UL 1008 Short Circuit Testing
Requirements for Transfer
Switches
A tutorial on UL’s short circuit testing
requirements regarding transfer switch
withstand and closing rating (WCR).
Ronald Schroeder
Director, Product Management and
Applications Engineering
UL1008 Short-Circuit Testing

What is WCR?

History of changes to UL1008 Short-Circuit
Testing Requirements

UL 1008 - 7th Edition Changes

What Now?

Conclusion
What is WCR?
Circuit Breakers (AIC) vs. Transfer Switches (WCR)

Ampere Interrupting Capacity (AIC) - Capability to safely
interrupt or break short circuit currents and disconnect the
power source from the load under overcurrent conditions.

Withstand Closing Rating (WCR) - Capability to safely endure
and close-on short circuit currents until overcurrent conditions
are interrupted. These WCR ratings are based on either:

Specific time durations (time based)

Coordination with specific circuit breaker or fuse types
(series rating)
Withstand and Closing Rating (WCR)


WCR of an ATS has an
inverse relationship to the
length of fault time - the
longer the fault time, the Current
(Amps)
lower the current it can
withstand and close-on
Typical Clearing Times

Fuses - 1/4 to 1/2 cycle

Thermal Mag Type
MCBs - 1.5 to 2 cycles

MCCBs w/Electronic
Trip – 2 to 3 Cycles

Power Breakers – 3 to
30 Cycles
Mechanical Design
Maximum Current Limit
Time Current Curve
Maximum ATS WCR
Time (Cycles or Seconds)
History of Changes to UL1008 Short
Circuit Testing Requirements

1972 - Original UL1008 Standard was issued in 1972 with short
circuit testing based on fuse and specific breaker ratings only

1989 - 4th Edition: Added an optional “Any Breaker” withstand and
closing rating (WCR)

1993 – Selective coordination requirements added to the NEC

2002 - 5th Edition: Added optional “Short-Time Current Rating Test”

2014 - 7th Edition of UL 1008 became effective November 1, 2014

Defined detailed guidelines for extending specific breaker qualifications

“Any" circuit breaker ratings were replaced with time duration markings shown
in seconds rather than cycles.

Short-time ratings (where applicable) were replaced with time duration markings
shown in seconds rather than cycles.

More descriptive text was added regarding how the ratings should be applied in
selecting appropriate over-current protection.
History of Changes to UL1008 Short
Circuit Testing Requirements

1972 - Original UL1008 Standard was issued in 1972 with short
circuit testing based on fuse and specific breaker ratings only

1989 - 4th Edition: Added an optional “Any Breaker” withstand and
closing rating (WCR)

1993 – Selective coordination requirements added to the NEC

2002 - 5th Edition: Added optional “Short-Time Current Rating Test”

2014 - 7th Edition of UL 1008 became effective November 1, 2014

Defined detailed guidelines for extending specific breaker qualifications

“Any" circuit breaker ratings were replaced with time duration markings shown
in seconds rather than cycles.

Short-time ratings (where applicable) were replaced with time duration markings
shown in seconds rather than cycles.

More descriptive text was added regarding how the ratings should be applied in
selecting appropriate over-current protection.
Changes in UL1008 short-circuit testing requirements and WCR label information ensure
transfer switches are adequately tested to safely withstand and close-on the short circuit current
shown on the label and therefore can be correctly applied in field applications
1972 Original UL 1008 Short Circuit Testing
Requirements
Switch
Withstand and Closing
Current Rating
Tested
with
CL Fuse
Original edition of UL1008 required
current limiting fuse and “specific
breaker” withstand and closing ratings
only

Breaker qualification typically included
testing a single breaker type and
additional breakers were approved by
comparing published trip curves
Tested with
Circuit
Breaker
“Any Fuse”
Within
designated
Classes:
J, L, T or
RK-5
“Specific
Breaker”
(Series
Rating)
10
10
10
10
0

0.5
1
1.5
3
Test Duration, Cycles (60 Hz)
Sample of Label Required on ATS
1989 “Any Breaker” Ratings Added in 4th
Edition
Switch
Withstand and Closing
Current Rating
Used with
CL Fuse
Used with
Circuit
Breaker
“Any Fuse”
Within
designated
Classes:
J, L, T or
RK-5
1989 - 4th Edition became effective and
added an optional “Any Breaker”
withstand and closing rating (WCR)

As description implies, this rating was
applicable to “Any Breaker” w/o an
adjustable short-time response

Test time durations:
“Any
Breaker”
(Time
Based)
10
10
10
10
0
“Specific
Breaker”
(Series
Rating)

