GEORGE FARRELL and FRANK VALVODA, P.E.

The Art of Protecting Electrical Systems, Part 11: Impedance in Systems with Rotating Machinery

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The Art of Protecting Electrical Systems, Part 6

This sixth part in our series discusses the need for short-circuit calculations and the current-limiting effect of some overcurrent protective devices. Whenever a short circuit occurs, every component in the system carrying the fault current must safely withstand the heating and magnetic stresses caused by the current. In addition, the protective device interrupting the fault current must do so safely and reliably. The National Electrical Code (NEC) requires two principal short-circuit ratings for electrical system components up to 600 volts: interrupting and withstand. Devices that interrupt current, especially overcurrent protective devices, may have one or both ratings.

The Art of Protecting Electrical Systems, Part 5

This fifth part in our series describes typical electrical system faults and the changes in transient voltage and power factor they produce. When a short circuit occurs, generators—whether utility or on-site—do not stop generating instantly, even if the drivers are immediately shut down. Because the only thing limiting the initial current a generator can deliver to its shorted terminals is its internal impedance, initial current flow may be 20 or more times the rated load current. Under fault conditions, generator impedance increases rapidly as a result of increased generator winding temperatures, voltage imbalance and changes in machine speed. As a result, the fault current decreases to a steady-state condition within a few cycles. Similarly, electric motor-driven equipment does not stop instantly.

The Art of Protecting Electrical Systems, Part 4: System Analysis

Editor’s Note: Protective equipment must withstand changes caused by short circuits in electrical systems. This fourth article in our series discusses aspects of short circuit calculations Engineers designing protection for electrical systems must consider the many changes that take place when a short circuit occurs. Protective equipment must be able to withstand the effects of short circuits, minimize damage and restore service as quickly as possible. The changes that occur during faults—many of which are interdependent—have not been brought to the attention of engineers.