Details for "UL 508 A Short Circuit Current Rating" - Phoenix Contact

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Short Circuit Current Ratings – Application Guide1The subject of Short Circuit Current Ratings, otherwise known as the SCCRrating, has been gaining increasing International awareness since thepublication of the 2005 Edition of ANSI/NFPA 70 by the National FireProtection Association (NFPA) in the United States. In this revision, Article409 – Industrial Control Panels was released to provide Inspectors a singlesource for requirements and to assist with verification of compliance to Article110.10 – Circuit Impedance and Other Characteristics. Widely recognized asthe National Electrical Code®, or NEC®, the NFPA 70 standard identifies safepractices for the installation and wiring of electrical equipment in the UnitedStates. Although the NEC® itself is not United States Law, conformance tothe requirements of the NEC® is generally mandated by State or Local Lawand is enforced by Electrical Inspectors and Authorities Having Jurisdiction(AHJs) throughout the country.What is a Short Circuit Current Rating (SCCR)?The Short Circuit Current Rating of an assembly or component represents the maximum symmetrical faultcurrent, at rated voltage, that the assembly or component can withstand without sustaining damage thatexceeds acceptance criteria or creating a hazardous operating condition. It is important that the SCCRrating not be confused with Interrupting Rating of a component. Interrupting Ratings apply only toequipment that is intended to interrupt current and not to the assembly in which the equipment is installedor the components it may be intended to protect. A common mistake, for example, is to assume that theInterrupting Rating of a primary overcurrent protective device such as a fuse or circuit breaker may beused as the SCCR rating of the panel in which it is installed.Where Does the Requirement for SCCR Originate?SCCR ratings have long existed in the United States for certain components such as fuseholders, butwere generally referred to as the Short Circuit Withstand Rating of the device and defined within therelevant product standard (e.g. UL 512 Standard for Fuseholders). The SCCR requirement wasexpanded with the release of the 2005 NEC® where Marking of the SCCR rating became required for:Equipment TypeNEC® ArticleHVAC Equipment440.4 (B)Industrial Control Panels 409.110Industrial Machinery670.3 (A)Meter Disconnect Switches 230.82 (3)Motor Controllers 430.8For the purpose of this Application Guide, the focus will be directed to Industrial Control Panels andIndustrial Machinery, for which the requirements for SCCR marking are very similar as demonstrated byFigure 1, summarized from the 2005 NEC®. Primary reference will be made to Article 409 – IndustrialControl Panels, newly published in the 2005 edition of the NEC®,Subsequently, in April 2006, the requirement for Marking the assembly SCCR rating of an IndustrialControl Panel became mandated by Underwriters Laboratories for all equipment Listed to the revised UL508A Standard for Industrial Control Panels. The modification of UL 508A by Underwriters Laboratorieswas made to bring the Standard into compliance with the new requirements of the 2005 NEC®.PHOENIX CONTACT P.O. Box 4100 Harrisburg, PA 17111-0001717-944-1300 Fax: 717-944-1625 e-mail: info@phoenixcon.com www.phoenixcon.com© 2008 PHOENIX CONTACT
Figure 1 – NEC® Articles 409.110 and 670.32NEC® 2005 Article 409 – Industrial Control Panels409.110 Marking. An industrial control panel shall bemarked with the following information that is plainly visibleafter installation:(3) Short circuit current rating of the industrial controlpanel based on one of the following:a. Short circuit current rating of a listed and labeledassemblyb. Short circuit current rating established utilizing anapproved method.FPN: UL 508A-2001, Supplement SB, is an example of anapproved method.Exception to (3): Short-circuit current rating markings arenot required for industrial control panels containing onlycontrol circuit components. (Exception added forclarification in the 2008 NEC®).NEC® 2005 Article 670.3 – Industrial Machinery670.3 Machine Nameplate Data.(A) Permanent Nameplate. A permanent nameplate shallbe attached to the control equipment enclosure of machineand shall be plainly visible after installation. The nameplate shall include the following information:(4) Short circuit current rating of the machine industrialcontrol panel based on one of the following:a. Short circuit current rating of a listed and labeledmachine control enclosure or assemblyb. Short circuit current rating established utilizing anapproved method.FPN: UL 508A-2001, Supplement SB, is an example of anapproved method.How to Determine a Panel Assembly SCCR RatingFirst, it is important to note that SCCR ratings are determined based upon Power Circuit components inthe assembly. For panel assemblies containing only Control Circuits, SCCR rating markings are notrequired for the assembly. An exception to Article 409.110(3) was added for clarification in the 2008NEC®. Of course, the final decision as to whether a panel assembly exclusively contains Control Circuitsis up to the AHJ in the locality of installation. For reference, the following definitions are provided from theNational Electrical Code®:Control Circuit. The circuit of a control apparatus or system that carries the electric signalsdirecting the performance of the controller but does not carry the main power current. (A controlcircuit is, in most cases, limited to 15 amps).Power Circuit. Components and conductors of branch and feeder circuits (e.g. the main powercircuit)Industrial Control Panel. An assembly of two or more components consisting of one of thefollowing:1. Power circuit components only, such as motor controllers, overload relays, fuseddisconnect switches, and circuit breakers.2. Control circuit components only, such as pushbuttons, pilot lights, selector switches,timers, switches, control relays.3. A combination of power and control circuit components.These components, with associated wiring and terminals, are mounted on or containedwithin an enclosure or mounted on a subpanel. The industrial control panel does not includethe controlled equipment.PHOENIX CONTACT P.O. Box 4100 Harrisburg, PA 17111-0001717-944-1300 Fax: 717-944-1625 e-mail: info@phoenixcon.com www.phoenixcon.com© 2008 PHOENIX CONTACT
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