what is the sccr of my industrial control panel? march 1st...

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

What is the SCCR of My Industrial Control Panel?March 1st & 2nd, 2017

2

DISCLAIMER

This document is intended to provide general technical information on a particular subject or subjects

and is not an exhaustive treatment of such subjects. Accordingly, the information in this document is

not intended to constitute application, design, software or other professional engineering advice or

services. Before making any decision or taking any action, which might affect your equipment, you

should consult a qualified professional advisor.

ROCKWELL AUTOMATION DOES NOT WARRANT THE COMPLETENESS, TIMELINESS OR ACCURACY

OF ANY OF THE DATA CONTAINED IN THIS DOCUMENT AND MAY MAKE CHANGES THERETO AT ANY

TIME IN ITS SOLE DISCRETION WITHOUT NOTICE. FURTHER, ALL INFORMATION CONVEYED

HEREBY IS PROVIDED TO USERS "AS IS." IN NO EVENT SHALL ROCKWELL BE LIABLE FOR ANY

DAMAGES OF ANY KIND INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS

PROFIT OR DAMAGE, EVEN IF ROCKWELL AUTOMATION HAVE BEEN ADVISED ON THE POSSIBILITY

OF SUCH DAMAGES.

ROCKWELL AUTOMATION DISCLAIMS ALL WARRANTIES WHETHER EXPRESSED OR IMPLIED IN

RESPECT OF THE INFORMATION (INCLUDING SOFTWARE) PROVIDED HEREBY, INCLUDING THE

IMPLIED WARRANTIES OF FITNESS FOR A PARTICULAR PURPOSE, MERCHANTABILITY, AND NON-

INFRINGEMENT. Note that certain jurisdictions do not countenance the exclusion of implied

warranties; thus, this disclaimer may not apply to you.

Excerpts from “UL Standard for Safety for Industrial Control Panels, UL508A” are copyright

Underwriter’s Laboratories Inc.

Excerpts from the “2011 National Electrical Code” are copyright National Fire Protection Association

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

UL 508A & NEC 409 Topics

Introduction

Definitions

UL 508A Supplement SB

RA Website & Database

Welcome

4

This is going to be exciting!

This is not intended as a

“One Man Show”…

Feel free to interrupt me & ask questions and for clarification

OK, not everyone is going to be excited, but this is cool stuff…


Definitions

6

Common Terminology

• Short-Circuit:

– An over-current that exceeds the normal full load current (10x, 100x, 1000x FLC)

• Short-Circuit Current Rating – SCCR:

• prospective symmetrical fault current at a nominal voltage to which an apparatus or

system is able to be connected without sustaining damage exceeding defined

acceptance criteria.

• Current Limiting:

– When a fuse or circuit breaker operates in its current limiting range, it will clear a short in

less than 1/2 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

Interrupt Rating

• • NEC® Article 100 Definition

• • Highest current an OVERCURRENT DEVICE

• (fuse or circuit breaker) is rated to safely interrupt.

• • Self protection rating only

• • NEC® 110.9 Interrupting Rating.

• • Requires the overcurrent device to have an

• interrupting rating not less than the maximum

• available fault current.

• • The maximum fault current must be calculated and

• varies based on system size/location.

7

Short Circuit Current Rating

• • NEC® Article 100 Definition

• • The highest current EQUIPMENT can withstand without extensive

• damage (fire or shock hazard).

• • Also known as component protection

• • May be based on a specific type of overcurrent device

• • NEC® 110.10 Circuit Impedance, Short-Circuit Current

Ratings, and Other Characteristics.

• • Requires the equipment to have a short circuit current rating

• not less than the maximum available fault current.

• • The maximum fault current must be calculated and varies

• based on system size/location.

8

9

NEC Article 409

• Article 409 covering “Industrial Control Panels”

was introduced in the 2005 Edition of National

Electric Code (NEC)

• Why introduce this new article?

– Control panels had been evaluated under several

different NEC articles

– Inconsistencies had led to improper use of control

components

– Article 409 provides a single set of requirements

that can be used by panel builders, installers and

inspectors

– Article 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 where it is installed.


NEC Article 409

• 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

Industrial Control Panel

11

An assembly of two or more components consisting of one of the following:

1. Power circuit components only, such as motor controllers, overload relays, fused disconnect switches, and circuit breakers

2. Control circuit components only, such as push buttons, pilot lights, selector switches, timers, switches, and control relays

409.110 Exception to (4): Short-circuit current rating markings are not required for industrial control panels containing only control circuit components .

