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Transformer grounding point.

    • 11 posts
    March 7, 2019 2:56 AM PST

    Just a information..

    having such a transformer 400v, low voltage which the Wye point is connected to ground with resistance as well as breaker or isolator. What is the purpose of the breaker or isolator in this. When it will be closed and when is it open...


    • 200 posts
    March 7, 2019 6:08 PM PST

    Thanks for the suggestion of this topic, Syed.

    It is advised to take a look on topic "grounding system":

    there you can find some of most used grounding systems with detailed information, including those very approximated of your diagram.

    The Wye-side of a Transformer is usually the Low-Voltage side to serve Residential, Commercial and even sometimes Industrial installations. The Wye-side has a common central point where all the 3 phases windings are connected, that is the Neutral, and it is also served to the same Users above on both Single-Phase (2-Conductors = 1 Phase + Neutral) or Three-Phase (4-Conductors = 3 Phases + Neutral).

    Usually (but not always) the Neutral is grounded near by the Wye-side of the power service Transformer. In some countries an additional conductor as Ground is also served by power company or such Ground conductor is mandatory by a safety code to be made by the User and so there are Single-Phase (3-Conductors = 1 Phase + Neutral + Ground) or Three-Phase (5-Conductors = 3 Phases + Neutral + Ground). However there are many countries and regions that such additional Ground conductor are not offered as service neither mandatory - in such cases the only Ground circuit available is the Neutral from the power service Transformer Wye-side.

    Your diagram, as it was drawn, when breaker/isolator is CLOSED, can be:
    -- LOW RESISTANCE GROUNDED (LRG): is used to reduce damage caused by high currents flowing during Ground Faults (GFs). They can be used on Low Voltage systems;

    -- HIGH RESISTANCE GROUNDED (HRG): similar to LRG systems, except that a Neutral Grounding Resistor (NGR) has a higher resistance value. This reduces the Ground Fault (GF) current to lower values, typically less than 10A on MV systems, and usually less than 5A on LV systems;

    [insert image: Figure 4. Resistance Grounded System=imgsnap102.png]

    -- Purpose of the breaker or isolator: if your diagram is correct, and UNGROUNDED systems (as when OPEN) are NOT RECOMMENDED for most modern electrical systems, the only purpose we could think is Maintenance;

    ADDED LATER: if it is a simple Isolator Switch the purpose stills Maintenance but if it is a Breaker (such as a controlled one or 4-Pole Breaker) it can be a solution to prevent Ground current from dividing through the Neutral conductor: switching the Neutral conductor  insures that no Neutral current can flow through a de-energized source. Please the the following topic:

    "MCCB switchgear"

    In hope that could answer your question.


    This post was edited by Alex de Moura at March 12, 2019 10:04 AM PDT
  • March 8, 2019 3:17 PM PST

    As Alex said, Maintenance is the only reason I have seen.  The disconnects are put in where there are redundant transformers.  For HRG, LRG, if there is a phase to ground fault, then the neutral is pulled up to 231V, above ground.  And if one has one's hands in the off-line redundant transformer that could be a shocking experience. yell 

    Following is based on 2017 NEC, Industrial practices:

    Ground fault for HRG stays on line forever.  They don't cause a trip. 

    Ground fault on an LRG, worst case will trip in a dozen milliseconds - plenty long enough to be a hazard.

    I have not seen any data for solidly grounded Wye systems.

    A normal practice is to put a two pole disconnect in the neutral wye point, with the second pole wired in the XFM feeder CB trip circuit.  This keeps one from inadvertently running with the neutral grounding point open.

    The neutral grounding disconnects are often seen in parallel generation as well.  Same reason. 

    One could also use a 4 pole disconnect.  I know of one engineer that says he specs them all the time.  However, I have never seen one.  I suspect the cost is prohibitive and I do not see that they are any safer that a disconnect with a second pole wired in the  CB trip circuit.

    • 200 posts
    March 12, 2019 10:06 AM PDT

    Syed, an image was added in the hope that could be of any help. Also additional info was posted. Thanks