30-Cycle Rated Switchboards - Selectivity Tradeoffs
By
Rick Schlobohm, P.E. – Senior Specification Engineer, GE
Franklin Bohac, P.E. – Specification Engineer, GE
Increased focus on selective coordination has resulted due to requirements in the 2005 and 2008
editions of the National Electrical Code (NEC). These requirements, which focus on emergency
systems (NEC Article 700), legally required standby systems (NEC Article 701), and Critical Operation
Power Systems (NEC Article 708), have also led the electrical designer to pursue selective
coordination for many other electrical systems. This paper will attempt to define and address the
need for 30-cycle withstand ratings for distribution equipment, not only for low voltage switchgear,
but also for low voltage switchboards when instantaneous selective coordination is specified. This
paper will also address the consequences, such as increased arc flash hazard categories, that
occur when a system is designed to achieve complete selective overcurrent protection through the
instantaneous range.
What is Selective Coordination?
The NEC in Article 100 defines selective coordination as the “localization of an overcurrent condition
to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent
protective devices and their ratings or settings”. A system designed to be selectively coordinated
simply implies that the overcurrent device closest to a fault or overload will open the circuit before
any upstream overcurrent device. An important aspect, which must be considered when designing
and specifying a selectively coordinated distribution system, is the minimum time level for which
selective coordination needs to be maintained and the maximum fault level at each piece of
equipment in the system. The two most common time levels specified for selectively coordinated
systems are 0.01 seconds and 0.1 seconds. The NEC Articles 700.27, 701.18, or 708.54 do not
provide a definition for selective coordination and do not indicate a preference for one or the other
common selectivity levels. This NEC selectivity requirement is generally, but not always, interpreted
as meaning that selective coordination must extend throughout the instantaneous region (0.01
seconds) of the device’s trip curve. Other standards such as Florida’s Agency for Health Care
Administration (AHCA) mandates selective coordination for time delays beginning at 0.1 seconds.
The 0.1 second standard has been adopted by many other authorities having jurisdiction (AHJ) and
is considered a good engineering compromise between the benefits of selective coordination and
the associated increase of arc flash hazard levels resulting from the time delays needed to achieve
complete selectivity among overcurrent devices. The 0.1 second standard will allow some overlap
in the time bands for the devices in the instantaneous time region. These overlaps will occur at the
higher fault levels. One item to understand in the selectivity discussion is the fact that even though
the main device and feeder device both have an instantaneous trip function and their time bands
overlap at high fault levels, they are selective up to the point of minimum instantaneous pickup for
the upstream device (Reference Point A in Figure TCC-1). Even though the instantaneous time
bands overlap for fault levels above Point A, the system is fully selective for faults and overloads
below that level. Traditional switchboards and other distribution equipment can be utilized in
designing to 0.1 second selective coordination if care is taken in the selection and adjustment of the
overcurrent protective devices.
When instantaneous selective coordination for the main distribution equipment is desired for time
levels of 0.01 second and greater, selectivity is easily achieved throughout this instantaneous range
by using conventional low voltage switchgear. This is accomplished by utilizing main devices
without an instantaneous trip function. This feature is also available in some vendor’s
switchboards, but the switchboards must be provided with internal bus that is braced for the
available fault current for 30-cycles and not just the 3-cycles as defined in the UL 891 standard for
switchboards. Without an instantaneous trip to isolate severe faults to a 3 cycle total clearing time,
the bus withstand needs to be rated for 30-cycles. This is the maximum time delay permitted by
the adjustment ranges of short-time and ground fault protection provided on fully adjustable UL
1066/ANSI rated low voltage power circuit breakers (LVPCB).
