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Transfer Switches: Understanding Why
UL 1008 Is a Critical Requirement
Jointly published by
Executive Summary
There is a growing need for
emergency backup power
systems in data centers,
hospitals and many other
facilities. A critical component
of many of those systems is
the transfer switch.
Transfer switches should
be certified to UL 1008
requirements. UL 1008
requires rigorous testing of
transfer switches. It is
designed to ensure reliability
and durability.
Despite the importance of UL
1008, most facility executives
are not aware of what UL
standard their transfer
switches are certified to.
It’s easy to take transfer switches for granted...
But it’s also risky.
With the growing emphasis on
emergency and standby power,
facility executives are taking a
closer look at the elements of
the backup power system. One
critical element of that system
is the transfer switch. Facility
executives count on their transfer
switches to perform when they
need to switch from the utility’s
electricity supply to backup
generators. Mostly, transfer
switches perform. But there have
been notable failures.
At an Amazon Web Services data
center in Virginia, an automatic
transfer switch didn’t work
properly in May 2010, when the
data center needed to shift from
utility power to its generators
after a vehicle hit a utility pole
nearby. As a result, some Amazon
customers lost cloud computing service for an hour. In 2010,
Codero’s Phoenix data center
experienced a power outage. The
generators started as expected,
2
but an automatic transfer switch
failed. In January 2010, a NaviSite
data center lost power when a
transfer switch failed during a
utility outage caused by a thunderstorm.
of switching devices do not have
to meet UL 1008 and thus do not
offer the assurance of having met
the strict testing requirements
needed for qualification to that
standard.
To ensure that transfer switches
perform as expected, it’s important that they comply with UL
1008. That standard sets out a series of rigorous requirements that
include “withstand & closing ratings or WCR” (which cover severe
fault currents, bolted faults and
short circuits within the electrical distribution system); tests to
ensure that the device can carry
rated currents; and endurance
tests specifying the number of
cycles that the transfer switch
must achieve and still perform its
intended function.
Most transfer switch failures
don’t make the news the way
the ones cited above did. But
failures do occur. In a survey cosponsored by Building Operating Management magazine and
ASCO Power Technologies, 20
percent of respondents whose
facilities have transfer switches
reported at least one failure of
a switch in the past five years.
(See Figure 1 on page 3.) In one
case, “both sides of the switch
closed at the same time during
a test. The switch exploded,” a
respondent reported. Another
respondent indicated the transfer
switch failure “caused elevators
to lock out.” A third commented
that the transfer switch “failed to
energize a critical Intensive Care
Unit circuit.”
But transfer switches are not
the only devices that are used to
transfer power. And other types
Figure 1. Have you had any failure of a transfer
switch in the past 5 years? R=647
Figure 4. Did any of the transfer switch failures
leave your facility without power? R=125
Yes No Yes
No
Not applicable 20%
80%
Transfer switch failures happen in a variety of ways.
But a third of respondents who experienced failures
reported that the switch completely failed and became
totally inoperational. (See Figure 2.)
Figure 2. Which of the following accurately
describes the transfer switch failure(s)? R=123
Controls failed but mechanical switch
continued to operate
Switch completely failed and became
totally inoperational
Switch mechanically locked up with
controls working
Failure of switch caused damage to
other equipment
Others
49%
33%
20%
5%
7%
(Responses add to more than 100 percent because
multiple answers were allowed.)
The transfer switches that failed were not
necessarily old units. (See Figure 3.) The same
number of failures was reported in the survey for
units less than five years old and units 15 or
more years old.
Figure 3. How long was the transfer switch(es) in
service when the first failure(s) occurred? R=107
Less than 1 year 1 year to less than 5 years
5 years to less than 10 years
10 years to less than 15 years
15 years to less than 20 years
20 years or more
7%
18%
29%
21%
9%
16%
(Responses add to more than 100 percent because
multiple answers were allowed.)
And transfer switch failures left a significant number
of facilities without power, the survey shows. Of
respondents who experienced failures, 42 percent
reported that their facilities lost power. (See Figure
4.) And more than half who lost power said that facility loads were without power for an hour or more.
