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An interesting issue with a TT supply

Alan Capon
An interesting real life example of why you need double pole switching with a TT supply. The scenario is a small village with a few customers supplied by a single pole mounted transformer. 


We got a call from an electrician working in a property who had measured 400V phase to earth, 210V neutral to earth, and 241V phase to neutral, and was understandably worried. Any suggestions of what I tracked the fault down as? This can happen on any TT supply, but is fortunately very rare - I have come across it three times in 20 years. 


Regards,


Alan.
  • 0
    perspicacious:

    There we go again, a or an before H here?


    Depends whether you pronounce it aitch or haitch. ?


  • 0
    There are plenty of three phase PMTs around.  Smaller PMTs tend to be single or split phase.  Generally up to 100kVA on a single pole and up to 200kVA on either an H pole or pole with a stub pole, Tf sits on steelwork between the two..  I have seen bigger than 200kVA but not very often.  IIRC pole mounted substations are exempt from business rates which I think are charged on ground mounted.
  • 0
    A few have the answer, with Geoff Blackwell spotting it first. The electrician was doing other work in the property, and found the problem while proving dead. 


    One of the other properties in the village had a single phase submain with an earth fault, and a non—operative main rcd feeding it.


    We used the famous “screwdriver in the flowerbed” to identify which phase had gone wrong, then tried a few properties our records said were on that phase, before settling on a riding stables with a three phase supply. As only one phase was affected by the fault, we removed one fuse from their cutout and asked them to call an electrician to locate their fault. The fault was located, and we drew our remaining fuses to allow the electrician to replace the customer’s main rcd, then replaced and re-sealed all three fuses. 


    Regards,


    Alan.
  • 0
    Thank you, Alan, for an interesting Sunday brain-teaser.


    Interesting that the fault was found by coincidence!
  • 0
    Looking at Davezawadi’s answer, it was an interesting problem. The earth fault meant that L3 to earth was only a handful of volts. All the phase to neutral voltages were correct, as were the phase to phase voltages, so everything worked normally. The advantage we had as the DNO, is that we had the ability to look at the whole village, and disconnect properties until we found out who had the problem. 


    Regards,


    Alan.
  • 0
    Chris Pearson:

    . . . Interesting that the fault was found by coincidence!


    Also concerning that we have no idea how long it had been there! It does show the value of disconnecting the neutral or at least measuring it before you begin work. 


    Regards,


    Alan. 


  • 0
    Why did you disconnected the stables when the fault was in a cottages submain?
  • 0
    But the joy of TT is that so long as  neutral is properly insulated and treated (as '7671 requires..), with the same respect as a live wire. this is not immediately dangerous, no equipment correctly wired L-N sees any damaging over-voltages, and no one even feels any tingles. It is rather like the first fault on an IT system. A second fault however is not safe..


    This example also suggests the stables have a better connection to terra-firma earth than the DNO.


    Had there been any PME users on that transformer it would have been another story altogether.


    I suppose if required you could make TT safer by having an alarm at the TX that calls in if there is significant current in the earth path in the manner of a sort of giant RCD for the whole street, but to raise a fault code, not to cut the power.

    Mike


  • 0
    Kelly Marie Angel:

    Why did you disconnected the stables when the fault was in a cottages submain? 


    The fault is at the stables - they have connected 'L3' to ground very solidly, and their RCD has not fired.

    Because the earth at the DNO transformer has a higher electrode resistance than the stables, the L3 voltage goes down to near terra-firma potential, and the star point of the transformer goes up, there is still 240V between them,

    (i.e. the two electrode to terra-firma resistances are now in series, with one to N one to L3, but the terra-firma earth between them is better connected to L3.)

    Now anyone on the same Tx sees 

    L3- E 0V,

    N -E  240V,

    L2 -E 400V

    L1-E  400V

    All the phase to phase and neutral voltages remain correct, just the main ground is moved from the centre of the star, to one corner.


    The original caller presumably had a single phase supply and had say L1, N and a local  terra-firma earth.

    Mike.



     


  • 0
    Had there been any PME users on that transformer it would have been another story altogether.

    I suppose that depends on how good an earth the PME'd installation's extraneous-conductive-parts happened to have. If it was the usual plastic pipes and so on, then the voltages would probably be very similar - but all their exposed- and extraneous-conductive-parts would have been a N voltage - so around 230V above true earth - so rather like a broken PEN situation (but the L-N voltage monitoring EVSE would have no chance). Probably not too much of a worry within the equipotential zone, but potentially a bit nasty on the edges...


     
    The advantage we had as the DNO, is that we had the ability to look at the whole village, and disconnect properties until we found out who had the problem.

    Would a simple clamp meter around all the conductors supplying a consumer indicate whether the problem lay in their installation - as there was likely a residual current of many amps? (Probably easier with ABC overheads...)


       - Andy.

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