When designing a project for selectivity, more thought and planning may be required to ensure
selectivity throughout the distribution system in addition to the protective devices at the main
distribution equipment. While perfect selectivity through the instantaneous range may be attained
using ANSI rated switchgear for the main equipment, the switchgear feeder breakers may not be as
selective with downstream molded case breakers as would be desired, or the large frame size may
force the designer to add another level of overcurrent devices which would hamper the overall
selective coordination of the system. Switchboards are available to achieve instantaneous
selectivity with a low voltage power circuit breakers (LVPCB) main not utilizing an instantaneous trip
function. These switchboards offer the designer the ability to have switchboard mounted feeder
breakers utilizing molded case breakers. Many of these molded case breakers will provide selective
coordination into the instantaneous range to higher fault values than can be obtained with
breakers rated to the ANSI standards. The use of molded case breakers in the main distribution
equipment as feeders can eliminate a layer of overcurrent devices that could be difficult to
selectively coordinate.
What is a 30-Cycle Withstand Rating?
This term can be simply defined as the ability of the distribution equipment to withstand the
mechanical and thermal stresses associated with a rated fault, for a time period of 30-cycles (0.5
seconds). The 30-cycle withstand rating has long been a requirement of low voltage switchgear
designed and tested to the ANSI C37.20.1 standard and labeled to the UL 1558 standard.
Switchgear short circuit ratings per these standards are based on two 30-cycle withstand tests with
a 15-second interval in between tests performed at 15% power factor at 635VAC maximum.
Switchboards however are only required to pass a single 3-cycle (0.05 second) short circuit test at
20% power factor and 600VAC maximum. Switchboards standards of UL 891 and NEMA PB-2 do
not include requirements or testing procedures for a withstand test greater than 3-cycles since the
circuit breakers used in these switchboards are UL 489 listed and all have an instantaneous trip
function. With an instantaneous trip function, the breaker is expected to open within 3-cycles to
prevent catastrophic damage to the distribution equipment. Figure TCC-2 references the 30-cycle
time period (0.5 seconds) by Point B and the 3-cycle time period (0.05 seconds) by Point C.
Even though UL 891 does not address a 30-cycle withstand rating, there are UL 891 listed
switchboards available with this higher duration withstand rating. These switchboards are tested to
the ANSI test standards and are witnessed by UL and thus can be labeled as suitable for
applications requiring a 30-cycle withstand rating.
When is a 30-Cycle Withstand Rating required?
As mentioned previously, the 30-cycle withstand rating is required for some equipment by the
standards for which they are designed and tested. The reason the ANSI standards for switchgear
require the 30-cycle withstand rating is that this equipment is designed to accept UL 1066/ANSI
C37 circuit breakers which are not required to have an instantaneous trip function. This is why any
overcurrent device mounted in a traditional UL 891 switchboard with a 3-cycle withstand rating
must have an instantaneous trip.
Figures TCC-1 and TCC-2 below show typical time current curves for a main and feeder device.
Figure TCC-1 shows a 4000A insulated case circuit breaker (ICCB) main and 1200A molded case
circuit breaker (MCCB) feeder both with instantaneous trips. Figure TCC-2 shows the same main
and feeder devices, however the main is now a low voltage power circuit breaker (LVPCB) without
an instantaneous trip. In figure TCC-1 you can see that the main device will detect the faults
greater than 33,000A at 0.01 seconds and due to the mechanical function of the breaker, will clear
these faults within 3-cycles. In figure TCC-2 the same fault will continue until cleared by the short
time function of the main device which will initiate breaker tripping in about 25-cycles (0.416
seconds) after the short-time pickup has been exceeded. Main 2 requires a switchboard with the
30-cycle withstand rating while Main 1 does not.