1. A combination of power and control circuit components. These components, with associated wiring and terminals, are mounted on, or contained within, an enclosure or mounted on a subpanel.

2. The industrial control panel does not include the controlled equipment.

State by State Adoption of the NEC

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SCCR Marking Requirements

• • 430.8 – Marked on motor controllers

• • 440.4(B) – Marked on HVAC equipment

• • 409.110(3) – Marked on Industrial Control Panels

• • 409.22 – Fault current cannot exceed marked SCCR

• • 670.3(A)(4) – Marked on industrial machinery control panel

• • 670.5 – Fault current cannot exceed marked SCCR

• • UL 508A, Supplement SB is an approved method to determine

SCCR for industrial control panels and industrial machinery

control panels

13

Fault Current Marking Requirement

• 110.24 Available Fault Current.

• • (A) Service equipment must be marked with

• the maximum available fault current and

• date of calculation

• • (B) If fault current increases due to system

• modification, the marking must be updated.

14

15

Short Circuit Faults / Current Limiting

A current limiting fuse will clear a fault within one half of a cycle.

Chuck Norris doesn’t clear faults, faults stay clear of Chuck Norris!


Short Circuit Faults / Current Limiting

Normal Load Current Short Circuit Current

Cu

rren

t (A

)

Available Short Circuit Current (Ipa)

Time

Clearing Time (tc)

Peak Let-Thru (Ipl)

Let-Thru (I2t)

Non-Current Limiting

A current limiting fuse will clear a fault within one half of a cycle.


“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)

Time for some Excitement!

19

Insufficient SCCR.mp4


UL 508A Supplement SB

“How to Comply”

Typical Short-Circuit Current Values

• • Short-Circuit Current at service entrance equipment varies by type/size of system:

• • Small residential systems (100A to 200A) – 10,000A to 15,000A or less

• • Small commercial building systems (400A to 800A) – 20,000A to 30,000A

• • Larger commercial and manufacturing building systems (2,000A to 3,000A) - 50,000A to

65,000A

• • Higher short-circuit currents are possible where low impedance (energy-efficient)

transformers are used (or where larger transformers (kVA) feed multiple services.

• • Commercial buildings directly connected to utility “grid system” – 200,000A or greater

• For Reference Only

21

Why Provide High SCCR?

• • Equipment standards only require the SCCR to be marked

• • NEC requires the marked value to be adequate for the maximum available

fault current

• • Although not required, high SCCR is often preferred

– • Flexibility of application

– • Multiple installations

– • Fault current unknown

– • Insufficient SCCR can delay installation

• Be Prepared!

22

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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: Can be determined using an approved (calculation) method

– Complete analysis according to UL 508A Supplement SB

• This is the recommended procedure for most panels and machines

– Other AHJ Approved Method

Who is the AHJ

Authority Having Jurisdiction (AH.}). An organization, office, or individual responsible for enforcing the

requirements of a code or standard, or for approving equipment, materials, an installation, or a

procedure.

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informational Note: The phrase "authority having jurisdiction,"

or its acronym AHJ, is used in NFPA documents in a

broad manner, since jurisdictions and approval agencies

vary, as do their responsibilities. Where public safety is

primary, the authority having jurisdiction may be a federal,

state, local or other regional department or individual such as

a fire chief; fire marshal; chief of a fire prevention bureau,

labor department, or health department; building official, electrical

inspector; or others having statutory authority. For insurance

purposes, an insurance inspection department, rating bureau,

or other insurance company representative may be the

authority having jurisdiction. In many circumstances, the

property owner or his or her designated agent assumes the role

of the authority having jurisdiction; at government installations,

the commanding officer or departmental official may be

the authority having jurisdiction.


Determining SCCR’s - 3 Basic Steps

• UL 508A Supplement SB

– “Short-Circuit Current Ratings for Industrial Control Panels”

• 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

26

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.

- Exception No. 3: “Enclosure air conditioners or multimotor and combination load equipment that is cord-and-attachment-plug connected or supplied from a branch circuit protected at 60 A or less 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

28

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 for achieving a high Panel

SCCR

Terminal Blocks - UL 1059

Assumed SCCR…

The assumed SCCR of Chuck Norris is…

29

Infinity times two!


