Fault characteristics in electrical equipment

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Courtesy of Eaton Corporation

Proper design and installation of electrical equipment minimizes the chance of electrical faults. Faults occur when the insulation system is compromised and current is allowed to flow through an unintentional path.

IEEE Std 493 Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems1 publishes data that reports that the vast majority of faults are classified as line-toground, or more simply “ground faults”.

Industry data collected from published reports indicates that many faults are low level [1][2][3][4] [5][6][7][8][9]. Those reports describe equipment meltdown when the faults were not cleared by the overcurrent device in a timely manner.

The reason that a low level ground fault can burn down a power distribution board while typically a phase-to-phase fault does not is that, counterintuitively, a low-current fault can release far more energy than can a high current fault. The reason is that for most overcurrent protective devices, lower level faults require more time to clear. As will be shown later, this slower clearing time causes much more energy to be released. This situation is particularly dangerous for ground faults as a ground fault has the potential to be a low-current fault.

While ungrounded distribution systems would reduce the likelihood of a ground fault, ungrounded systems are almost never recommended due to the potential for damaging high line-to-ground voltages that can occur during intermittent ground faults [10].

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