Understanding
NEC Article 409 &
UL508A Short-Circuit
Current Ratings
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.
1
UL 508A & NEC 409 Topics
Introduction
Definitions
UL 508A Supplement SB
RA Component Solutions
RA Website & Database
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NEC 409 Code Change
•
•
2005 Edition of National Electric Code (NEC)
contains a new Article 409 covering “Industrial
Control Panels”
Why introduce this new article?
– Control panels have been evaluated under several
different NEC articles
– Inconsistencies have led to improper use of control
components
– New article provides a single set of requirements
that can be used by panel builders, installers and
inspectors
– NEC 409 provides for design and evaluation
process to insure safe installation and application of
control panels
• Panel SCCR must cover the actual available
fault level.
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NEC 409 Code Change
Panel SCCR
50kA RMS Symmetrical
480V Maximum
•
•
New Article 409 covers “Industrial Control
Panels”
“Industrial Control Panel” Definition
– Industrial Control Panels, intended for general
use, ordinary locations, 600V or less.
• Clause 409-110 (3) “Markings”
Industrial Control Panels must be
evaluated & marked for their
Short-Circuit Current Rating (SCCR)
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Definitions
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Common Terminology
•
Short-Circuit:
– An over-current that exceeds the normal full load current (10x, 100x, 1000x FLC)
•
Short-Circuit Current Rating – SCCR:
– The maximum short-circuit current an electrical component can sustain without the
occurrence of excessive damage when protected with an over-current protective device
•
Interrupting Rating (Breaking Capacity):
– The maximum short circuit current at rated voltage that a over-current protective device
can safely interrupt and clear.
•
Current Limiting:
– When a fuse or circuit breaker operates in its current limiting range, it will clear a short in
less that ½ cycle, typically ¼ cycle
– Performance standards such as UL specify let-thru characteristics (Ip & I2t) at rated fault
levels and voltages
•
Power Circuit:
– Conductors and components of branch and feeder circuits.
•
Control Circuit:
– A circuit that carries the electric signals directing the performance of a controller, and
which does not carry the main power circuit. A control circuit is, in most cases, limited to
15 amperes
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“Feeder Circuit” Definition
•
“Feeder Circuit” covers all
power conductors and
components from incoming
feeder disconnect, …. to the
line side of the last branch
SCPD.
“Feeder Circuit”
“Branch” SCPD
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“Branch Circuit” Definition
•
“Branch Circuit” covers all
power conductors and
components from the loadside of the branch circuit, ….
to the controller load-side
connections.
“Branch Circuit”
“Branch” CPD
•
Terms used for protection
devices:
– Over-current Protection
Device (OPD)
– Branch Circuit Protection
Device (BCPD)
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UL 508A Supplement SB
“How to Comply”
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How to determine SCCR of a panel?
• Three potential options:
• Option 1: Test each panel design and document in a follow-up procedure
– Costly, time consuming, too many variations
• Option 2: Purchase previously tested constructions
– Practical for repeatable machine/panel design and production
• Option 3: Complete analysis according to UL 508A Supplement SB
– Recommended procedure for most panels and machines
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Determining SCCR’s - 3 Basic Steps
•
UL 508A Supplement SB
–
–
•
“Short-Circuit Current Ratings for Industrial Control Panels”
Goes into effect in “April 2006”
SB4.1: Short-Circuit Current Rating (SCCR)
–
a)
b)
c)
Shall be determined by the following evaluation:
First, establish the short circuit current ratings of individual power circuit
components as specified in SB4.2
Second, modify (limit) the available short-circuit current within a portion of a
circuit in the panel due to the presence of current limiting components as
specified in SB4.3
Third, determine the overall panel short-circuit current rating as specified in
SB4.4
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SB4.2.1 – Individual Power Circuit Components
SB4.2.1 All power circuit components, including
disconnect switches, branch circuit protective
devices, branch circuit fuse holders, load
controllers, motor overload relays, terminal
blocks, and bus bars, shall have a shortcircuit current rating expressed in amperes
or kilo-amperes and voltage.
–
–
Exception No. 1: Power transformers,
reactors, current transformers, dry-type
capacitors, resistors, varistors, and voltmeters
are not required to have a short circuit current
rating.
Exception No. 2: The “S” contactor of a wyedelta motor controller is not required to have a
short-circuit current rating.
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SB4.2.2 – Establish SCCR of Components
SB4.2.2 The short-circuit current rating of a component shall be established by one of the
following methods:
a) The short-circuit current rating marked on the component or on instructions provided
with the component;
b) The short-circuit current rating determined by the voltage rating of the component and the
assumed (assigned) short-circuit current from Table SB4.1; or
c) The short-circuit current rating….