(See Figure 5.)
42%
56%
2%
Figure 5. How long were your facility loads
without power? R=53
A minute or less
More than a minute to 30 minutes
More than 30 minutes to 1 hour
More than 1 hour to 2 hours
More than 2 hours to 6 hours
More than 6 hours to 1 day
Greater than 1 day
11%
26%
13%
13%
21%
11%
6%
(Responses add to more than 100 percent because
multiple answers were allowed.)
According to the survey, failures can be traced to
a variety of issues, including power surges, lack of
maintenance, age of equipment and poor design.
(See Figure 6.)
Figure 6. What were the major reasons for
transfer switch failure(s)? R=125
Power surge
Lack of maintenance
Age
Poor design
Poor application Operator error Other
22%
21%
18%
14%
7%
3%
31%
(Responses add to more than 100 percent because
multiple answers were allowed.)
Jacob L. Huske, PE, vice president of Environmental Systems
Design, says that the most common causes of failures of
contactor-based transfer switches that he has heard about were
poor maintenance, or a misapplication of the transfer switch that
resulted in power outages due to nuisance tripping of a circuit
breaker external to the transfer switch during a transfer.
“I have found that many transfer switch failures are the result of
shorted-out rectifiers,” observes Christopher A. Wade, principal
consultant for Resilient Mission Critical Solutions. Previously,
Wade oversaw the global strategy, planning, engineering and
facilities operations for mission critical data centers for a Fortune
100 company.
Another potential problem Wade mentions is human error. In one
case, he says, the transfer switch was left in manual mode for a
couple of months following completion of maintenance work. So,
when needed, the transfer switch did not operate. “Needless to
say, that was a career limiting situation,” notes Wade.
3
Ensuring Transfer Switch Reliability
Despite failures, transfer switches
can be extremely reliable, provided
that they are properly selected and
maintained. The starting point is to
ensure that the switch is qualified
and certified to the correct UL standard. For transfer switches, that’s
UL 1008.
“Transfer switch equipment tends
to be very reliable when properly
applied and maintained, so failures
of UL 1008 transfer switches are
rare,” says Huske.
UL 1008 was established in 1970 by
Underwriters Laboratories to guard
against transfer switch failures
and resultant potential fires. A
performance standard as well as a
design and construction standard,
UL 1008 requires transfer switches
to undergo rigorous testing by an
independent testing and certification agency.
According to the Seventh Edition
of UL 1008, published in July 2012,
the standard applies to automatic
transfer switches (ATS), manual
transfer switches, closed transition
transfer switches, and hybrid transfer switches. UL 1008 also covers
fire pump transfer switches, bypass/
isolation switches, softload transfer
switches and transfer switches used
as service equipment.
4
To meet UL 1008 listing requirements, a transfer switch must meet
the construction requirements
specified in the standard and pass
a series of stringent tests. From the
facility executive’s perspective, UL
1008 transfer switches offer safe
and dependable transfer of electrical power from the utility main to
backup emergency power generators during mandatory compliance
testing and when emergency power
is actually needed.
“UL 1008 is almost always the standard used for enclosed, contactortype transfer switch equipment
applied in the United States, including equipment installed in mission
critical facilities,” explains Huske.
“Where transfer switch equipment is used for transferring
critical load in legally required
emergency power applications
or healthcare facilities, UL 1008 is
generally the only standard that
the authority having jurisdiction
(AHJ) will accept.”
The dependability and safety of UL
1008 transfer switches are “very
important,” says John Ferguson,
director of facilities for Regional
West Medical Center in Nebraska.
“As a hospital, we have to keep our
operations safely going, even in
power outage situations, to protect
the lives of our patients and our
staff who are caring for them.”
Ferguson isn’t alone in seeing
the importance of UL 1008 listed
transfer switches. Harris of Cushman & Wakefield calls the UL 1008
automatic open transition transfer
switches his client uses for its life
safety generator “very important.”
