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Thread: Ungrounded System - EGC

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    Ungrounded System - EGC

    For an ungrounded system with a delta-wye secondary:

    Is an EGC required?
    Is the EGC connected through the supply transformer? Such that the EGC in the building is connected to the ground(?) of the supply line.
    If the EGC is required, whats the point of it? If there is a single phase line to ground fault then not enough current will flow to trip the breaker.

    Im studying for the PE exam, any input is appreciated.

    Thanks!

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    yes, required
    yes, bonded thru the xfmr

    if you have different phases faulted on different frames w/o bonding = no trip
    if an individual comes in contact with both frames he is the sc path

    same fault scenario as above
    if only one frame is bonded and the other is not
    an individual becomes the fault path if he contacts the unbonded frame and earth/ground

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    Quote Originally Posted by TheSmoothestCriminal View Post
    For an ungrounded system with a delta-wye secondary:

    Is an EGC required?
    Yup, sure is.

    Is the EGC connected through the supply transformer? Such that the EGC in the building is connected to the ground(?) of the supply line.

    Yes- only difference is there is no connection from the grounding block to an X0 on the trafo. Everything else is the same like on a grounded system.



    If the EGC is required, whats the point of it? If there is a single phase line to ground fault then not enough current will flow to trip the breaker.

    Picture it like this:

    First, When one phase faults, the frame will be energized and will pass some level of current to earth, building steel, ect- that amount of current depends on the size and voltage of the system. Larger the systems the more capacitance between conductors and earth and as such more current flow. Large enough currents can shock or even kill.


    Second- even if the currents are small enough (ie say 4ma), you still need an EGC. Reason being that if a phase faults down in a piece of equipment it will not open an OCPD and remain there until fixed. During this time if a phase faults down else where, you now have two objects in the same building at 240 or 480 volts potential relative to each other. Its possible for a person to touch the faulted equipment, have current flow through him, through the floor/earth/ect all the way to the other piece of faulted equipment. That current could be very large depending on the path resistance. Similarly there could be 2 pieces of faulted equipment in the same room, or a fault to the plumbing and something next to a sink- a person could contact one with one hand and another object with the other hand- that would make 240 or 480 across the chest. However, it doesnt stop there. If there is anything interconnecting the two pieces of faulted equipment- or even some high impedance path through building material- the current may be high enough to create enough heat to start a fire yet the current is low enough (from the resistance) to never trip the breakers associated with those circuits. If the the current path is through coaxial cable, a signaling circuit, ect it certainly is possible for the shields on those cables to burn. Even low levels of current passing through wood for years on end can lead to pyroforic carbonization.*

    Summed up you do not want 2 or more objects in a building be it plumbing, conduit, switch boxes, motors, tools, ect to be at 240 or 480 volt potentials relative to each other.


    Third- You need a good reference for your ground detector. The whole point of an ungrounded system is service continuity and as such a chance to repair a fault before another one occurs- thus the code mandates a ground detector. That detector needs a reliable circuit- and by having a low impedance circuit to ever piece of metal that could become contact by a phase conductor the detector will instantly and reliably signal a fault has occurred in the system.
    I'm in over my head...

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    I think you have received good explanations. Just remember "ground" refers to several different things unfortunately. When we talk about "ungrounded systems" we are only talking system grounding. You still need bonding and earthing of metal parts.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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    *

    I just want to give a real life example of where not bonding different objects supplied by the same ungrounded system was detrimental. In Norway most homes and businesses were/are supplied by an ungrounded system- that is 3 wires on the pole (phase 1, 2, 3) and the pole transformer neutral (were present) effectively isolated from earth.

    Homes receive either 2 or 3 phases- businesses usually 3. There is no neutral or ground wire supplied to the service. Just two or three "hot" wires. Each structure has its own grounding system with EGCs, a ground bar in the panel, bonding of pipes to the ground bar and of course earth electrode(s). Each structure has a complete grounding system just like the NEC.


    The issue comes that none of the grounding systems in each separate building is interconnected to one another via a low Z path. Only through the earth via earth electrodes and anything else in contact with the soil.


    The final outcome is that over time one phase would fault in one building going unnoticed, and then another phase would fault down in another building served by the same pole transformer. This did not always result in OCPDs operating, and 230 volts potentially would exist between two buildings with current flowing between them. This current is believed to be the result of many fires, and possibly one major reason behind the increased number of home fires in Norway.


    Point being that ungrounded systems need everything bonded in as much as grounded systems.
    I'm in over my head...

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    Quote Originally Posted by electrofelon View Post
    I think you have received good explanations. Just remember "ground" refers to several different things unfortunately. When we talk about "ungrounded systems" we are only talking system grounding. You still need bonding and earthing of metal parts.
    I agree, well said-
    I'm in over my head...

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