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……

• Exception 4: Next slide…….

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

UL508A Supplement BSB4.2.3 Applying high fault ratings

• High fault current ratings can only be used when specified branch circuit

protection device is provided, except…

3

M

140G

Exception No. 4: When the specified branch circuit protection related to the high fault short-

circuit current rating is a non-current limiting overcurrent device, a current-limiting

overcurrent device is able to be used at the same high fault rating where the interrupting

rating of the current-limiting overcurrent device is equal to or greater than the specified

overcurrent device.

65kA conditional or

high fault rating with

breaker

Non current limiting breaker

M

Replace non-current limiting

device with a current limiting

device and claim 65kA

Current limiting fuse

Exception no. 1 allowed substitution of a current limiting fuse provided the i2T peak value was not greater then the fuse listed

Exception no. 3 allowed substitution the current limiting circuit breaker of a motor controller if the current limiting breaker I2T peak

value is not greater

33

Replacements for Current Limiting Fuses?

34

SB4.3 - Feeder Components that Limit SC Current

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…

• Current Limiting Device must be in the feeder circuit• Can only raise downstream BRANCH circuit components (not overcurrent protective

devices e.g., fuses/circuit breakers Interrupt Rating or combination motor controllers

SCCR)

35

Short Circuit Current Rating (SCCR) –Supplement SB4.3.1

The short circuit rating shall be determined by either the formula method in SB4.3.1(a) or by using Tables SB4.3 or SB4.4 Power transformer available short circuit current

SB4.3.1(a) Formula Method

• Method A: Calculation with Formulas

• Single-phase Transformers

• Transformer Full-Load Current (IFL) = (Transformer kVA × 1000) / Voltage*

• Short Circuit Current (ISC line-to-line) = ((Transformer Full Load Current (IFL )) /

Transformer Impedance (Z)

• Three-phase Transformers

• Transformer Full-Load Current (IFL) = (Transformer kVA × 1000) / (Voltage** × 1.732)

• Short Circuit Current (ISC line-to-line-to-line) = ((Transformer Full Load Current (IFL )) /

Transformer Impedance (Z)36

37

Let Through Current

What is the let through current of Chuck Norris?

Zero, nothing gets through Chuck Norris!


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 Control

b) Control Transformers

c) 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 breakerwhen 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

65kA SCC

Available

Branch Circuit PD

65kA SCCR

Motor Controller

18kA SCCR

17kA Ip Let-

through

a) 65kA Panel SCCR


Current Limitation – UL Listed Circuit Breakers

“Current Limiting Circuit Breakers”

65kA Available

17kA Let Through


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

65kA SCC

Available

Branch Circuit PD

65kA SCCR

10kA Ip Let-

through

b) 65kA Panel SCCR

Motor Controller

10kA SCCR


Current Limitation – UL Listed Fuses

“Current Limiting Fuses”

65kA Available(Use 100kA column)

10kA Let Through

UL508A Second Edition Supplement BAddition of fuses added to Table SB4.2

• Test values for fuses UL 248-2 were added

• Apply current limiting rules to raise SSCR ratings

4

M M M

Current limiting device

in the feeder (100kA)

5kA or less peak let thru current

5kA branch

circuits

5kA panel rating increased to 100kA by applying current limiting rules

See Page 92 of the BussmannShort-Circuit Current Rating

(SCCR) Document

UL508A – Table SB4.2 updated for J fuse types

4

20 amp fuse or smaller have let thru values 5kA or less

Apply current limiting rules to raise SSCR ratings

M M M

20 amp Class J

5kA branch

(no High fault rating)

5kA

peak let

thru


Current Limiting Devices – Fuses or CB’s 25kA Available

SCCR: 50kA

10kA Motor Controller

50kA Panel Rating

• Current Limiting Fuse in

a “Feeder” Circuit:

– 60A Class J Fuse limits Ip

to 8kA, to 50kA

threshold

– 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


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

10kA

65kA 70kA

10kA Panel Rating

100kA


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

200kA

65kA 70kA

65kA Panel Rating

100kA


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.

65kA

200kA

65kA 70kA

200kA

1

1 1

2

2

2

100kA

65kA Panel Rating


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

unless an exception applies

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

53

The Global SCCR Tool…

..\Rockwell-Automation-Resource-Guide-v1.0-KE.pdf

what is the sccr of my industrial control panel? march 1st...

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