And Ken Cooper, chief engineer at
Sands Regency Casino Hotel, says
that UL 1008 transfer switches
perform “very well, as long as they
have proper maintenance” and are
tested periodically.
From Wade’s point of view, there
are two major benefits of using UL
1008. One is that it simplifies the inspection process on code-required
emergency power systems. “The
reliability of backup power systems
is impacted by switchgear design,”
says Wade. “The UL 1008 standard
has typically been specified for
legally required standby and emergency power systems (e.g., systems
used for critical loads in hospitals).
It is applicable for equipment up to
600 V (it does not address medium
voltage), and is written for the
simple standalone transfer switches. The basic UL 1008 standard does
not address the reliability requirements as the system becomes more
complex with multiple sources of
power (utility and generators) and
with the introduction of tie circuit
breakers.”
Figure 7. Which of the following UL Standards is
your transfer switch(es) certified to? R=633
UL 67
UL 98
UL891
UL1008 Not sure
Others
1%
2%
2%
5%
92%
1%
(Responses add to more than 100 percent because
multiple answers were allowed.)
The other major benefit of UL 1008
revolves around the device testing
requirements of the standard. “UL
1008 really relates to testing devices to validate that they will operate
safely and reliably as intended,” says
Wade. “UL 1008 is designed to be a
self-contained standard for a switch
assembly and has very stringent
requirements for endurance testing. It contains overload, endurance
and interruption testing criteria in
addition to all mechanical construction requirements to ensure safe
operation of the switch.”
Wade points out that the mechanical interlocking requirements of
UL 1008 are significant. Mechanical interlocking is a useful safety
feature for the open transition
transfer of two sources. “Some
modern circuit breakers surpass
the endurance requirements in the
industry standards,” Wade says.
“Therefore, the key specification
item is the circuit breaker endurance. Mechanical interlocking can
be specified for open transition
transfer on simple systems where
the switchgear arrangement makes
it physically possible. However, mechanical interlocking is not possible
in many switchgear arrangements,
and electrical interlocks have successfully been employed for many
years. Either method of interlocking
(mechanical or electrical) prevents
the accidental paralleling of the two
sources, thus increasing safety.”
Despite the importance of UL 1008,
the results of the survey suggests
that most facility executives are unfamiliar with it. Among respondents
with transfer switches, only 5 percent said that their switches were
certified to UL 1008. (See Figure 7.)
The overwhelming majority — 92
percent — said that they weren’t
sure about the standard to which
their devices were certified. This
is eye opening information when
the transfer switch is considered
by most people knowledgeable on
power distribution systems to be
the “heart” of power systems.
As noted earlier, 20 percent of
survey respondents experienced
transfer switch failures in the past
five years. In those cases, 97 percent
of survey respondents were not
sure what certification those failed
switches had, if any.
One reason that some facility executives may not know which standard their devices are certified to is
that transfer switches can last a very
long time. For example, Dave Lopes,
chief engineer for ABM Engineering,
has some experience with failed
transfer switches and is replacing
them with UL 1008 listed units. He’s
unsure whether the replaced units
were UL listed “because they were
vintage units.” He guesses the original switches were installed around
1970 to protect power transfer to
the commercial high-rise’s elevators, the main building emergency
riser, and the emergency generator.
5
Importance of UL 1008 Testing
UL 1008 Testing Requirements
that a UL 1008 listed transfer switch
can operate after withstanding and
closing-on a severe fault current,
bolted fault, or short circuit within
the electrical distribution system.
Why is UL 1008 so important? UL
1008 listing helps assures facility
executives that transfer switches
will operate effectively and safely,
as proven by stringent independent
third-party testing. UL itself may
conduct and monitor the tests, or
another third-party laboratory may
do so under UL supervision.
