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high impedance faults

Discussion in 'Electrical Engineering' started by CD, Oct 28, 2003.

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  1. CD

    CD Guest

    Hi all,

    What exactly is a high impedance fault? I'm trying to figure it out, so I
    can get a better picture as to why it doesn't always trigger overcurrent
    protective devices and such.

  2. anon

    anon Guest

    Not a lightweight concept!

    Searching "hirop" or hirop and electrical protection systems may lead to a
    It would probably be worthwhile having a good grip on 3 phase HV protection
    systems before you dig into HIROP.

    [email protected] for direct response
  3. Rowbotth

    Rowbotth Guest

    Every fault will have some resistance to ground (or whatever the return
    path might be). As with any other path for current flow, some are
    better than others. A high resistance fault will be one which has a
    very high resistance path for the fault current to get back to source.

    One example would be:

    Snow at very low temperatures. Snow, at low temperatures, is a
    relatively poor conductor. A few years ago, locally an overhead 25kV
    line had one phase broken by equipment passing below. The broken phase
    conductor fell to earth, landed in the snow, and the path was so poor
    that the recloser on the system operated 7 times before the current path
    got to be good enough for two consecutive trips to be close enough
    together for the relay tro know that it was "seeing" an actual fault,
    and it should trip.

    (There was sufficent resistance in the return path for current flow that
    it actually limited the amound of current which flowe3d, so the clearing
    device did not recognioe this as an actual phase to ground fault, in
    spite of the fact that the phase conductor was resting on the ground.)

    Does this help?

  4. daestrom

    daestrom Guest

    As the name implies, they are simply faults with more than zero impedance.
    The problem can be when protective devices/relaying doesn't sense enough
    fault current to operate. For example, a broken phase hanging in a tree or
    across a structure. Perhaps the impedance is high enough that the fault
    currents don't immediately trip the protection devices.

    But you might have nearly full rating of the line being dumped into a fault
    (something that can't necessarily 'use' that kind of power safely).
    Burning, fire, bad things. If the fire progresses of course, the fault gets
    'worse' and eventually trips.

    This is one reason why a variety of protective relaying is used, not just
    short-circuit protection. Phase imbalance, differential relaying and
    ground-fault are some ways to detect this type of fault and deenergize the
    equipment before too much damage.

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