0.5
1
1.5
3
Test Duration, Cycles (60 Hz)

1-1/2 cycles for short-circuit currents
< 10 kA and switches rated < 400
Amp

3 cycles for higher short-circuit
levels and switches rated > 400 Amp
1989 “Any Breaker” Ratings Added in 4th
Edition


New “Any Breaker” ratings:

More flexibility coordinating ATS
with over current protective devices

Less concern w/ future changes in
overcurrent protection

Since the “Any Breaker” rating was
optional, not all ATS products had this
rating

ATS manufacturers (like ASCO) with
robust designs enabled them to
qualify for “Any Breaker” ratings
“Specific Breaker” ratings:

Higher WCR but limited to circuit
breakers listed on label

Need numerous breakers listed for
application flexibility.
Sample of Label Required on ATS
1989 “Any Breaker” Ratings Added in 4th
Edition


New “Any Breaker” ratings:

More flexibility coordinating ATS
with over current protective devices

Less concern w/ future changes in
overcurrent protection

Since the “Any Breaker” rating was
optional, not all ATS products had this
rating

ATS manufacturers (like ASCO) with
robust designs enabled them to
qualify for “Any Breaker” ratings
“Specific Breaker” ratings:

Higher WCR but limited to circuit
breakers listed on label

Need numerous breakers listed for
This shows
the advantage of having BOTH a specific breaker rating for higher
application flexibility.
WCR values, AND an “any breaker” rating to allow additional breakers
Sample of Label Required on ATS
1993 NEC Adds Selective Coordination
Requirements
 Selective
Coordination: localization of an
overcurrent condition to isolate outages
to the circuit affected by selecting
appropriate overcurrent protective
devices and settings
 1993-
First added to the NEC for elevator
circuits and (3) subsequent articles were
added in 2005 and 2008:

2005-Article 700.27 – Emergency
Systems Coordination

2005-Article 701.18 – Legally Required
Standby Systems

2008-Article 708.54 - Critical Operations
Power Systems (COPS)
 UL
recognized applications where
transfer switches would be required to
withstand and close-on short-circuit
currents for time durations greater than
0.1 secs (6 cycles for 60Hz).
*
*Source: NEC Handbook 2011
2002 Short-Time Current Rating Test
UL 1008, 5th Edition
Added to UL1008,
June 2002
Switch
Withstand and Closing
Current Rating
Used with
CL Fuse
“Any Fuse”
Within
designated
Classes:
J, L, T or
RK-5
“Specific
Breaker”
(Series
Rating)
“Any
Breaker”
(Time
Based)
X kA
@ A cycles
10
10
Test Duration, Cycles (60 Hz)
B
<<X kA
@ C cycles
10
A
10
10
3
10
10
1.5
10
10
1
<X kA
@ B cycles
10
10
0.5
10
10
0
Short-Time
Current Test
Used with
Circuit
Breaker
C
2002 Short-Time Current Rating Test
UL 1008, 5th Edition
Utility Service
ATS Must Withstand and
Then Close-On Same
Magnitude of Short
Circuit Current
Fault Current
Delivery
UL 1008 Standard - Passing Criteria:
F2
F1
Emergency Generator

Alternate Source Main Contacts Must Exhibit
Continuity After Test

Enclosure Door Must Remain Closed

Phase-to-Enclosure Fuse Must Be Intact

ATS Must be Operable by Intended Means

No Breakage of Switch Base…

Power Cables Can’t Pull Free of Terminal Lugs

For Short Time Test: ALL Contacts Must Exhibit
Continuity AND Pass Temperature Rise! Must be
‘Like New’!
Short
Circuit
Test
(non-short
time)
Short
Time
Test
2002 Short-Time Current Rating Test
UL 1008, 5th Edition
Short time test:
 Longer short circuit test time = more energy ATS must endure
 Plus, successful temperature rise test requires minimal main
contact damage (‘like new’)
 Therefore, the short-time WCR < short-circuit WCR
 ATS manufacturers’ options for short-time WCR compliance:
A. Don’t offer short-time ratings
B. Qualify existing products for short-time ratings
C. Develop a separate transfer switch design to provide short-time ratings
 ASCO’s robust standard 600-4000Amp switches qualified for
short-time ratings. In addition, a separate design family was
developed for even higher short-time ratings, including 30
cycles (0.5secs)
2014 UL1008, 7th Edition
The 7th Edition of UL 1008 has resulted in significant
changes to short-circuit testing ratings shown on the
transfer switch:

Specific guidelines were added for qualifying additional
circuit breakers to be shown on the short circuit label
markings.