Optionally, to achieve a short-time
rating, the transfer switch must
withstand a fault current of up to
100,000 amperes for 0.05 seconds,
or 3 cycles. A series of tests assures
The two primary tests to assure
that a UL-1008 transfer switch’s
contact will remain closed during a
fault current until the downstream
protective device clears the fault
are called a “withstand and closing
tests.” The transfer switch’s ability
to handle these crucial functions is
specified by its “withstand and closing rating (WCR)” currents, which
is the maximum current that can
be carried by that transfer switch
for a specific period of time. This
testing meets National Electrical
Code (NEC) Articles 517, 700, 701
and 708. NEC Article 708 is particularly important because it focuses
on systems designated as “critical
operations power systems.”
Other standards that automatic
transfer switches may be required
to meet are National Fire Protection Association’s (NFPA) Life Safety
Code and other applicable NFPA
standards such as NFPA 99 & 110.
Transfer switches installed internationally are qualified to IEC60947-61, which has similar requirements as
UL1008
“Temperature rise testing” is
performed on UL 1008 transfer
switches after short time tests.
This testing confirms the contacts
carrying the rated current are doing
so without overheating. Following
this testing, the UL-certifying official
visually inspects the transfer switch,
looking for any signs of damage. To
achieve a short-time rating under
UL 1008, contacts must be practically “like new”in order to pass the
temperature rise test. The withstand and close-on tests and temperature rise tests are conducted
on the same set of contacts. Parts
cannot be switched out during any
phase of testing.
Endurance performance
Endurance testing is a key element
of UL 1008, according to electrical engineer John Drengenberg,
consumer safety director at UL. The
standard has tables specifying the
number of cycles transfer switches
at each ampere level must achieve
in order to meet the standard and
receive UL 1008 certification, notes
Drengenberg.
Any UL 1008 transfer switch can
transfer at least 3,000 times, with
at least 1,000 of those operations
under at least 100 percent of rated
End Side Bar
6
load. UL 1008 transfer switches
rated at lower amperes must be
able to complete 6,000 transfers
under 100 percent load or greater.
A UL 1008 transfer switch undergoing monthly and annual testing
may be operated roughly 15 times
a year, according to Drengenberg.
“In 100 years, that UL 1008 transfer
switch will have been through just
1,500 cycles, so they are designed,
built and tested to last when they
are tested and certified by UL
1008.”
Drengenberg says that UL inspectors make unannounced visits to
factories producing UL 1008 transfer switches and pull a sample off
the line. Drengenberg says UL also
wants to hear about operational
failures. “We like to get as much
information as possible,” he says.
Figure 8. Who ensured that your transfer switch(es)
was certified to its appropriate standard? R = 649
You
Design Engineer who designed your facility
Installing Contractor
Authorities who approved occupancy certificate
No one
Not sure
10%
49%
47%
26%
1%
15%
(Responses add to more than 100 percent because
multiple answers were allowed.)
Finding Expertise
Scott Blackman, facilities management and commissioning regional
manager at Syska Hennessy, recommends that facilities executives
consider tapping the resources of a
reputable engineering firm to help
them select the proper transfer
switches for their business’ needs.
“The facility manager has tremendous experience across a broad
range of operations for his or her
building,” says Blackman. “The
advantage an engineering firm
offers is that it has multiple clients
with various business needs using
transfer switches in many different
buildings. That expertise allows
the engineer to refine the transfer
switch selection process, based
on his or her experience with what
works and what doesn’t for numerous buildings and situations.”
Huske agrees: “There are many
application-specific considerations
that must be addressed for any type
of transfer equipment that will be
used, and the best way to avoid
problems is to have someone with
extensive experience assist.”
The survey shows that most facility
executives rely on outside parties
to ensure that transfer switches
are certified to the appropriate UL
standard. (See Figure 8.)
No matter who is involved in helping select transfer equipment, it’s
important for the facility executive
to take steps to ensure that the correct UL standard has been followed.
It’s a red flag if transfer switch
literature says the device merely
“meets UL standards,” Drengenberg
says. “(The device) may even have
undergone independent testing
but it might not be UL at all. They
may not have tested under load
conditions,” he points out. Similarly,
facility executives should be wary of
“per UL 1008 standards” and similar
terms that suggest a product complies with UL 1008 but does not say
that outright.
