how to determine short-circuit current ratiings
how to determine short-circuit current ratiings
Electrical safety is vital. Learn how
to use product standards and the NEC to
determine SCCRs for your industrial control panels, components and assemblies.
by Dan Neeser
I
t’s important to understand short-circuit current
ratings (SCCR) for industrial control panels. Industrial control panel is a general term that describes
an assembly of two or more components that can
be power, control or both, but doesn’t include the controlled equipment.
UL 508A, the UL Standard for Safety for Industrial
Control Panels and the National Electrical Code (NEC ®)
require industrial control panels that contain power circuit components (supply main line power to loads such
as motors, heating, lighting, appliances or receptacles)
to be marked with an SCCR, previously referred to as a
withstand rating.
As referenced in UL 508A and the NEC, industrial
control panels that contain only control circuit components do not have to be marked with an SCCR. For clarification, this would be an assembly that just controls the
performance and does not carry any main line power.
However, if the industrial control panel contains the
branch-circuit overcurrent protective device (considered
a power circuit component) supplying the control circuit, then the SCCR must be marked based on the interrupting rating of the overcurrent protective device.
How to
Determine
Short-Circuit
Current Ratings
32
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Why Is SCCR Important?
The SCCR represents the highest short-circuit (fault)
current that equipment such as industrial control panels
can safely withstand without causing a fire or shock hazard. Exceeding the SCCR can cause catastrophic and
violent equipment component and enclosure failure.
Therefore, it’s important to identify (calculate) the
maximum fault current that can be present where the
industrial control panel or other equipment is installed
and assure the maximum fault current is less than the
marked SCCR of the industrial control panel to comply
with code.
Note that there is not necessarily a relationship between applying an assembly within its marked SCCR
and low arc-flash hazard from an electrical safe work
practices standpoint.
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how to determine short-circuit current ratiings
How to Determine SCCR
Product standards are used to determine the SCCR for
components, such as contactors and motor starters (industrial control equipment); variable-frequency drives
(power conversion equipment); terminal blocks or power
distribution blocks; and switches.
For industrial control equipment, the standard is UL
508; for power conversion equipment, the standard is
UL 508C; for terminal blocks, the standard is UL 1059;
for power distribution blocks, the standard is UL 1953;
and for switches, the standards are UL 508 (motor controllers) or UL 98 (branch circuit disconnects).
The SCCR often is dependent on a specific type of
overcurrent device. For overcurrent devices, the associated rating is called the interrupting rating, or the maximum fault current an overcurrent device can clear safely.
The interrupting rating is determined in accordance
with UL 489 for circuit breakers and UL 248 for fuses.
For assemblies, the SCCR can be determined by the
testing and listing process in accordance with the product standard. For instance, panelboards are tested and
listed in accordance with UL 67; switchboards in accordance with UL 891; and motor control centers in accordance with UL 845.
For industrial control panels, two alternatives are
available to determine an assembly SCCR: testing or
analysis method. UL 508A, Supplement SB identifies
an approved process that can be used to determine the
SCCR for industrial control panels by analyzing the
components in the industrial control panel.
If desired as an alternative, testing of industrial control panels can be performed to determine the SCCR
of industrial control panels. However, this often is cost
prohibitive unless large usage of the same industrial
control panel is expected.
For heating and cooling systems (HVAC equipment),
UL 1995, the UL Standard for Safety for Heating and
Cooling Equipment, recently was updated to require
HVAC equipment to be marked with an SCCR, effective
July 2012. This can be determined by testing or by analysis
in accordance with the requirements added to UL 1995
which are similar to that of UL 508A, Supplement SB.
how to determine short-circuit current ratiings
The overall process is to:
1. Find the lowest rated SCCR of any power circuit
component.
2. Increase the rating of branch circuit components if
possible by using a current-limiting device in the feeder
circuit if present.
3. Compare this value to the lowest interrupting rating of overcurrent protective devices in the industrial
control panel.
The lowest component SCCR (after steps 1 and 2) or
the lowest interrupting rating of overcurrent protective
devices (step 3) results in the overall assembly SCCR.
For power circuit components, the SCCR is determined by either the product markings or the instruction
sheets. If there are no product markings, a default value
can be used as indicated in UL 508A, Supplement SB
(see figure 1).
SCCR can be raised to that value (50 kA, 100 kA or
200 kA) (see figure 3).
The process is similar for current-limiting circuit
Figure 2. When a current limiting device is located in the feeder circuit, it can be
investigated to determine if it can raise branch circuit component ratings.
breakers, with the exception that manufacturer-specific
peak let-through is used
If the transformer is 10 kVA or less for transformers in the feeder circuit, as long as the load-side components and overcurrent protective devices are rated 5
kA or greater, the interrupting rating of the overcurrent
protective device on the line side of the transformer can
be applied to the load-side components and overcurrent
protective devices.
