copyright © 2006 rockwell automation, inc. all rights reserved. 1 understanding nec article 409...

Copyright © 2006 Rockwell Automation, Inc. All rights reserved. 1

UnderstandingNEC Article 409 & UL508A Short-Circuit Current Ratings

3 Copyright © 2006 Rockwell Automation, Inc. All rights reserved.

UL 508A & NEC 409 Topics

Introduction

Definitions

UL 508A Supplement SB

RA Component Solutions

RA Website & Database


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.


NEC 409 Code Change

• 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)

Panel SCCR

50kA RMS Symmetrical

480V Maximum


Definitions


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


“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


“Branch Circuit” Definition

• “Branch Circuit” covers all power conductors and components from the load-side 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)


UL 508A Supplement SB

“How to Comply”


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


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)– Shall be determined by the following evaluation:a) First, establish the short circuit current ratings of individual power circuit

components as specified in SB4.2

b) 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

c) Third, determine the overall panel short-circuit current rating as specified in SB4.4


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 short-circuit 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 wye-delta motor controller is not required to have a short-circuit current rating.


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….

PowerFlex 70 Drive User Manual 100-C Contactor Nameplate & Instruction Sheet


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


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.


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……

SCCR: 100kA, 600V with UL Class J or CC

Fuses


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…


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

Secondary components are not required to have a

specific SCCR and do not affect the

SCCR of OPD.

1 kVA, 15A Max.

SCC Available: Limited

OPD SCCR


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)Common Controlb)Control Transformersc)Power Supplies

• Control Circuit BCPD SCCR must cover the panel SCCR!

> 65kA

65kA Panel SCCR


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 short-circuit current rating not less than the published peak let-through 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.

Feeder CB

“Current Limiting”

65kA SCCR

40kA SCC

Available

Branch Circuit PD

MCCB = 65kA SCCR

Branch Controller

30kA SCCR

25kA Ip Let-

through

a) 65kA Panel SCCR


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.

Feeder Fuse Switch

200kA SCCR

60A Class J

40kA SCC

Available

Branch Circuit PD

65kA SCCR

8kA Ip Let-

through

b) 65kA Panel SCCR

Branch Controller

14kA SCCR


Current Limitation – UL Listed Fuses

“Current Limiting Fuses”


Current Limiting Devices – Fuses or CB’s

• Current Limiting Fuse in a “Feeder” Circuit:– 60A Class J Fuse limits

Ip to 8kA, – Default SCCR of a

power block is 10kA

25kA Available

• Feeder “low” let-thru affect on the Branch– Motor Controller SCCR

is 10kA

Note: All SCPD’s must cover the

available fault level

SCCR: 50kA


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.

In other words, use the lowest rating of a component or branch protective device to determine branch rating

65kA

18kA

65kA 70kA


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; or3) The modified short-circuit current rating determined from SB4.3.1 – SB4.3.3 for each branch circuit supplied by the associated feeder component.

65kA

18kA

65kA 70kA

200kA

1

1 1

2

2

3

50kA


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 current-carrying 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.”


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


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 [email protected]

2. Contact by calling Rockwell Automation Technical Support Line at 1-440-646-5800, select Allen-Bradley Brand, then select Industrial Control.

copyright © 2006 rockwell automation, inc. all rights reserved. 1 understanding nec article 409...

Documents

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definitions slide

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protection association

current rating sccr