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Three phase earth

Discussion in 'Electrical Engineering' started by Barry, Oct 12, 2006.

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  1. Barry

    Barry Guest

    Hi all,

    What is the reason for the yellow phase being earthed or used as the star
    point in a three phase system? I have seen this several times in the North
    of the uk. I am told that it is something to do with impedances?

    Many thanks.....Barry
  2. John McLean

    John McLean Guest

    I'm not aware of any arrangements in the UK; which have this configuration
    from a public utility. and most certainly not in Scotland which is "north of
    the UK".
    It's not a standard colour code either for the UK.The earth cable here, is
    coded Green/Yellow, (was Green self colour to BS 7671).
    Is this perhaps a single phase, construction transformer output, you are
    referring to; which is rated for 110/55/110 Volts, by means of a centre
    tapped transformer?

  3. Barry

    Barry Guest

    Hi John,

    Thanks for your reply. I have seen this a couple of times when I've been out
    troubleshooting and told that its something to do with earth fault
    impedances? L2 (yellow as it was) is the star point. By the way the colours
    I am reffering to are not the wire colours but the "old" phase rotation
    colours ie (R) L1 (Y) L2 and (B) L3.
  4. John McLean

    John McLean Guest

    The other possibility; is that the transformer is an earthing transformer to
    another; and for a bulk electrical supply. Without seeing the circuitry, I'm
    at a loss.

  5. Barry

    Barry Guest

    Hi Guys,

    Thanx for your responses. A little about me then!! I am 55 years old OUCH ,
    and have always been an electronics engineer. I have been in the
    "electrical" industry for a little over fourteen years now. I am Technical
    Manager for my company and part of my job is sorting out customers problems
    when everything else fails, the buck stops here.

    This goes back a few years now and I have seen the yellow phase earthed a
    couple of times. I remember that on one occasion it was down in a valley at
    a sub station in Scotland serving a distillery (no samples were given
    unfortunately). The problem was with a power factor transducer which was
    connected between the red and yellow phases for reference, it didn't work !!
    It transpired that the yellow phase was pulled to ground. Connecting between
    the blue and red phases restored normal operation.

    It was then I was told of the earth fault impedence but I'm still not sure
    of the logic of this? Its not really important I suppose but its just one
    of those things that come to mind from time to time as unsolved.


  6. Are you sure it wasn't a delta winding, where there is no star point
    to earth?
  7. Barry

    Barry Guest


    I don't think that there was a VT involved although it was a long time ago
    and a lot has happened since then. I would be interested to know if you get
    an answer.


  8. Setanta

    Setanta Guest

    Dont understand what you are saying here:

    What type of earthing system are you talking about?

    Is there balance between phases ...i.e. no N current?

    Connecting between
  9. Setanta

    Setanta Guest

    All the coils are either connected end to end as in delta


    one end of each coil are connected together as in star

    If the star point is subsequently earthed then the system Neutral and Earth
    are at the same potential

    For a three phase supply that hasnt got an equal impedance on each phase
    there will be a neutral current and a resulting shock risk as the neutral
    voltage is different than the assumed zero at the star point

    Is this what you are talkin about
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