Equipment Short-Circuit Current Rating and...

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© 2013 Eaton. All rights reserved.

Equipment Short-Circuit Current Rating and Available Fault Current

Dan Neeser – Field Application Engineer

Eaton’s Bussmann Business

[email protected]

2© 2013 Eaton. All rights reserved.

Agenda

• Interrupting Rating & Series Rating

• Short-Circuit Current Ratings

• Maximum Fault Current Marking

• Fault Current Calculations

• UL 508A – Industrial Control Panels

Supplement SB - SCCR

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Interrupting 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.

• Similar Requirements in OSHA 1910.303(b)(4)


Interrupting Rating

• NEC 110.9: Device interrupting rating MUST be equal to or greater than the maximum available fault current.

• Chapter 1 Video Clips - Interrupting Rating

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1st

Determine available short-circuit current (Isca) at lineside

terminals of each OCPD.

2nd

Apply OCPDs with adequate Interrupting Rating.

Isca 3

MM

MSB

Isca 1

Isca 4 Isca 5

Isca 6

Isca 2

Interrupting Rating - Proper Application


Must select circuit breaker with interrupting rating adequate for point of application – varies by voltage and circuit breaker type.

Interrupting Rating - Proper Application

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High Interrupting rating of current-limiting fuses at full voltage reduces concerns about proper interrupting rating at point of application

I.R. - Proper Application


A combination of: - Circuit breakers / circuit breakers

OR- Fuses / circuit breakers …

that can be applied at available fault levels above the interrupting rating of the load side circuit breaker, but not above that of the main

or line side device.

Downstream device will ALWAYS be a circuit breaker

Series Rated Systems

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Series Rated

Combination

65,000 A.I.R.200A Circuit Breaker

65,000 A.I.R.

ISC=65,000 A

20A Circuit Breaker

ISC=65,000 A

10,000 A.I.R.

Series Rated Systems: CB-CB


Series Rated

Combination

200,000 A.I.R.

ISC=200,000 A

20A Circuit Breaker

ISC=300,000 A

10,000 A.I.R.

Series Rated Systems: Fuse-CB

LPJ 400 SP

300,000 A.I.R.

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• NEC® 240.86(A) Selected Under Engineering Supervision in Existing Installations• The series rated combination devices must be selected

by licensed, professional engineer. • Requires Documentation and Stamp. • Series combination rating and upstream device must be

field marked on end use equipment. • Downstream circuit breaker must remain passive.

• NEC® 240.86(B) Tested Combinations• The combination of line side and load side devices must

be tested and marked on the end-use equipment (panelboards & switchboards).



• Additional Limitations and Requirements

• Labeling Requirements

• Manufacturer – NEC® 240.86(B)

• Field Marking – NEC® 110.22(B)&(C)

• Motor Contribution Limitations

• NEC 240.86(C)

• Motor FLA cannot exceed 1% of IR of protected CB.

• Lack of Selective Coordination

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Current Limitation

• A current limiting fuse will clear a fault within one half of a cycle.• Thermal energy is

proportionate to the square of “RMS” current multiplied by the time (Irms2t)

• Mechanical stresses are proportionate to the square of “peak” current multiplied by the time (Ip2t)


Current Limitation

• Conductor protection for fault of 45kA/480V (26kA with conductor) with non-current-limiting device (1 cycle) vs. current-limiting device (>1/2 cycle)

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Short-Circuit Current Ratings

• 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.

• Similar Requirements in OSHA 1910.303(b)(5)


Motor Starter - SCCR

• High Fault Test: starter protected by an instantaneous trip circuit breaker (MCP) that only provides Type 1 protection

• High Fault Test: starter protected by Low-Peak® fuses that provides Type 2 (No Damage) protection

Fault

480V - 22,000A

Fault

480V - 22,000A

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• 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

SCCR Marking Requirements


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.

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How to Comply?