The "Any" circuit breaker ratings were replaced by “time
based” ratings shown in seconds rather than cycles.

Short-time ratings (where applicable) are also shown in
“seconds” instead of “cycles”.

More descriptive statements have been added regarding
how the ratings should be applied in selecting appropriate
over-current protection.
2014 UL1008, 7th Edition
The 7th Edition of UL 1008 has resulted in significant
changes to short-circuit testing ratings shown on the
transfer switch:

Specific guidelines were added for qualifying additional
circuit breakers to be shown on the short circuit label
markings.

The "Any" circuit breaker ratings were replaced by “time
based” ratings shown in seconds rather than cycles.

Short-time ratings (where applicable) are also shown in
“seconds” instead of “cycles”.

More descriptive statements have been added regarding
how the ratings should be applied in selecting appropriate
over-current protection.
Qualification of Specific Circuit Breakers
Prior to UL1008, 7th Edition

The most significant change in the 7th Edition is the addition of a
method for qualifying additional circuit breakers to be shown on the
WCR label markings.

Prior to the 7th Edition, the method for listing specific breakers was not
defined in the standard and was based on a comparison of the
“published” maximum instantaneous clearing time between the tested
breaker and non-tested circuit breakers.

If the non-tested circuit breaker’s published clearing time was equal to
or less than the tested circuit breaker’s published clearing time, the
non tested breaker could be listed.

Most switch manufacturers then published a formidable list of specific
breaker types on their WCR label at that time.

Unfortunately, this method could not guarantee that all listed breakers
would coordinate with the ATS to clear the short circuit safely.
Qualification of Specific Circuit Breakers
Prior to UL1008, 7th Edition
 Recall
Time Current Curve = Max
ATS WCR
i
t
earlier the WCR of an ATS is
related to the time current curve.
Qualification of Specific Circuit Breakers
Prior to UL1008, 7th Edition
 Recall
Time Current Curve = Max
ATS WCR
i
42kA Specific Breaker Rating
with Breaker A
42kA
Breaker
A
Test Time =
10ms
Pub Clearing Time 22ms
t
earlier the WCR of an ATS is
related to the time current curve.
 ATS
was short circuit tested at 42kA
with breaker A which cleared the fault
in 10ms but has a “published”
clearing time of 22ms.
Qualification of Specific Circuit Breakers
Prior to UL1008, 7th Edition
 Recall
Time Current Curve = Max
ATS WCR
i
42kA Specific Breaker Rating
with Breakers A and B
earlier the WCR of an ATS is
related to the time current curve.
 ATS
was short circuit tested at 42kA
with breaker A which cleared the fault
in 10ms but has a “published”
clearing time of 22ms.
 Breaker
B was approved for specific
breaker listing based on comparison
of the published clearing time of 22ms
or less.
42kA
Breaker
A
Breaker
B
Test Time =
10ms
Pub Clearing Time 22ms
t
Qualification of Specific Circuit Breakers
Prior to UL1008, 7th Edition
 Recall
Time Current Curve = Max
ATS WCR
i
42kA Specific Breaker Rating
with Breakers A and B
earlier the WCR of an ATS is
related to the time current curve.
 ATS
was short circuit tested at 42kA
with breaker A which cleared the fault
in 10ms but has a “published”
clearing time of 22ms.
 Breaker
B was approved for specific
breaker listing based on comparison
of the published clearing time of 22ms
or less.
42kA
Breaker
A
 However,
there is no guarantee
Breaker B will actually clear in 10ms
@ 42kA and could possibly take 60%
or longer, or 16-25ms.
Breaker
B
Test Time =
10ms
Test Time = 16ms
Pub Clearing Time 22ms
t
Qualification of Specific Circuit Breakers
Prior to UL1008, 7th Edition
 Recall
Time Current Curve = Max
ATS WCR
i
42kA Specific Breaker Rating
with Breakers A and B
X
42kA
Breaker
A
Breaker
B
Test Time =
10ms
Test Time = 16ms
Pub Clearing Time 22ms
If Breaker B has an
actual clearing time of
16ms, ATS would see
60% more energy than it
would for 10ms, which
may be above the ATS
WCR
t
earlier the WCR of an ATS is
related to the time current curve.
 ATS
was short circuit tested at 42kA
with breaker A which cleared the fault
in 10ms but has a “published”
clearing time of 22ms.
 Breaker
B was approved for specific
breaker listing based on comparison
of the published clearing time of 22ms
or less.
 However,
there is no guarantee
Breaker B will actually clear in 10ms
@ 42kA and could possibly take 60%
or longer, or 16-25ms.
 Notice
short circuit energy level with
Breaker A was below WCR limit of the
ATS, but the potential energy level w/
Breaker B would exceed the rating of
the ATS.
Qualification of Specific Circuit Breakers
Post UL1008, 7th Edition
Time Current Curve = Max
ATS WCR
i
42kA Specific Breaker Rating
with Breaker A
42kA
Breaker
A
Test Time =
10ms
t
Effective 11/1/2014 circuit breakers
to be listed must either be shortcircuit tested with the transfer
switch per UL1008 requirements or
they can be added to the WCR
label based on the following
paragraph 9.13.3.7(c):
“1) The duration of time that
current passes through the
contacts of the switch shall be
measured from the short-circuit
withstand and short-circuit
closing test data. The shortest
duration shall be selected from the
evaluation.”
Qualification of Specific Circuit Breakers
Post UL1008, 7th Edition
???
42kA