He recommends looking for the UL
inside a circle logo and the word
“listed” as well as the exact transfer
switch words cited in the standard.
UL 1008 transfer switches are specifically labeled as “non-automatic
transfer switch,” “automatic transfer switch” or “transfer and bypassisolation switch.” These labeling
requirements are spelled out in UL’s
Electrical Construction Equipment
Directory, commonly referred to
as the “Green Book.” UL also places
a code on the switch that identifies the manufacturer, according
to Drengenberg. And literature or
specifications should say something
like “UL1008 Listed” or “tested and
certified by UL 1008.”
7
Figure
9. How
are your
transfer
Figure
9. How
are your
transfer
switch(es)
maintained?
R=631
switch(es) maintained?
R=631
Using
thirdthird
party
service
providers
66%66%
Using
party
service
providers
Using
in-house
expertise
36%36%
Using in-house expertise
Using
original
manufacturer’s
service
organization
22%22%
Using original manufacturer’s service organization
Other
1% 1%
Other
(Responses
add to
more
thanthan
100 100
percent
because
(Responses
add
to more
percent
because
multiple
answers
werewere
allowed.)
multiple
answers
allowed.)
Put UL 1008 in writing
To avoid any misunderstanding, facilities executives who are purchasing transfer switches on their own
are spelling out UL 1008 in their
master construction specifications.
The Unified Facilities Guide Specification on automatic transfer switches cites UL 1008 (2012 edition) in its
specifications for the Army Corps of
Engineers, Naval Facilities Engineer-
ing Command (NAVFAC), Air Force
Civil Engineer Support Agency
(AFCESA) and NASA projects. It
is also referenced in the MIL-PRF17773C(SH), which is the Navy
standard for automatic bus transfer
switches installed on Navy combat
vessels.
Proactive Steps
Although proper UL certification for
transfer switches is essential, it’s not
the whole story. Maintenance and
testing are also crucial to ensure
that the transfer switch will respond
when needed.
“All transfer switch equipment
manufacturers publish recommended schedules for maintenance
and testing, so obtaining and
understanding these recommendations is the best place to start,”
suggests Huske. “Facility executives
also should engage a qualified service company to regularly perform
inspections and maintenance on
transfer switch equipment in their
buildings.”
88
Regional West Medical Center in
Nebraska relies on a third party to
do load-bank testing, as well as any
repair needs uncovered, when the
emergency backup system must
be operated for four hours to meet
Joint Commission requirements,
according to Ferguson.
Harris has his transfer equipment maintained under a thirdparty contract. Monthly testing
is performed in-house, but it is
witnessed under the third-party
agreement at least once a year.
Cooper, of the Sands Regency
Casino Hotel, also relies on a thirdparty electrical contractor to make
sure his transfer switches and circuit
breakers perform as expected for
data center/call center operations.
The majority of survey respondents
use third-party service providers
to maintain their transfer switches,
though many also rely on in-house
expertise. (See Figure 9.)
Figure 10. How often do you test transfer
switches? R=630
Monthly
Quarterly
Semiannually
Annually
Less often than once a year
Not sure
Figure 11. What are the steps taken when
maintaining your transfer switch(es)? R=632
46%
16%
12%
26%
5%
8%
(Responses add to more than 100 percent because
multiple answers were allowed.)
Contact inspection
Infrared scans of live electrical components
Lubrication of joints
Testing of sub-systems
Removal of dirt/debris from electrical path
Not sure
69%
53%
48%
47%
47%
23%
(Responses add to more than 100 percent because
multiple answers were allowed.)
On-site
Testing
Automatic transfer switches are
electromechanical devices that have
moving parts. Those moving parts
can seize if they’re left in the same
position for months or years.
Operating those moving parts
regularly helps to ensure they will
continue to move smoothly.
The survey shows that monthly
is the most common frequency
for testing transfer switches.
(See Figure 10.)
That’s the case at Regional West
Medical Center, where the emergency backup power system and its
transfer switches are put through an
hour of performance testing monthly, as well as periodic load-bank testing for four-hour runs per Joint Commission requirements, according to
Ferguson. Two licensed electricians
are on hand and, although he’s had
some problems with the emergency
power system, “there have been no
problems with transfer switches,”
says Ferguson.
