Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 21 replies
  • Subscribers 29 subscribers
  • Views 1729 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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.
Parents
  • 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.



     


Reply
  • 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.



     


Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines