mechanical interlock between two switches/circuit lina. He has been with Square D for over eighteen years and has served
as the engineering manager for custom switchboard design supporting
breakers? Is a key interlock being used?
the design of systems for power transfer and distribution in industrial,
4. Understand that any additional keys beyond those commercial, and retail areas. Chad is a member of NEC Code-Making
required to operate the interlocks will defeat the trans- Panel 13 which has responsibility for Articles 445, 700, 701 and 702.
Neon Lighting,
A Professional Advantage
The book contains guidelines for installers and inspectors, tips on using the 2005 NEC and the Canadian Electrical Code,
standards and listing information from UL, CSA, and ITS, and gives important definitions and terms.
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