The National Fire Protection Association Standard 110 for emergency
and standby power systems says,
“the monthly test of a transfer
switch shall consist of electrically
operating the transfer switch from
the standard position to the alternate position and then a return to
the standard position.”
NFPA 110 also requires the entire
emergency power supply system
to be exercised at least monthly. As
many emergency generators are
diesel, running under light or no load
conditions can cause wet stacking,
which is carbon buildup in the cylinders and exhaust system, a potential
fire hazard. Generator loads of 30
percent or more often are required
by NFPA for monthly testing, which
means the transfer switches also are
being tested.
Experts suggest the best way to test
the automatic transfer switch and
the emergency generator to meet
NFPA 110 requirements is to operate
the transfer switch’s test toggle and
confirm that it is properly signaling
the generator to start and run. Once
proper signaling is verified, the building load is transferred.
Load banks offer a valuable tool for
improving the reliability of an emergency power system. They are used
to test the system without interrupting the critical loads. In the survey,
however, only 28 percent of respondents reported using load banks for
testing transfer switches. Facility
executives who aren’t now using
load banks should evaluate whether
to incorporate them into a planned
maintenance strategy.
Maintenance is another key to ensuring that the transfer switch will function reliably.
Asked how their transfer switches
were maintained, 23 percent of survey respondents said they were unsure, raising the question of whether
they understand the importance of
regular maintenance. (See Figure 11.)
“We know maintenance is paramount. Building managers need to
inspect UL 1008 transfer switches
monthly,” Drengenberg says.
9
Methodology
The ASCO Transfer Switches Survey was sent via email to a
random sample of 15,000 Building Operating Management
subscribers on Feb. 14, 2013. Reminders were sent to nonrespondents on Feb. 20, and Feb. 22, 2013. A total of 147 subscribers chose to opt out of the survey or failed to respond due
to an invalid email address, yielding a final sample of 14,853.
The survey was closed for responses on March 1, 2013. With
1,134 qualified responses returned and with a net sample of
14,853, the rate of response for the email survey was computed to be 7.6 percent. The overall estimated margin of error
for this study is +/- 2.89 percent at the 95 percent confidence
level.
What type of facilities are you
responsible for? R = 1,133
Educational (K-12, Colleges/Universities)
Commercial Office
Medical/Healthcare
Data Centers
Government
Industrial
Retail
Hospitality
Other
34%
33%
21%
13%
13%
10%
8%
6%
5%
Remember
the business
model
The type of UL 1008 transfer switch to use
depends on many factors, but possibly the
most critical one is the organization’s business model.
To that end, Blackman recommends regularly commissioning and evaluating transfer
switches to see if they still satisfy the business
model. “Today’s business is constantly changing and the infrastructure, including transfer
switches, often is overlooked,” he notes.
He recalls a client with open-transition
transfer switches that were performing as designed to an older business model. However,
lights flickering and other interrupting factors
of regular testing were problematic under the
new business model. As a result, the organization opted to retrofit with closed-transition
switches.
For new construction or renovation projects,
facility executives have an important role to
play in making sure the organization gets
what it needs. “Facility executives must know
what the actual function of the facility is going to be – data center, hospital, office building – and work with the design engineers to
determine what is needed,” says Wade.
10
And, when automatic transfer switches are
needed in the United States, facilities executives must be sure they are UL 1008 listed.
Otherwise, they probably will not pass inspections by the authorities having jurisdiction
over the facility.
UL 1008 and UL 891
Different Standards for Different Devices
Some facility executives may believe that UL 1008 and UL 891 can
be used interchangeably. That isn’t
the case, say experts.
“From my perspective they’re totally different standards,” says Steve
Eich, vice president, Environmental
Systems Design.
