LOW IMPEDANCE FAULT AND HIGH IMPEDANCE FAULT BASICS AND TUTORIALS

Low impedance faults or bolted faults can be either very high in current magnitude (10,000 A or above) or fairly low, e.g., 300 A at the end of a long feeder. Faults that can be detected by normal protective devices are all low impedance faults.

These faults are such that the calculated value of fault current assuming a ‘‘bolted fault’’ and the actual are very similar. Most detectable faults, per study data, do indeed show that fault impedance is close to 0 V.

This implies that the phase conductor either contacts the neutral wire or that the arc to the neutral conductor has a very low impedance. An EPRI study performed by the author over 10 years ago indicated that the maximum fault impedance for a detectable fault was 2 V or less. Figure 23.2 indicates that 2 V of fault impedance influences the level of fault current depending on location of the fault.


As can be seen, 2 V of fault impedance considerably decreases the level of fault current for close-in faults but has little effect for faults some distance away. What can be concluded is that fault impedance does not significantly affect faulted circuit indicator performance since low level faults are not greatly altered.

High Impedance Faults
High impedance faults are faults that are low in value, i.e., generally less than 100 A due to the impedance between the phase conductor and the surface on which the conductor falls. Figure 23.3 illustrates that most surface areas, whether wet or dry, do not conduct well.

If one considers the fact that an 8-ft ground rod sunk into the earth more often than not results in an impedance of 100 V or greater, then it is not hard to visualize the fact that a conductor simply lying on a surface cannot be expected to have a low impedance.

These faults, called high impedance faults, do not contact the neutral and do not arc to the neutral. They are not detectable by any conventional means and are not to be considered at all in the evaluation of fault current indicators (FCIs) and most other protective devices.

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