If the transformer is rated 5 kVA or less with a 120 V
maximum secondary, as long as the load-side components
and overcurrent protective device ratings are rated 2 kA
or greater, the interrupting rating of the overcurrent
protective device on the line side of the transformer can
be applied to the load-side components and overcurrent
protective devices (see figure 4).
The final check is verifying the
lowest interrupting rating of the
overcurrent devices in the industrial
control panel. If this is lower than
any power circuit component after
the steps above, the assembly SCCR
is lowered to this value.
Recent Changes to
UL 508A Supplement SB
Figure 1. The SCCR is determined by either the product markings or the instruction
sheets. If there are no product markings, a default value can be used as indicated
in UL 508A, Supplement SB. This chart shows the SCCR of power circuit components
for power distribution blocks.
In addition, the SCCR for motor controllers can be
determined by UL 508. Many manufacturers publish the
SCCR for their motor controllers on the UL website.
It’s possible to raise the ratings of branch power circuit components — those on the load side of the branchcircuit overcurrent device (the overcurrent device closest
to the load). Any component or overcurrent device on
the line side of the branch-circuit overcurrent device is
in the feeder circuit.
When a current-limiting device (fuse or circuit breaker marked current-limiting or a transformer 10 kVA or
less) is located in the feeder circuit, it can be investigated
to determine if it can raise branch circuit component
ratings (see figure 2). However, branch-circuit overcurOverview of UL 508A, Supplement SB
rent device interrupting rating can’t be raised by a curThree main considerations are used to determine the rent-limiting feeder overcurrent protective device.
SCCR of an industrial control panel according to UL
If the peak let-through value for current-limiting fus508A, Supplement SB (analysis method):
es in the feeder circuit (at a given fault current of 50 kA,
A. Power circuit components.
100 kA or 200 kA) as indicated in UL 508A, Table SB
B. Current-limiting devices in the feeder circuit.
4.2 (taken from UL 248) is less than the branch circuit
C. Overcurrent protective devices.
component SCCR, then the branch circuit component
Figure 3. This illustrates a current-limiting fuse in the feeder circuit that can be used to increase a branch circuit component SCCR, if the peak let-through of the current-limiting fuse from Table SB 4.2 in the feeder circuit
at a given fault current level is less than the branch circuit component SCCR. However, the interrupting rating of
the branch-circuit overcurrent protective device can’t be raised.
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Three basic changes have been made
to the latest edition of UL 508A, Supplement SB dated February 4, 2010.
First, the term motor controller in
the default SCCR Table SB 4.1 has
been clarified to indicate that it also
includes combination motor controllers, float and pressure operated
motor controllers, power conversion
equipment and solid-state motor
controllers.
Second, intermediate peak letthrough values at fault currents of
100 kA for Class J and T fuses were
added to Table SB 4.2. This allows
peak let-through values other than
the typical case size of 30 A, 60 A,
100 A, 200 A, etc., to be used at fault
values of 100 kA. Intermediate values at 50 kA or 200 kA aren’t available.
The third change was clarification
of SCCRs vs. interrupting rating in
UL 508A Supplement, SB 4.3.2 and
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how to determine short-circuit current ratiings
Figure 4. Transformers in the feeder circuit can be used to raise the transformer
secondary components and overcurrent protective device ratings.
4.3.3. This revision changed the term short-circuit current rating to interrupting rating when referring to overcurrent protective devices. This helps to clarify that current-limiting fuses or current-limiting circuit breakers
can’t be used to raise the interrupting rating of branchcircuit overcurrent protective devices.
Several more proposed changes to UL 508A Supplement SB are in the preliminary review stage.
Summary
SCCRs are required to be marked on industrial control panels and other equipment in accordance with the
product standards and the NEC. The SCCR for components typically is determined by testing.
The process for determining the SCCR of industrial control panels typically is determined by UL 508A,
Supplement SB, because testing can be cost prohibitive
in low volume applications.
The process for UL 508A, Supplement SB analyzes
the SCCR for power circuit components, determines if
these ratings can be raised by current-limiting devices in
the feeder circuit, and compares the lowest component
rating with the lowest interrupting rating of overcurrent
devices to determine the assembly SCCR.
For Additional Information
For more information on industrial control panels and SCCR, see Section 4
of the Cooper Bussmann “Selecting Protective Devices” handbook. Download it for free by clicking on the “register for free download link” and
submitting the required information at http://bit.ly/SPDhandbook.
Dan Neeser is a field application engineer with Cooper Bussmann.
He has been associated with Cooper Bussmann since 1996 and specializes in training on the design and application of overcurrent protective
devices in electrical distribution systems in accordance with the National Electrical Code ®, and equipment in accordance with the various product standards. He participates in IEEE, NEMA, NFPA, and
IAEI activities. Prior to his position with Cooper Bussmann, he was
a sales engineer for a large electrical manufacturer focusing on construction project sales. He received his B.S.M.E. from the University of
North Dakota, and is a committee member of NFPA 79.
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