UL 1008 is specific to transfer
switches, which are designed for a
specific function: to transfer power
from one source to another “flawlessly and quickly,” says John Drengenberg, consumer safety director
for UL. UL 1008 contains rigorous
requirements for testing the functionality of transfer switches. Applications that involve emergency
power for institutional buildings
must have a UL 1008 transfer switch
per the National Electrical Code
(NEC).
“UL 1008 is the cover-all standard
for automatic transfer switches
(ATS) — especially in life safety
applications,” says John Yoon, P.E.,
McGuire Engineers. “Chances are if
you are getting an off the shelf automatic transfer switch, it is in the
vast majority of instances a UL 1008
listed ATS. Most AHJs are looking
for that UL 1008 label when they
review electrical installations for
code compliance. It’s simple onestop shopping for people (AHJs,
end users, engineers) who need a
simple code compliant solution for
99 percent of situations.”
UL 891 covers switchboards.
Switchboards, by contrast with
transfer switches, can perform a
variety of functions, depending on
their intended use. “A switchboard
is a collection of components that
is used to distribute energy,” says
Drengenberg. The components that
are used in a particular switchboard
— which might include circuit breakers, transformers or motor controls
— should comply with the relevant
UL standard for that type of device.
(See “Other Switching Devices” on
Page 13.)
One function that a switchboard
can perform is to transfer power
from one source to another. One
way to accomplish that function
is by including an actual transfer
switch within the switchboard,
says Drengenberg. In that case, the
transfer switch should comply with
UL 1008. Switchboards can also be
designed so that an operator could
disconnect the utility power and
turn on the feed from the emergency generator. But that’s a more
time-consuming way to handle
transfers from one power source to
another.
“UL 1008 is very, very specific,” says
Yoon. “UL 1008 is for standalone
transfer switches, where you’re
trying to connect a crucial load
to two power sources, a primary
source and an alternate source. UL
1008 doesn’t deal with how you
protect that device from abnormal conditions elsewhere in an
electrical distribution system.” It is
expected that there are overcurrent
protection devices elsewhere (for
example, fuses or circuit breakers)
that will interrupt power before the
automatic transfer switch (ATS)
fails catastrophically. UL 1008 also
doesn’t give guidance on how to
transfer power between more than
two sources with the same device.
“These situations are the exception,” says Yoon. “Most end users
who need an ATS have no need
for anything other than a UL 1008
listed device.” In cases where UL
1008 doesn’t provide adequate
functionality, UL 891 comes into
play. UL 891 doesn’t cover how you
transfer from one source of power
to another, but rather covers how
individual components are put
together into an assembly (switchboard).
11
Common UL 1008 Switches
Understanding UL 1008 Switch Options
The simplest and least expensive
option, a manual transfer switch,
requires facility personnel to
hand-change their state from the
utility power source to the backup
generator. Although manual transfer switches are used, automatic
transfer switches are far more common, according to Scott Blackman,
facilities management and commissioning regional manager at Syska
Hennessy.
Christopher A. Wade, principal consultant for Resilient Mission Critical
Solutions, explains that transfer
switches can be put into four general groups.
Open-transition transfer switches
are “break-before-make” devices.
The connection to one source is
opened before connection to the
second source is closed.
“This causes a total power interruption for a short period of time,”
explains Wade. “During re-transfer
from the emergency generator to
the utility power, the switch needs
to keep the contacts open for onehalf to three seconds to allow time
for the residual voltage in inductive
load devices (motors, transformers,
etc.) to decay. This is called ‘delayed
transfer.’”
According to Wade, open-transition
transfer switches are the least
expensive; most designs have a mechanical interlock to prevent utility
paralleling.
Fast closed-transition transfer
switches provide a “make-beforebreak” switching action and feature
a very short timeframe (less than
100 milliseconds) in which utility
and generator are paralleling elec-
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tricity during the transfer period.
“Closed transition transfer switches
utilize a control system and are connected to the generator set similar
to that of open-transition transfer
switches,” explains Wade. “Some
fast closed-transition transfer
switches have an active synchronizing function in the control so
that failures to synchronize can be
avoided.”
These switches prevent momentary
breaks in power during generator
set testing and when closing back
to a restored utility source.