Service Equipment

Isc = 60,142 A

Isc = 27,532 AIsc = 42,153 A

Max Avail. Fault Current = 58,524 A

Date Determined/Calculated: 9/2010

Isc = 18,752 AIsc = 38,525 A

Required per NEC 110.24

Motor ControllerSCCR = 25kA

Industrial Control PanelSCCR = 30kA

Industrial Machinery PanelSCCR = 65kA

HVACSCCR = 40kA

All equipment must comply with: NEC 110.9 (IR) &110.10 (SCCR)

Engineer – Calculate

Contractor – Label


IR/SCCR Inspection Check-List

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Fault Current Calculations

• To determine the fault current:• Draw the one-line diagram

• Identify sources of short-circuit current (utility, generation, motors)

• Include system component information (tranformers, conductors, busway, overcurrent devices are not considered)

• Use calculation method (ohmic, per unit, point-to-point, software)

• Use “typical” fault current values as a reference only.


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

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Short-Circuit Current Factors

• Typically highest at service point

• Decreases in downstream equipment (due to impedance of transformers and conductors)


Calculation Example

• 480V/3000A Service supplied from 2500 kVA Transformer

• Find Isc at Transformer Secondary & Service Equipment

480V, 3000A Service EquipmentService Transformer

2500 kVA

13.2kV – 480V

5% Impedance

7 – 600 Cu kcmil/phase

25 Feet in PVC

Isc = 60,142 A

Isc = 58,524A

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IFLA =KVA X 1000

EL-L X 1.732

KVA = KVA of Transformer

EL-L = Secondary Phase to Phase voltage

Calculation Example

• Step One: Calculate Secondary FLA of Transformer (3 phase)


2500 KVA Transformer

13.2kV - 480V, 3 Phase

Z = 5%

ISC

IFLA =2500 X 1000

480 X 1.732

IFLA = 3007 A

Calculation Example

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Mult =100

% Z

Mult =100

5

Mult = 20

Calculation Example

• Step Two: Calculate Multiplier• Z = Impedance of Transformer, refer to transformer

nameplate or manufacturer data


ISC = IFLA X Mult

ISC = 3007 X 20

ISC = 60,140 A

Calculation Example

• Step Three: Calculate Secondary Short Circuit Current

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480V

Switchgear


25 Feet in PVCISC = 60,140A

Calculation Example

• Calculate Short Circuit Current at 480V Switchgear


ISC 480V SWGR = ISC X Mult

Calculation Example

• Isc known from previous calculation• Calculate multiplier for Cable

• Step One: Calculate f value

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f =1.732 X L X ISC

C X EL-L

L = Length of conductor

ISC = Short-circuit current at beginning of circuit

C = “C” Value Constant for Cable

From Table. Multiply by # of runs

EL-L = Phase to Phase Voltage

Calculation Example

• Step One: Calculate f value


Calculation Example

• C Value – 600 kcmil, CU, PVC = 28,033

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f =1.732 X 25 X 60,140

7 X 28,033 X 480= 0.0276

Calculation Example

• Step One: Calculate f Value• L = 25 Feet

• ISC = 60,140 A

• C = 7 X 28,033 - See “C” Value Table

• EL-L = 480V


Mult =1

1 + f

• Step Two: Use Calculated f Value• Calculate Multiplier

Calculation Example

Mult = = 0.97311

1 + 0.0276

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ISC 480V SWGR = ISC X Mult

ISC 480V SWGR = 60,140 X .9731

ISC 480V SWGR = 58,522 A*

* Add motor contribution if present

Calculation Example

• Calculate Short Circuit Current at 480V Switchgear


What if the System Changes?


3000 kVA

13.2kV – 480V

5% Impedance


25 Feet in PVC

Isc = 72,171 A

Isc = 70,587A


2500 kVA

13.2kV – 480V

4% Impedance


25 Feet in PVC

Isc = 75,178 A

Isc = 72,667A

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Cooper Bussmann Short-Circuit Calculator

• New version• Apple or Android Apps

• Web (run from homepage)

• Old version• Download for Window XP


• Choose three phase or single phase system

Calculating available fault current and creating 110.24 labels has never been easier!