“3) The circuit breaker time-current
characteristic shall be evaluated in
the instantaneous trip region at the
current measured for the shortcircuit withstand and short-circuit
closing tests. If the circuit breaker
maximum clearing time, at this
current on the time-current
characteristic, is less than or equal
to the shortest test duration as
determined in (1), the circuit
breaker is acceptable for addition
to the markings as specified in
5.2.5.1.e following:”

In this case the max instantaneous
clearing time is greater than the
actual time durations of shortcircuit tests conducted on the
transfer switch and the breaker
type is not suitable for listing at the
specific breaker rating, but may be
acceptable under the time based kA
rating
Qualification of Specific Circuit Breakers
Post UL1008, 7th Edition

“3) The circuit breaker time-current
characteristic shall be evaluated in
the instantaneous trip region at the
current measured for the shortcircuit withstand and short-circuit
closing tests. If the circuit breaker
maximum clearing time, at this
current on the time-current
characteristic, is less than or equal
to the shortest test duration as
determined in (1), the circuit
breaker is acceptable for addition
to the markings as specified in
5.2.5.1.e following:”

In this case the max instantaneous
clearing time is greater than the
actual time durations of shortcircuit tests conducted on the
transfer switch and the breaker
type is not suitable for listing at the
specific breaker rating, but may be
acceptable under the time based kA
rating
Breaker B – 16.7ms
Clearing Time in the
Instantaneous Trip
Region
16.7ms
42kA
Qualification of Specific Circuit Breakers
Post UL1008, 7th Edition

Transfer switch manufacturers now have two choices for listing
specific breaker ratings on their WCR label:
1)
Test with every specific breaker listed on the WCR label since actual
clearing times are typically much faster then the published trip curves
indicate
2)
Conduct time duration tests which will encompass the published clearing
times of all the breakers to be listed.

Both breaker qualification methods are expensive and exhaustive

Using a breaker for method 2 is no longer practical with the new UL
guidelines for qualifying additional breakers, because the actual
clearing times are significantly faster than their published clearing
times.

ASCO has robust power transfer switch designs and has conducted
longer time duration tests on our switches to encompass the
published clearing times of the breakers to be listed on the WCR label.
UL1008, 7th Edition
The 7th Edition of UL 1008 has resulted in significant
changes to short-circuit testing ratings shown on the
transfer switch:

Specific guidelines were added for qualifying additional
circuit breakers to be shown on the short circuit label
markings.

The "Any" circuit breaker ratings were replaced by “time
based” ratings shown in seconds rather than cycles.

Short-time ratings (where applicable) are also shown in
“seconds” instead of “cycles”.

More descriptive statements have been added regarding
how the ratings should be applied in selecting appropriate
over-current protection.
UL1008, 7th Edition
Old Label
The 7th Edition replaced the term “ANY" circuit
breaker with time duration markings on the WCR
label shown in seconds. Time based ratings are
more conducive for comparison to the time
current curves published by breaker
manufacturers
New Label
The UL specified test time durations
are 0.025 secs for short-circuit currents
up to 10 kA on switch ratings of 400
amp or less and 0.050 secs for most
other short-circuit levels.
UL1008, 7th Edition
The 7th Edition of UL 1008 has resulted in significant
changes to short-circuit testing ratings shown on the
transfer switch:

Specific guidelines were added for qualifying additional
circuit breakers to be shown on the short circuit label
markings.

The "Any" circuit breaker ratings were replaced by “time
based” ratings shown in seconds rather than cycles.