Soft closed-transition transfer
switches have an automatic synchronizer that provides control
signals to the generator set so it can
synchronize with the utility service.
The switch closes the generator to
the utility and then gradually ramps
the facility loads.
The primary advantage of this
transfer switch is that the on-site
power system “is actively synchronized to the utility service, therefore
voltage and frequency disruptions
during the transfer process are kept
to an absolute minimum,” points
out Wade.
“Transfer time is adjustable and
can vary from a few seconds to
several minutes,” explains Wade.
This transfer switch typically is used
for large businesses, resort complexes, data centers, governmental
centers or facilities participating
in interruptible or load-shedding
programs, according to Wade.
Soft closed-transition transfer
switches are costly, as they require
more equipment.
Bypass/isolation transfer switches
allow power in a transfer switch to
be isolated without interrupting
power to the facility.
“This allows the automatic transfer
switch to be conveniently maintained,” explains Wade. Bypass/
isolation transfer switches are frequently specified in mission critical
facilities serving UPS equipment in
data centers and healthcare facilities.
John Ferguson, director of facilities
at Regional West Medical Center in
Nebraska, has about 20 open transition automatic transfer switches
installed to make sure the hospital’s
essential services are not compromised. “We’ve upgraded our
switches since they were installed in
the 1980s and 1990s,” says Ferguson. “I would love to have closed
transition units, but so far have not
gotten funding for them.”
Other Switching Devices
Facility Executives Should Know Requirements
When medium voltage transfers are
necessary, electrically-operated circuit breakers may be used provided
the complete product is qualified
and certified to UL1008A, which is
the standard for medium voltage
transfer switches.
“In optional standby applications, including data centers,
power transfer between utility and
standby generator sources is often
accomplished using electrically
operated circuit breakers in customized switchboard and switchgear
systems, which are typically constructed to the applicable switchboard and switchgear standards,
rather than UL 1008,” explains
Jacob L. Huske, PE, vice president
of Environmental Systems Design.
“The transfer equipment will still be
required to be listed and labeled per
the National Electrical Code (NEC)
to be accepted by the AHJ. However, other standards, such as UL
891 and UL 1558, are typically used
in those cases.”
While switchboards and switchgear
can be used to transfer power, it
is important to note that they are
not covered by UL 1008. Facility
executives should be aware of the
requirements for those devices.
UL 891 dead-front switchboards are
rated at 600V or less and intended
for use under NEC and ANSI/NFPA
70 for use where short-circuit currents are not more than 200,000
amperes. Electrical characteristics
of assemblies under this standard
address rated voltages, continuous
current and short-circuit current
ratings. They also must meet rated
peak withstand current, conditional
short-circuit current, fused shortcircuit current, diversity factor and
frequency. UL 891 also has test
specifications and routine tests.
UL 1558 addresses metal-enclosed
low-voltage power circuit breaker
switchgear assemblies that contain
such devices as low-voltage power
circuit breakers, other interrupting
devices, switches, control, instrumentation and metering as well as
protective and regulating equipment.
UL 1558 low-voltage circuit breakers are not transfer switches,
though some literature may incorrectly call them something such as
“circuit breaker transfer switches.”
These are addressed in the National
Electrical Code and must also meet
ANSI C37.20.1 (Standard for MetalEnclosed Low Voltage Power Circuit
Breaker Switchgear) and ANSI
C37.51 (Standard for Conformance
Testing of Metal-Enclosed LowVoltage AC Power Circuit Breaker
Switchgear Assemblies) to comply
with UL 1558.
Enclosed and deadfront switches
mounted in enclosed panelboard or
deadfront switchboards are manually operated by external handles.
These transfer devices are designed
for use in optional standby systems.
They are sometimes referred to
as double-throw transfer switches
and are specifically excluded by UL
1008. However, they are covered
extensively under UL 98. Panelboards for control and protection
of electrical lighting and appliance
branch circuits, as well as power circuits, including feeder distribution
panelboards, are covered by UL 67.
End Side Bars
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