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Thread: Ground Wires and MV Ductbanks

  1. #11
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    190713-0940 EDT

    charlie b:

    My guess --- it is to provide reasonably good earthing of conductive parts of the manhole to local earth. Thus, leakage current to these conductive parts would be less likely to produce shocking current from the conductive parts to local earth (others areas of the manhole). Done for human safety.

    It takes only a small current, mA range (1 to 5 is detectable), to shock a person.

    Whether the 3 hot wires are low impedance earthed somewhere or not there is leakage current via some means, and therefore the hot wires are in someway referenced to earth.

    With respect to EGC circuits, if leakage current within a piece of equipment is modest, possibly up to 10 A, then the voltage drop on the EGC at the equipment relative to likely earth potential at the equipment may be only about 1 or 2 V. From 10 A and 0.1 to 0.2 ohms in the EGC.

    But what happens when there is a dead short from a hot 120 wire to the EGC? It approaches 60 V.

    .

  2. #12
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    In my opinion, since the grounding grid horizontal wire has to be minimum 4/0 -from mechanical point of view-this cable could be a sort of a connection between the both cable ends' ground grids. However, it has to be more connections between these grids.

  3. #13
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    Charlie
    Don't know if this was resolved for you...found this in a Power Systems text.
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