100-C Contactor Nameplate & Instruction Sheet
PowerFlex 70 Drive User Manual
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Table SB4.1 - Unmarked Components
• 508A SB - Table SB4.1
– “Assumed” SCCR’s
– Line up with UL minimum test SC
test requirements to attain listing
– These minimum ratings limit “high
fault” panel ratings
– High Fault SCCR’s on all
components & combinations are top
priority
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SB4.2.2 – Establish SCCR of Components
SB4.2.2 The short-circuit current rating of a component
shall be established by one of the following
methods:
a) The short-circuit current rating marked on the
component or on instructions provided with
the component;
b) The short-circuit current rating determined by the
voltage rating of the component and the assumed
(assigned) short-circuit current from Table
SB4.1; or
c) The short-circuit current rating for a load controller,
motor overload relay, or combination motor
controller that has been investigated (tested) in
accordance with the performance requirements,
including short-circuit test requirements for
standard fault currents or high fault currents
specified in the Standard for Industrial Control
Equipment, UL 508, and described in the
manufacturer’s procedure.
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SB4.2.3 – High Fault SCCR of Components
SB4.2.3 A high fault short-circuit current rating for a motor controller,
an overload relay, or a combination motor controller, as specified in
SB4.2.2 (a) or (c), shall only be used as the short-circuit current rating of
the component when the specified branch circuit protective device is
provided.
• Exception 1:
– When the specified BCPD related to the high fault SCCR is a Class
CC, G, J, L, RK1, RK5 or T fuse, a fuse of different class is able
to be used at the same high fault rating where the Ip let-through
current and I2t of the new fuse is not greater than that of the
specified fuse. See Table SB4.2 for maximum let-through currents
(Ip) and I2t
• Exception 2:
– The specified BCPD is able to be provided in the field when the
panel is marked in accordance to SB5.1.2.
• Exception 3:
– When the specified BCPD related to the high fault SCCR is a listed
circuit breaker marked “current limiting”, a different current-limiting
circuit breakers is able to be used at the same high fault
rating……
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SCCR: 100kA, 600V with
UL Class J or CC Fuses
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SB4.3 - Feeder Components that Limit SCC
SB4.3 – Feeder components that limit the short-circuit current available
•
SB4.3.1 – For branch circuits supplied by a power transformer with an isolated
secondary winding, the a power transformer with an isolated secondary…
•
SB4.3.2 – For branch circuits supplied by a Listed circuit breaker marked
“current limiting” in the feeder circuit, the short circuit current rating…
•
SB4.3.3 – For branch circuits supplied by a Class CC, G, J, L, RK1, RK5, or T fuse
in the feeder circuit, the short-circuit current rating…
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Control Transformer Circuits
Control Circuit: “A circuit that carries the electric signals
directing the performance of a controller, and which does
not carry the main power circuit. A control circuit, in
most cases is limited to 15A.”
Control Transformer: “A transformer whose secondary
supplies power to control circuit devices only
(excluding loads).
a) Typically limited to 1kVA with a secondary current less
than 15A
b) The OPD provides over-current and short circuit
protection for the control transformer branch circuit
c) Secondary components are not evaluated for SCCR
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.
OPD
SCCR
1 kVA, 15A
Max.
SCC Available:
Limited
Secondary components
are not required to have
a specific SCCR and do
not affect the
SCCR of OPD.
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SB3.2 Over-current Protection of Control Circuit
SB3.2.1 For control circuits tapped from the feeder
circuit, the over-current protection for the
common control or for the primary of a control
transformer or power supply shall be provided
with branch circuit protective device having a
short-circuit current rating not less than the
overall panel short-circuit current rating.
•
Control circuit taps off the feeder circuit must be
protected:
a)
b)
c)
•
> 65kA
Common Control
Control Transformers
Power Supplies
Control Circuit BCPD SCCR must cover the panel
SCCR!
65kA Panel SCCR
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SB4.3.2 – Current-Limiting w/ Circuit Breakers
SB4.3.2 For branch circuits supplied by a Listed circuit
breaker marked “current-limiting” in the feeder circuit, the
short-circuit current rating on the line side of the circuit
breaker shall be one of the following:
a) The short-circuit current rating of the feeder circuit breaker
when all of the components in the branch circuit have a shortcircuit current rating not less than the published peak letthrough current of the circuit breaker, see Figure SB4.1, and
the short-circuit current rating of all branch circuit
protective devices on the load side are not less than the
short-circuit current rating of the feeder circuit breaker;
b) The smallest short-circuit current rating of any branch circuit
protective device on the load side of the feeder circuit
breaker, when the conditions of SB4.3.2(a) exist except the
short-circuit current rating of the branch circuit protective
devices on the load side are less than the short-circuit current
rating of the feeder circuit breaker.
c) The smallest short-circuit current rating of any branch circuit
on the load side of the feeder circuit breaker, when the
conditions of SB4.3.2(a) or SB4.3.2(b) are not met.
