Issue XI
GE Industrial Solutions Products Services Markets Resources

A Primer on Low Voltage Motor Control Centers Understanding Short Circut Motor Contribution Calculating Short Circuit Current without a Computer

A Motor Control Center is a factory assembled grouping of motor starters and controllers in a common structure.  It is made up of bussed vertical sections connected together by a common horizontal main bus.

Each section may contain compartmentalized starters, feeders, transformers, adjustable frequency drives and panelboards just to mention a few.  Depending upon the size, most starters and feeders may be plug in type.  These units are typically rated up to 600 volts AC.

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Motor's contribution is the current generated by a motor or motors during a short circuit condition.  It represents a small but important value that is needed to determine the maximum short circuit current available and thereby establishing the short circuit rating of electrical equipment.  Regardless of the size or voltage rating of a motor, it can be demonstrated that motor contribution is present during a fault.

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A Short Circuit analysis is used to determine the magnitude of short circuit current the system is capable of producing and compares that magnitude with the interrupting rating of the overcurrent protective devices (OCPD).  Since the interrupting ratings are based by the standards, the methods used in conducting a short circuit analysis must conform to the procedures which the standard making organizations specify for this purpose.

In the United States, the America National Standards Institute (ANSI) publishes both the standards for equipment and the application guides, which describes the calculation methods.

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