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Using one line diagrams, this study is performed in order to determine the magnitude of currents flowing throughout the power system at critical points during various time intervals after a "fault" occurs. These calculations are required for determining the interrupting ratings of your equipment and the coordination of protective relays, circuit breakers and fuses. Accurate one-line diagrams and utility positive and zero sequence source impedance information are required for this study.
Why do we need it?
Circuit breakers and fuses have overcurrent ratings and short circuit interrupting ratings. Overcurrent rating identifies the amount of electrical current the device may withstand without a fuse blowing or circuit breaker tripping. The short circuit rating is the maximum electrical current the device can tolerate before it fails. In the event of a fault a device will either trip or fail depending on the manufacturer's short circuit interrupting rating. In this short-circuit study we will calculate the fault current at various locations in the plant and compare to the short circuit interrupting ratings of the devices used at each location.
An Electrical Equipment Evaluation Study is an optional extension of a Short Circuit Study. This study compares equipment ratings with calculated short-circuit and operating conditions.
Why do we need it?
Avoid extensive equipment, system damage and personnel injury because of under-rated equipment in the event of a fault. This study will identify under-rated equipment and recommend replacements. The National Electrical Code requires that all short circuit interrupting devices have ratings sufficient for the current which must be interrupted.
Bolted fault currents are converted into arc currents for additional analysis.
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