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40kA
SCC
Available
Feeder CB
“Current Limiting”
65kA SCCR
25kA Ip
Letthrough
Branch Circuit PD
MCCB = 65kA SCCR
Branch Controller
30kA SCCR
a) 65kA Panel SCCR
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SB4.3.3 - Current Limiting w/ Fuses
SB4.3.3 For branch circuits supplied by a Class CC, G, J, L,
RK1, RK5, or T fuse in the feeder circuit, the short-circuit
current rating on the line side of the fuse shall be one of the
following:
a) The short-circuit current rating of the feeder fuse when all of
the components in the branch circuit have a short circuit
current rating not less than the peak let-through current
corresponding to the specific fuse class employed from Table
SB4.2 based on the current rating of the fuse and the
available short-circuit current on the line side of the fuse, and
the short-circuit current rating of all branch circuit
protective devices on the load side are not less than the
short-circuit current rating of the feeder fuse;
b) The smallest short-circuit current rating of any branch circuit
protective device on the load side of the feeder fuse, when
the conditions of SB4.3.3(a) exist except the short-circuit
current rating of the branch circuit protective devices on the
load side are less than the short circuit current rating of the
feeder fuse.
c) The smallest short-circuit current rating of any branch circuit
on the load side of the feeder fuse, when the conditions of
SB4.3.3(a) or SB4.3.3(b) are not met.
40kA
SCC
Available
Feeder Fuse Switch
200kA SCCR
Copyright © 2006 Rockwell Automation, Inc. All rights reserved.
60A Class J
8kA Ip
Letthrough
Branch Circuit PD
65kA SCCR
Branch Controller
14kA SCCR
b) 65kA Panel SCCR
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Current Limitation – UL Listed Fuses
“Current Limiting Fuses”
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Current Limiting Devices – Fuses or CB’s
• Current Limiting Fuse
in a “Feeder” Circuit:
25kA
Available
SCCR: 50kA
– 60A Class J Fuse limits
Ip to 8kA,
– Default SCCR of a
power block is 10kA
• Feeder “low” let-thru
affect on the Branch
– Motor Controller SCCR
is 10kA
Note: All SCPD’s
must cover the
available fault level
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SB4.4 Overall SCCR of the Panel
SB4.4.1 For each branch circuit provided with
branch circuit protection within the industrial
control panel, the smallest short-circuit
current rating of all power circuit
components on the load side of a branch
circuit protective device and the control
circuit over-current protection in SB3.2.1
shall be determined and compared with the
short-circuit current rating of the branch
circuit protective device. The smaller of the
two ratings shall be assigned to the line
side of the branch circuit protective
device.
18kA
65kA
70kA
65kA
In other words, use the lowest rating of a
component or branch protective device to
determine branch rating
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Short Circuit Panel Ratings
SB4.4.4 The overall short circuit current rating of the panel
shall be one of the following:
a)
For an industrial control panel consisting of a
single branch circuit without branch circuit
protection….
b)
For an industrial control panel consisting of a
single branch circuit including branch circuit
protection….
c) For an industrial control panel consisting of multiple
branch circuits, and feeder components within
the panel, such as disconnecting switches, bus
bars, terminal blocks, and feeder over current
protective devices, the short-circuit current rating
shall be the lowest of the following:
1) The lowest short circuit current rating of any
branch circuit in accordance with SB4.4.1 that
has not been modified by SB4.3.1 – SB4.3.3;
2) The short circuit current rating of any feeder
component not covered by SB4.4.4(c)(3) and
any control circuit over current protection
connected to the feeder as in SB3.2.1; or
3) The modified short-circuit current rating
determined from SB4.3.1 – SB4.3.3 for each
branch circuit supplied by the associated feeder
component.
18kA
2
200kA
2
50kA
3
70kA
65kA
65kA
1
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SB5 Markings
• SB5.1 General
– SB5.1.1 The nameplate rating of an industrial control panel shall include:
“Short circuit current: ___kA rms symmetrical, ___V maximum” or the equivalent.
– SB5.1.2 An industrial control panel marked with a high fault short circuit current rating
and is not provided with the required branch circuit protective device as specified in the
Exception to SB4.2.3 shall be marked with the type and size of branch circuit protection
required to be installed in the field. This marking shall be included as part of the
marking in SB5.1.1.
• SB5.2 Cautionary markings
– SB5.2.1 An industrial control panel with a short circuit current rating based on the high
fault short circuit current ratings of one or more components as specified in
SB4.2.3 shall be marked with the word “WARNING” and the following statement: “Risk
of Fire or Electric Shock – The opening of the branch-circuit protective device
may be an indication that a fault current has been interrupted. All currentcarrying parts and other components protected by this device should be
examined and replaced if damaged. If burnout of a current element of an overload
relay occurs, the complete overload relay must be replaced.”
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Panel SCCR – 4 Checks to Validate the Results
1. Panel SCCR will be determined by the smallest
SCCR of any feeder or branch over-current
protective device or component
2. Branch circuit protection devices must cover
panel available fault level
3. All components and controllers with SCCR’s
based on high-fault ratings must be used with
the specified branch circuit protective device
4. And finally, the panel SCCR must cover the
available fault current supplying the panel!
Available Fault:
40kA @ 480V
Panel SCCR
65kA rms Symmetrical
480V Maximum
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Need Additional Assistance with Your Application??
If there are any questions or issues that were not addressed during this
presentation, additional assistance can be attained via RA Technical
Services Team. They can be contacted by the following means:
1. Contact by e-mail at raictechsupport@ra.rockwell.com
2. Contact by calling Rockwell Automation Technical Support Line at 1-440646-5800, select Allen-Bradley Brand, then select Industrial Control.
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