1 2 3

• Click on Add to my

System

• Select a component

you want to add

Create a System

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• Select Add Transformer


1 2 3

• If you have a known primary fault

current you can add it here. If not,

select assume infinite

• Enter values into the appropriate fields

Add a Transformer


• After adding a Transformer,

you will be asked if you want

to add motor contribution


1 2 3

• You can complete a quick calculation

by selecting a % of your

transformer’s FLA or add it manually

• Add these calculations to your

system

Add Motor Contribution

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• Select Add To My System


1 2 3

• Add Conductor Run or Buss Run

• Add the correct values and Add To System

Add Other Components


• View system one‐line

diagram


1 2 3

• Email one‐line

diagram

• Select a specific fault to create a label

System Summary

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• Select a fault and click on Create Label


1 2

• Enter the email address, project name,

fault name, and label size, then Send Label

Create a Label


• Tap on Fuse Sizing Diagram

Sizing fuses for mains, feeders, and branch circuits has never been easier!

1 2 3

• Tap on the circuit to protect

• Review each section and the fuse suggestions for the selected location

Fuse Sizing

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• The User guide provides useful

information about FC2

• There is a user guide selection at the bottom

if each page

• Based on where you are in the app, you will

be provided a specific

overview of FC2’s

functionality

• While the FC2 is very

intuitive, there may be

additional questions

• There is a Contact Us selection at the

bottom of each page

• From this location,

you can select to send

an email for technical

assistance or

customer service

support

User Guide & Contact Us


Cooper Bussmann FC2 Web Version - Example

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How to Determine SCCR?

• Short-Circuit Current Rating (SCCR)• Can be established by testing during the listing

and labeling process

OR

• Can be determined using an approved (calculation) method

• UL 508A Supplement SB is an approved method (industrial control panels and industrial machinery control panel)

• AHJ Approved Method

• NRTL field evaluation can also be used.


Industrial Control Panel SCCR

• Industrial Control Panel. 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;

3)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.

The industrial control panel does not include the controlled equipment.

48

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UL 508A Supplement SB

• What Needs to be Analyzed per UL 508A, Supplement SB?• All power circuit components (SB 4.1)

• Feeder and branch circuit components that supply power to loads (motors, lighting, heating and appliances)

• Includes disconnect switches, fuses, circuit breakers, load controllers, overload relays, power distribution/terminal blocks, bus bars, etc.

• Control circuit components are not required to be analyzed

• Pushbuttons, pilot lights, selector switches, timers, control relays, etc.


Power vs. Control Circuits

Control circuit components don’t have to be analyzed

Power Transformer

Control Transformer

*

*Control Circuit, but affects SCCR

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• How to Determine Component SCCR (SB4.2.2)?• The SCCR marked on the component or on instructions.• The SCCR determined by the voltage rating of the

component and the assumed short circuit current from Table SB4.1.

• or• The SCCR for a load controller, motor overload relay, or

combination motor controller that has been investigated 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.



• What are the Rules (Sweep 1)?

• Lowest component SCCR limits assembly SCCR unless:

• Combination ratings can be used to increase branch circuit component ratings (SB 4.2.2)

• Component with marked/specified OCPD

• Check with component manufacturer for combination ratings

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Combination Ratings - Branch

• Use combination ratings with specified/marked OCPD to fix low rated branch components

Component SCCR OCPD I.R.

