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.
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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