CURRENT IN AMPERES
CURRENT IN AMPERES
1000
1000
MAIN-2
ComponentName MAIN-1
GE
PowerBreak II, EGTU
SS
Trip 4000.0 A
Plug 4000.0 A
Settings Phase
LTPU/LTD (0.5-1 x In) 0.50X (2000A); Min CB
STPU (1.5-9 x LTPU) 1.5X (3000A)
STD (ST01 - ST11) ST02-Min (I^2t Out)
INST (2-9 x In) 9X (36000A)
Override (HSIOC) Fixed (50200A)
100
MAIN-1
100
FDR-1
10
B
1
0.10
0.01
C
0.5 1
10
100
1K
10K
Figure TCC-1
A
ComponentName FDR-2
GE
SK, MVT Plus/PM
SKL
Trip 1200.0 A
Plug 1200.0 A
Settings Phase
LTPU (0.5-1.0 x P) 0.5 (600A)
LTD (1-4) 1
STPU (1.5-9 x LTPU) 1.5 (900A)
STD (1-4) 1 (I^2t Out)
INST (1.5-10 x P) 1.5 (1800A)
0.10
0.01
100K
SWBD-1.tcc Ref. Voltage: 480V Current in Amps x 1 SWBD-1.drw
1
T IM E IN S E C O N D S
10
FDR-2
T IM E IN S E C O N D S
ComponentName FDR-1
GE
SK, MVT Plus/PM
SKL
Trip 1200.0 A
Plug 1200.0 A
Settings Phase
LTPU (0.5-1.0 x P) 0.5 (600A)
LTD (1-4) 1
STPU (1.5-9 x LTPU) 1.5 (900A)
STD (1-4) 1 (I^2t Out)
INST (1.5-10 x P) 1.5 (1800A)
ComponentName MAIN-2
GE
WavePro, EGTU
WPS-40
Trip 4000.0 A
Plug 4000.0 A
Settings Phase
LTPU/LTD (0.5-1 x In) 0.50X (2000A); C-1
STPU (1.5-9 x LTPU) 2X (4000A)
STD (ST01 - ST11) ST11-Min (I^2t Out)
0.5 1
10
100
1K
10K
100K
SWBD-2.tcc Ref. Voltage: 480V Current in Amps x 1 SWBD-2.drw
Figure TCC-2
There are UL 489 listed breakers available on the market that are advertised with switchable
instantaneous trip functions. When many of these breakers are investigated, the switchable
function does not switch the instantaneous off, but simply increases the instantaneous pickup to a
fixed high level setting. The reason these breakers are capable of being provided in a traditional UL
891 switchboard with only a 3-cycle fault withstand rating is that they continue to retain an
instantaneous trip function.
Breakers available without an instantaneous trip function are not listed to UL 489, but rather are
low voltage power circuit breakers listed to UL 1066 and built per the standards of ANSI C37. These
true UL 1066/ANSI rated low voltage power circuit breakers are available in GE Spectra and
PowerBreak switchboards as main devices without instantaneous trip functions when these
switchboards are provided with a 30-cycle withstand rating to accommodate these breakers.
If the designer of an electrical system chooses to achieve the 0.01 second selectivity by eliminating
the instantaneous function on the main device, he can choose to use UL 891 switchboards that are
tested for a 30-cycle withstand. However, the designer must be aware of consequences of an
increased arc flash hazard that is present with this design approach.
How does meeting selectivity affect Arc Flash Hazard risk categories?
The awareness of arc flash hazards has made tremendous strides in recent years. The standard for
electrical safety in the workplace started in the 1970s when OSHA asked the NFPA to create a
standard. This standard became NFPA 70E. Awareness in the general industry started increasing
when the 2002 edition of the NEC was published requiring arc flash warning labels on equipment
that was likely to be worked on or maintained while energized. Around the same time IEEE 1584
was published which provided calculations to quantify arc flash thermal energy release and the aid
in the determination of appropriate personal protective equipment (PPE) to protect workers from
arc flash. While this paper is not intended to be a tutorial on arc flash, there are a few facts that
must be remembered.
An arc flash is a violent event which releases thermal and mechanical energy from an arc, which
can be as hot as the sun’s surface and can vaporize copper to 67,000 times its solid volume
propelling from the incident area through a pressure wave with deafening sound. An arc flash
energy magnitude is determined by the magnitude of the current in the arc, the time to clear the
arc, and the distance of the worker from that arc, in addition to other variables which are not under
the control of the designer. The hazard risk categories for personal protective equipment (PPE) are
certified for the level of heat energy applied over a given area to allow the worker to survive the
incident with no worse than “curable” (second degree) burns on his torso.
