NEC® Article 409 and UL 508A Short-Circuit Current Ratings (SCCR) 1 Using Fuses to Increase Your SCCR Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. What is a SCCR? • A Short Circuit Current Rating is the maximum current a device or panel can safely withstand for 3 AC electrical cycles (50 msec.) or while an overcurrent protective device operates. • SCCR is usually expressed in RMS kA at the system voltage. Example: 18 kA at 480 VAC • RMS is the Root Mean Squared AC current. • The SCCR of a fuse or circuit breaker is the same as its Interrupting Rating (IR). Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 2 NEC® Article 409 - Industrial Control Panels • Article 409 was added to the NEC® in 2005 • Most states and Authorities Having Jurisdiction (AHJs) have adopted the 2005 NEC. • Article 409 requires all industrial control panels to be marked with an SCCR. National Electrical Code and NEC are registered trademarks of the National Fire Protection Association (NFPA), Quincy, MA. Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 3 NEC SCCR Requirements • The 2005 NEC requires SCCR markings on all: Industrial Control Panels - NEC Article 409 Motor controllers - NEC Article 430.8 HVAC controllers - NEC Article 440.4(B) Industrial machinery - NEC Article 670.3 Meter disconnects - NEC Article 230.82(3) National Electrical Code and NEC are registered trademarks of the National Fire Protection Association (NFPA), Quincy, MA. Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 4 SCCR of Industrial Control Panels • The SCCR of a panel is not necessarily equal to the interrupting rating of the main overcurrent protective device (fuse or circuit breaker). • The SCCR of a panel is dependent on the “weakest link” rating within the panel. • The SCCR of the Industrial Control Panel must be greater than the maximum possible fault current at the panel. Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 5 How Do You Determine SCCR? • NEC Article 409.110 states that the SCCR of an industrial control panel is the: • SCCR of a Listed and labeled assembly, or • SCCR based on use of an “approved” method UL 508A Supplement SB is given as an example of an approved method – As of April 25, 2006, all UL 508A Listed panels must be marked with SCCR Method approved by local electrical inspector (AHJ) – Engineer approved Apparent RMS method. National Electrical Code and NEC are registered trademarks of the National Fire Protection Association (NFPA), Quincy, MA. Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 6 UL 508A Supplement SB • Provides method to determine SCCR of panel based on the types and ratings of components. • Does not require UL testing. However, as of April 25, 2006, UL 508A Listed panels must use as default method. • SCCR is based on SCCR of lowest rated component, or • If current-limiting devices (i.e. fuses) are used in the feeder circuit, the SCCR rating of the panel may be higher than the lowest rated component. Current-limiting fuses Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 7 UL 508A Supplement SB SCCR Method UL 508A Supplement SB describes three steps to determine the SCCR of industrial control panels: 1. Determine the SCCR of all individual power components and UL 2. 3. approved series ratings. Modify the SCCR of certain series combinations if current-limiting devices are used in the feeder circuit. The SCCR of the panel is equal to the lowest rated SCCR of any: – – – – Overcurrent protective device UL Approved series SCCR rating Modified combinations from Step 2 Power component not affected by Step 2 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 8 Example Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 9 UL 508A Supplement SB SCCR Method UL 508A Supplement SB describes three steps to determine the SCCR of industrial control panels 1. Determine the SCCR of all individual power components and UL approved series ratings. 2. Modify the SCCR of certain series combinations if current limiting 3. devices are used in the feeder circuit. The SCCR of the panel is equal to the lowest rated SCCR of any: – – – – Overcurrent protective device UL Approved series SCCR rating Modified combinations from Step 2 Power component not affected by Step 2 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 10 UL 508A Supplement SB – Step 1 of 3 Step 1 - Determine the SCCR of all power components and UL approved series ratings. There are 4 sub-steps to Step 1: a. Examine all individual components and record their SCCRs. b. If SCCR is not marked or known, use UL508A default Table SB4.1 c. Check and record any UL approved series ratings. d. Note the minimum SCCR in the feeder circuit and each branch circuit. Let’s begin with Step 1a… Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 11 UL 508A Supplement SB – Step 1 of 3 Step 1 - Determine the SCCR of all power components and UL approved series ratings. There are 4 sub-steps to Step 1: a. Examine all individual components and record their SCCRs. b. If SCCR is not marked or known, use UL508A default Table SB4.1 c. Check and record any UL approved series ratings. d. Note the minimum SCCR in the feeder circuit and each branch circuit. Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 12 UL 508A Supplement SB – Step 1 of 3 Step 1a. Examine all individual components and record their SCCRs. Branch Circuit #1 Branch Circuit #2 Branch Circuit #3 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 13 UL 508A Supplement SB – Step 1 of 3 Step 1 - Determine the SCCR of all power components and UL approved series ratings. There are 4 sub-steps to Step 1: a. Examine all individual components and record their SCCRs. b. If SCCR is not marked or known, use UL508A default Table SB4.1 c. Check and record any UL approved series ratings. d. Note the minimum SCCR in the feeder circuit and each branch circuit. Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 14 UL 508A Supplement SB – Table SB4.1 Step 1b. If SCCR is not marked or known, use UL508A default Table SB4.1 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 15 UL 508A Supplement SB – Step 1 of 3 Step 1b. If SCCR is not marked or known, use UL508A default Table SB4.1 10 kA Branch Circuit #1 Branch Circuit #2 Branch Circuit #3 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 16 UL 508A Supplement SB – Step 1 of 3 Step 1 - Determine the SCCR of all power components and UL approved series ratings. There are 4 sub-steps to Step 1: a. Examine all individual components and record their SCCRs. b. If SCCR is not marked or known, use UL508A default Table SB4.1 c. Check and record any UL approved series ratings. d. Note the minimum SCCR in the feeder circuit and each branch circuit. Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 17 UL 508A Supplement SB – Step 1 of 3 Step 1c. - Check and record any UL certified and approved series ratings. • The contactor in Branch Circuit #2 with a marked SCCR of 5 kA is UL Listed with a SCCR of 100 kA and provides “Type 2” no damage coordination when protected by UL Class J or CC fuses. Branch Circuit #2 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 18 UL 508A Supplement SB – Step 1 of 3 Step 1 - Determine the SCCR of all power components and UL approved series ratings. There are 4 sub-steps to Step 1: a. Examine all individual components and record their SCCR’s. b. If SCCR is not marked or known, use UL508A default Table SB4.1 c. Check and record any UL approved series ratings. d. Note the minimum SCCR in the feeder circuit and each branch circuit. Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 19 Step 1d. - Feeder Circuit before Step 1 after Step 1 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 20 Step 1d. - Branch Circuit #1 before Step 1 after Step 1 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 21 Step 1d. - Branch Circuit #2 before Step 1 after Step 1 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 22 Step 1d. - Branch Circuit #3 before Step 1 after Step 1 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 23 Example – after Step 1 Step 1d. Note the minimum SCCR in the feeder circuit and each branch circuit. Branch Circuit #1 Branch Circuit #2 Branch Circuit #3 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 24 UL 508A Supplement SB SCCR Method UL 508A Supplement SB describes three steps to determine the SCCR of industrial control panels 1. Determine the SCCR of all individual power components and UL approved series ratings. 2. Modify the SCCR of certain series combinations if current limiting devices are used in the feeder circuit. 3. The SCCR of the panel is equal to the lowest rated SCCR of any: – – – – Overcurrent protective device UL approved series SCCR rating Modified combinations from Step 2 Power component not affected by Step 2 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 25 UL 508A Supplement SB – Step 2 of 3 Step 2 - Modify the SCCR of certain series combinations if current-limiting devices are used in the Feeder Circuit. • If current limiting fuses are used in the feeder circuit, compare the Ipeak of the fuse from UL 508A Table SB4.2 to the lowest rated SCCR component or minimum SCCR in each branch circuit. • The Ipeak of the fuse must be less than or equal to the lowest rated SCCR of any component or SCCR in each branch circuit. Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 26 UL 508A Supplement SB – Table SB4.2 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 27 UL 508A Supplement SB – Step 2 of 3 Step 2 - Modify the SCCR of certain series combinations if current-limiting devices are used in the Feeder Circuit. Question: What is the Ipeak of the 60 Ampere current limiting Class J fuses used the feeder circuit? Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 28 UL 508A Supplement SB – Table SB4.2 So for our Example… Available RMS fault current Maximum Ipeak See UL 508A Supplement SB Table SB4.2 for complete listing of all Current Limiting Fuses Ip x 103 (or Ipeak) values listed in Table SB4.2 represent available RMS fault currents. Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 29 UL 508A Supplement SB – Step 2 of 3 Step 2 - Modify the SCCR of certain series combinations if current-limiting devices are used in the Feeder Circuit. Question: What is the Ipeak of the 60 Ampere current limiting Class J fuses? Answer: • The maximum Ipeak of the 60 Amp Class J fuses at an available RMS fault current of 200 kA is 16 kA • The maximum Ipeak of the 60 Amp Class J fuses at an available RMS fault current of 100 kA is 10 kA • The maximum Ipeak of the 60 Amp Class J fuses at an available RMS fault current of 50 kA is 8 kA Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 30 Step 2 continued Step 2 - Modify the SCCR of certain series combinations if current-limiting devices are used in the Feeder Circuit. Question: What is greatest available fault current the 60 Ampere Class J fuses will limit the Ipeak to 10 kA or less? Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 31 Step 2 continued Step 2 - Modify the SCCR of certain series combinations if current-limiting devices are used in the Feeder Circuit. Question: What is greatest available fault current the 60 Ampere Class J fuses will limit the Ipeak to 10 kA or less? Answer: The 60 Ampere Class J fuses will limit the maximum Ipeak to 10 kA when the available fault current is 100 kA. Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 32 Step 2 continued • Therefore, the modified SCCR of the power distribution block with 60 Amp Class J fuses is 100 kA. before Step 2 after Step 2 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 33 As a result of Step 2… After Step 2, the minimum SCCR of the Feeder and Branch Circuits are now as shown on this circuit diagram. Branch Circuit #1 Branch Circuit #2 Branch Circuit #3 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 34 UL 508A Supplement SB SCCR Method UL 508A Supplement SB describes three steps to determine the SCCR of industrial control panels 1. Determine the SCCR of all individual power components and UL 2. approved series ratings. Modify the SCCR of certain series combinations if current limiting devices are used in the feeder circuit. 3. The SCCR of the panel is equal to the lowest rated SCCR of any: – – – – Overcurrent protective device UL approved series SCCR rating Modified combinations from Step 2 Power component not affected by Step 2 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 35 As a result of Step 3… Since Branch Circuit #1 contains the lowest rated SCCR, the SCCR of the entire panel is 14 kA. Branch Circuit #1 Branch Circuit #2 Branch Circuit #3 Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 36 SCCR Label for our Example Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 37 How to Increase Short-Circuit Current Ratings 1. 2. 3. Use components and approved combinations with higher rated SCCRs. Use current limiting Class J, RK1, or Class CC fuses whenever possible. Replace low interrupting rated overcurrent protection with higher rated fuses and fusible switches. • According to UL 508A Articles SB3.2 and SB4.4, the SCCR of the panel cannot be greater than the interrupting rating of any overcurrent protective device in branch circuits or in the primary of control circuits. Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 38 How to Increase Short-Circuit Current Ratings • In the example given, if the MCCB in Branch Circuit #1 was replaced with a fusible disconnect switch such as Littelfuse LFFS030CC with Littelfuse Class CC fuses, the minimum SCCR of Branch Circuit #1 would increase to 50 kA, and the SCCR of the entire panel would increase to 50 kA. LFFS030CC Disconnect Switch UL Class CC fuses Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 39 How to Increase Short-Circuit Current Ratings • If UL 508A Supplement SB method does not yield desired results, • Consider Listing the panel by a NRTL (Nationally Recognized Testing Laboratories) • If acceptable to the AHJ, consider approval using the equivalent available RMS (up-over-and-down) method by a registered PE. Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 40 Important Safety Reminder • OSHA 1910.132(d) requires employers to identify electrical hazards in the workplace, inform and train workers how to avoid hazards, and provide them with personal protective equipment. • Fault current studies must be performed regularly and compared with the SCCR’s of all industrial control panels and equipment… • When industrial control panels are installed, moved, or modified. • When Arc-Flash Hazard assessments are performed to meet OSHA and NFPA regulations and standards. Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 41 For More Information… Please contact Littelfuse POWR-GARD Engineering Services for more information on: z z z z NEC Article 409 Requirements UL 508A SCCR Consultation Services Arc-Flash & Electrical Safety Safety Audits z z z Engineering Services Technical Support Littelfuse Products 1. Contact our Technical Hotline at: 800-TEC-FUSE -or- techline@littelfuse.com 2. Visit the POWR-GARD Technical Center website at: www.littelfuse.com/technicalcenter Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 42 Questions & Answers Confidential and Proprietary to Littelfuse, Inc. © 2006 Littelfuse, Inc. All rights reserved. 43