Short-time ratings (where applicable) are also shown in
“seconds” instead of “cycles”.

More descriptive statements have been added regarding
how the ratings should be applied in selecting appropriate
over-current protection.
WCR Labelling Changes
UL1008, 7th Edition
Short-time ratings will still be shown
as short-circuit amperes, volts, and
time duration markings shown in
seconds instead of cycles.
UL1008, 7th Edition
The 7th Edition of UL 1008 has resulted in significant
changes to short-circuit testing ratings shown on the
transfer switch:

Specific guidelines were added for qualifying additional
circuit breakers to be shown on the short circuit label
markings.

The "Any" circuit breaker ratings were replaced by “time
based” ratings shown in seconds rather than cycles.

Short-time ratings (where applicable) are also shown in
“seconds” instead of “cycles”.

More descriptive statements have been added regarding
how the ratings should be applied in selecting appropriate
over-current protection.
UL1008, 7th Edition WCR Label Changes
ATS w/o Short-Time Rating
Old Label
The original text sometimes caused
confusion with interpreting the
coordination of the short circuit ratings
with the overcurrent protection.
New Label
More descriptive statements have been
added regarding how the ratings should be
applied in selecting appropriate over-current
protection. Notice if the circuit breaker is
coordinated with a switch w/o a short time
rating, the short-time delay adjustment must
be set to zero.
UL1008, 7th Edition WCR Label Changes
ATS with Short-Time Rating
Old Label
The original text was also the same on the
WCR label even if the switch had a short- time
rating. This again caused confusion in
interpreting the short circuit ratings.
With the 7th Edition, the text is different for a
switch with a short-time rating. Note the
switch must have a short-time rating, which
must be coordinated with the short-time
rating of the breaker.
New Label
What Now?

The “Any Breaker” rating has been replaced in the 7th edition by
a time based rating of .05 seconds (for switches >400 Amp) or
.025 seconds for switches rated 400A and under for short circuit
currents <10,000 Amp.

This forces the system designer to determine the instantaneous
clearing time of the breaker from the published trip curves to
confirm it is less than or equal to the time based rating of the
transfer switch.

Typically .05 seconds is sufficient to cover most breakers
available today. If not, or if the breaker has a “short time
response”, the short-time rating of the transfer switch must be
applied which is typically at a lower kA level.

If the transfer switch has no short-time rating, CL fuses w/ faster
clearing times may have to be installed ahead of the switch for
coordination purposes which adds cost and physical size to the
project.
What Now?

It is important to note the time based short-circuit and short time
ratings are both optional for product qualification under UL1008

Not all transfer switch manufacturers offer these two optional
ratings

These optional ratings offer more convenience and flexibility in
selecting an acceptable circuit breaker to coordinate with the
transfer switch WCR

ASCO 7000 Series Power Transfer Switches now offer four
Withstand and Closing Ratings to provide maximum flexibility:
Current
Limiting Fuse Rating
Specific
Breaker Rating
Time
Based Breaker Rating
Short
Time Rating (600-4000 Amp Switches)
ASCO Product Configurations and Ratings
WCR Values
Standard Product
Short-Time Product
WCR Values
Time Duration and Current (kA)
Ampacity and Frames
Ampacity and Frames
Time Duration and Current (kA)
Secs
3
.05
8
.13
18
.3
30
.5
42
150-600
Current (kA)
50
36
85
100
100
85
65
18
.3
30
.5
50
36
30
600-1200
H
P
1000-1200
G
Q
600-1600
65
50
800-1200
100
65
1600-2000
100
2600-4000
125*
4000
Cycles
Secs
J
ATS 600-800
BPS 6001200
1600-3000
42
3
.05
G
G
S
U
85
Current (kA)
Cycles
65
100
* 3.6 cycles ( 0.6 Sec) Duration
Conclusion

Significant changes in UL 1008 short testing requirements since its
inception have had a significant impact on:

Short-circuit testing methods

Markings on WCR labels

Transfer switch product designs

7th Edition now clarifies specific breaker qualification criteria, which will
impact the quantity of breakers shown on most switch equipment labels

7th Edition requires additional information on product labels which helps
authorities having jurisdiction determine compliance with UL1008

7th Edition aims to mitigate the mis-application of transfer switches
based on the incorrect assessment of breaker coordination

ASCO 7000 Series Power Transfer Switches:

are tested and qualified to both the optional time based short-circuit and the
short time requirements specified in UL1008

offer more flexibility in coordinating the transfer switch and over-current
devices