Looking back at our examples in Figure TCC-1 and TCC-2 which show how removing the
instantaneous trip function on the main can increase selectivity, we will now look at arc flash
hazard categories for each of these main versus feeder protection schemes. For the simple system
shown in Figure TCC-1 with a 4000A main with an instantaneous trip in a UL 489 switchboard and
an available fault current of 65kA the arc flash level is calculated to 14 cal/cm², which would be
within the capabilities of Category 3 PPE. Category 3 PPE consists of cotton underwear, flame
resistant (FR) shirt and pant and flame resistant (FR) coveralls. This same piece of equipment shown
in Figure TCC-2 designed for instantaneous selectivity (0.01 second) with a main device without
instantaneous trip protection would have an arc flash level of 73 cal/cm² which exceeds the
capabilities of all hazard risk categories. There is no appropriate PPE recommended by NFPA 70E
for arc flash energies that exceed 40 cal/cm², because the blast effects of arc flash are a bigger
concern than the thermal effects and there is no recognized protection against the blast effects of
an arc flash for these high levels. Reference Figures AFL-1 and AFL-2 for respective Arc Flash and
Shock Hazard equipment labels for the examples in TCC-1 and TCC-2.
The electrical designer now has a tough decision to make with regard to the need for instantaneous
selectivity and the corresponding increase in the arc flash hazard category for the equipment. The
scenario presented in the example of TCC-2 and AFL-2 utilized a switchboard with a 30 cycle bus
withstand rating, an ANSI rated main circuit breaker without instantaneous and a molded case
circuit breaker feeder rated at 1200A. Many designers would consider true ANSI rated switchgear
construction for this type of application instead of a rated and tested switchboard. Using
switchgear does not necessarily solve the selectivity or the arc flash hazard risks. Most circuit
breaker manufacturers have published tables and guidelines that list the various combinations of
overcurrent protection devices whose instantaneous trips have been found to be selective with
each other along with the corresponding maximum fault values for which the combinations
achieve instantaneous selectivity. Many of the combinations require the use of molded case
feeders because the same level of selectivity is not achieved with the ANSI rated devices. Using
switchgear could hinder the ability to selectively coordinate the downstream combination of
devices. The arc flash incident energy will be different when switchgear is applied due to the
difference in the input data used in the calculations for low voltage switchgear versus
switchboards. The gap measurement (conductor phase spacing gaps) used for switchboard
construction is 25mm but increases to 32mm for switchgear. The working distance measurement
used in the calculations also is different between the equipment types. For switchboards the input
data is 18” while for switchgear the data used is 24”. While these differences in input data for the
different types of equipment do change the calculated incident energy, the low voltage switchgear
does not present a lower hazard risk category. The incident energy available when utilizing
switchgear in lieu of a switchboard for the scenario presented in TCC-2 is 43 cal/cm². This value is
still greater than the thermal energy rating of Category 4 PPE and so no suitable PPE is available.
The electrical designer has options, but must weigh the benefits of instantaneous selectivity with
corresponding arc flash hazards of each design option.
Figure AFL-1
Figure AFL-2
What does this all mean?
When an electrical designer makes a decision to have a selectively coordinated system, he must
first decide what level of selectivity is required. If selectivity is desired beginning at 0.1 seconds,
then traditional switchboards can be utilized with careful selection and specification of the
overcurrent protective devices. If the design requires selectivity through the instantaneous range
beginning at 0.01 seconds, the designer must then be aware of requirements in equipment
selection when the instantaneous trip function is eliminated for the main device. If the main device
does not have an instantaneous trip function, a 30-cycle withstand rating must be specified for the
distribution equipment. With true 30 cycle rated switchboards now available, the designer is not
required to specify ANSI rated switchgear to apply main breakers without instantaneous trips. One
must also consider the safety aspects of the design and the corresponding differences in arc flash
hazard categories for the various types of distribution equipment to be utilized for the project. The
designer must work with the owner, architect and AHJ and make them aware of the consequences
of the deign criteria. They should work as a team to provide the best compromise between the
benefits of selective coordination and arc flash safety hazards that increase with some designs.