PFC on a TT system

Discussion in 'Electrics UK' started by too old to learn, 13 Oct 2009.

  1. too old to learn

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    Hi,
    I have been round my friends house as he has a TT system and I have never worked on these installations before. (yeah yeah, pathetic I am! lol)
    Whilst doing the PFC the result was 11 Amps.
    Is this correct for a TT system?
    I am guessing it is as the Ze was 20.9 ohms.

    I was just expecting similar readings to a TNS system where it is above 1KA.

    Guess this is the reason TT supplys (I know all do now to the 17th) needed time delayed RCD's to the 16th edition

    Thanks
     
  2. toasty

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    I don't understand the question.

    If the Ze is 20.9ohms, and the supply voltage is 230v, then surely the max fault current to earth can't be greater than about 11amps (230/20.9)

    With TNS systems, the earth impedance is much much much lower, so of course the fault current to earth is a lot higher.

    Is that what you mean? Sorry, but assuming you are a sparky, I would have thought it obvious.

    Dan
     
  3. Taylortwocities

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    The figure you put in the Schedule of Tests, and the one you need to determine, will be the higher of PSC (Prospective Short Circuit Current ) and PFC.
    In TT the PFC will not be very high as you it is always limited by the mass of earth.
     
  4. too old to learn

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    Yes I know that!

    My thoughts are that if there is a PEFC or PSFC then there is only 11A runnign down the live. This is therefore not enough to trip a ring breaker.

    Therefore the need for RCD's.

    Main point is that I have never done a TT suppply and I am using his house as practice. was just surprised at first to see such a low reading comapred to TNS system.

    So i guess this value is correct.

    Therefore I can Write this down as the PFC on the test sheet

    thanks
     
  5. Taylortwocities

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    Cool, just checking :cool:
     
  6. too old to learn

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    sorry Taylorwocities, you got there just before me.

    Least i know for future reference that thsi is what i wirte in.

    Thanks
     
  7. Spark123

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    Your PFC will almost certainly be phase to neutral on a TT system (PSCC - prospective short circuit current).
    The PEFC (prospective earth fault current) will be low - in your case 11A. PFC (prospective fault current) is the greatest fault current which can flow in an installation i.e. the PSCC or PEFC, which ever is greater.
    If there was a PSCC of 11A then your lights would be very dim!
    Same as on any TN-S, TN-CS, single or 3 phase system it is the max fault current which can flow - to make sure your fuses/MCBs are up to the job (or backed up).
     
  8. too old to learn

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    well i have been and measured the PSFC (betwenn live and neutral) and the result was 1.18KV. this is the answer that I was first expecting and therefore why i asked the first question.
    Thereofre this is the value that I would put down on my test form.

    One question I have now? how come if my PSCC was 11A my ligths would be like my brain, very dim :) ha ha
     
  9. londonboy

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    I may be wrong but don't you put worst case figures down?

    ie,
    lowest for IR
    lowest for pfc
    highest for zs
     
  10. RF Lighting

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    You put the highest for Ipfc

    That is the worst case scenario - i.e. the higher the fault current, the more damage could be done to the installation, such as melted circuit breakers etc.
     
  11. londonboy

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    Yeh course! I was thinking about not enough current flowing to disconnect breaker, obviously this is to do with Ze.

    Leave off its gone 5 o clock ;)
     
  12. RF Lighting

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  13. Spark123

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    1.18KV :eek:

    Your lights would be dim as the impedance of the supply conductors (phase and neutral) would have to be very high to give you a figure as low as 11A. Think about impedances in series acting like a potential divider with your current using apparatus - the volt drop would be massive!
     
  14. too old to learn

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    Sorry i meant 1.18kA :LOL:
     

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