Combination ratings of overcurrent protective devices and components can be used

Nameplate

480

SCCR:

Voltage:

100kA


Combination Ratings - Contactor

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Combination Ratings - MMP

• OL Protection Only

• Fuse or Circuit Breaker Required for SC Protection

• Suitable for Group Installations

• Straight Ratings (480V)


Combination Ratings – Type E/F

• Combination Motor Controller

• SC & OL Protection

• Line Side Adapters Required

• Slash Ratings (480/277V)

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Combination Ratings - ASD

• 200kA with Class CC/J/T Fuse

• Suitable for Group Installations

• 65kA with Type E CMC (480/277V)


Combination Ratings - Feeder

• Use UL Listed power distribution block with high SCCR when used with specified fuses or circuit breakers and required load side conductors

• Must have feeder circuit spacing (Listed PDB) if in feeder circuit

Nameplate

480

SCCR:

Voltage:

100kA


Combination ratings of PDBs is cost effective fix

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59© 2013 Eaton. All rights reserved.59

Combination Ratings - PDBs

PDBFS220 (4 – 14 Load Side Conductors) – 100kA with 175A Class J fuse or less



• What are the Rules (Sweep 1)?

• Lowest component SCCR limits assembly

SCCR unless:

• Feeder components are used that limit the short-

circuit current reducing the need for higher branch

circuit component SCCR (SB 4.3)

• Current limiting overcurrent protective devices

• Transformers rated 10kVA or less

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Feeder Circuit

Branch Circuit

Branch Circuit

Feeder vs. Branch Circuits


Use of CL Devices - OCPD

• CL Device must be in the feeder circuit• For CL Fuses or Circuit Breakers

• Use PEAK Let-through, Not RMS Let-Through (more conservative)

• Fuses - cannot use manufacturer specific fuse data –only tables based on performance requirements from UL 248 (more conservative - apply to all manufacturers)

• Circuit Breakers - must be Listed and marked “current-limiting” and use published let-thru curves (most circuit breakers are not CL)

• Can only raise downstream BRANCH circuit components (not overcurrent protective devices –fuses/circuit breaker IR or Combination motor controllers SCCR)

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Fuse LT – UL vs Manufacturer

• LPJ-100SP

• UL Limits• 50kA = 12kA• 100kA = 14kA• 200kA = 20kA

• Manufacturer Let Through Curves• Ip LT:

• 50kA = 9kA• 100kA = 12kA• 200kA = 14kA

• Irms LT:• 50kA = 4kA• 100kA = 5kA• 200kA = 6kA


• Current limiting feeder OCPD can be used to increase branchcomponent SCCR.


Use of CL OCPD can only fix components

F

B

Use of CL Devices - OCPD

Nameplate

480

SCCR:

Voltage:

14kA

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Use of CL Devices - Transformers

• CL Device must be in the feeder circuit

• For Transformers• If 10kVA with secondary devices (components and

overcurrent devices) of 5kA or higher, apply the IR of the primary overcurrent protective device

• If 5kVA (120V Secondary) with secondary devices (Components and overcurrent devices) of 2kA or higher, apply the IR of the primary overcurrent protective device



10 kVA

Use of CL Devices - Transformers

• Transformers can be used to limit the short-circuit current available• If 10kVA & sec comp = 5kA+• If 5kVA/120V & sec comp = 2kA+• Then assign entire circuit I.R. of

primary overcurrent device

Use of Small Transformers (10kVA or less) can increase secondary component/OCPD ratings

Nameplate

480

SCCR:

Voltage:

200kA

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• What are the Rules (Sweep 2)?• Lowest overcurrent protective device interrupting

rating (or SCCR for some devices) always limits assembly SCCR

• All Feeder and branch circuit overcurrent devices

• Tested series combination ratings or series ratings (fuse-circuit breaker or circuit breaker-circuit breaker) are NOT allowed.

• Branch circuit overcurrent devices tapped from the feeder circuit supplying a control circuit.

• Supplemental protective devices protecting the control circuit transformer in a motor branch circuit.


Overcurrent Protection Devices

• Branch Circuit Protective Devices• Fuses

• Circuit Breakers

• Application Limited Devices

• Supplemental Protectors/Fuses• Not suitable for branch circuit

protection (protecting a load)

• Can only be used in control circuits

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Why High SCCR?

• NEC & 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


Fix It - Solutions

• High I.R. Protective Devices• Class R

• Class J

• Class J Drive Fuse

• Class T

• Class CC

• CubeFuse

• High Speed

• High IR/CL CBs

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