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Eaton: Short circuit current rating (SCCR) Basics, Short circuit current rating (SCCR) Grundlagen

What is the Short-Circuit Current Rating (SCCR)?

The Short-Circuit Current Rating (SCCR) is a parameter for electrical components or assemblies. According to the definition in the National Electrical Code (NEC), the SCCR is the maximum short-circuit current that a component or system must be able to safely withstand. 

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Why is the SCCR needed?

Calculating the SCCR makes it possible to assess whether a main circuit can withstand a short-circuit load until the upstream protective device trips. It also allows engineers to identify what the maximum short-circuit load is at the installation site. Short-Circuit Current Ratings are an important criterion for the safety of industrial and commercial switchgear systems and control panels in accordance with the UL standard. When used correctly, switchgear systems should be able to withstand a short-circuit without exposing employees to a fire hazard, flying debris, an arc fault or an electric shock.

 

How does the rating affect the North American market?

Of course, when exporting machines containing electrical equipment to North America, the applicable standards and rules there must be observed. This means that machines and systems must achieve the specified SCCR value at the connection point.

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Which standards require a Short-Circuit Current Rating?

The North American National Electrical Code (NEC), the US Federal Office for Occupational Safety and Health Administration (OSHA) and the Underwriters Laboratories (UL) create guidelines relating to the SCCR.

  • OSHA standards require all electrical switchgear systems to have an SCCR suitable for new and existing switchgear systems.
  • The NEC (section 409.22) prohibits the installation of switchgear systems in which the potential short-circuit current at the connection point exceeds the SCCR of the respective switchgear system.
  • The NEC requires machine control panels to have an SCCR marking.

The NEC safety standard also lists other requirements, including that documentation or a dated marking (depending on the type of switchgear system) for the available fault current (also called the available short-circuit current) must be provided at the installation point of a control panel or switchgear system. These modifications allow installers, testers, and inspectors to verify that the SCCR is equal to or greater than the available short-circuit current for the installed device, meaning it also complies with the other sections of standards which deal with device installations.

How is the SCCR calculated for a switchgear system?

There are two ways to calculate the SCCR for a control panel or switchgear system: tests or analyses. Because tests are often expensive, analysis is the most common way to calculate the SCCR. Supplement SB to the North American standard UL 508 A provides an analysis method which considers the "lowest-rated component" as the basis for calculating the SCCR of systems.
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Manufacturer let-through charts for current-limiting breakers; The example above is for Eaton’s Moeller Series NZM breaker, type NZMH2-VE250-NA, molded case circuit breaker - 240V, 480V, 600V.

Six steps to calculate the SCCR value:

  1. Identify all the components in the electrical circuit: This includes, load break switches, miniature circuit breakers, overload relays, motor controls (contactors) and terminal blocks.
  2. Determine the SCCR value for all the components in the electrical circuit: The SCCR value is usually specified on the component, combination values can be found on the UL website (e.g. for motor controller combinations) or in the manufacturer's instructions. If the manufacturer doesn't provide any information, the SCCR value can be calculated using Table SB 4.1 in UL 508 A.
  3. Determine the SCCR value for each branch circuit: The SCCR value for the branch circuit is equal to the lowest-rated component SCCR value.
  4. Determine the SCCR value for the feeder circuit
  5. Consider the overcurrent protection devices included in the branch circuit
  6. Determine the lowest-rated value for all the components or branch circuits

The SCCR of a switchgear system is equal to the lowest-rated value for the SCCRs of all the components, the potentially increased SCCRs of components in the branch circuits or the short-circuit breaking capacity of the overcurrent protection devices. For more information on how to calculate the SCCR, see the whitepaper "Current-limiting components: Improving Eaton's Short-Circuit Current Rating (SCCR)."

The SCCR for a machine should be calculated during the machine control panel's early engineering phase—this works out to be the most efficient. Eaton offers a variety of resources to make it easy to calculate the SCCR.

 

Conclusion

Calculating the SCCR value for systems is an essential part of the applicable safety standards in the North American market. If you want to sell machines and systems there, the SCCR value must be calculated and marked on the system. But calculating the SCCR isn't witchcraft—and if it's worked out early enough in the planning phase, it can also be increased relatively easily.

 

White paper about the Short Circuit Current Rating

Research indicates that more than half of OEMs design their machines to a typical minimum equipment SCCR of 5kA. However, with the new NEC code, valid after January 2017, it is possible that many equipment installations will not be code compliant or approved. This white paper explores the standards and best practices for determining equipment SCCR and the approved methods to increase ratings.

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