How rto Find PFC using Value ZE

Discussion in 'Electricians' Talk' started by lsuk, Aug 1, 2007.

  1. lsuk

    lsuk New Member

    hello,

    whats the method to find the value of the PFC if the Ze is of a certain value,

    much appreciated


    reagrds
     
  2. Squarewave

    Squarewave New Member

    Well, if PFC is V over Ze, then.....think!? ;)
     
  3. BusterGonad

    BusterGonad New Member

    hello,

    whats the method to find the value of the PFC if the
    Ze is of a certain value,

    much appreciated


    reagrds

    Why would you want to do it this way?
     
  4. sparky Si-Fi

    sparky Si-Fi Screwfix Select

    hello,

    whats the method to find the value of the PFC if the
    Ze is of a certain value,

    much appreciated


    reagrds

    try Ohms law ??
     
  5. Morris Minor

    Morris Minor New Member

    V
    <u>_</u>
    I =
    R

    If you have any knowledge, think about that formulae.
     
  6. Morris Minor

    Morris Minor New Member

    OOps....Didnt print properley. Just think I equals V over R, and give it a bit off thought.

    Must admit we use this method sometimes.
     
  7. sparky Si-Fi

    sparky Si-Fi Screwfix Select

  8. seneca

    seneca Screwfix Select

    Just divide the volts by the Ze.
     
  9. ban-all-sheds

    ban-all-sheds New Member

    OOps....Didnt print properley.


    Of course you have to measure R somehow, as it isn't Ze...
     
  10. Squarewave

    Squarewave New Member

    OOps....Didnt print properley.


    Of course you have to measure R somehow, as it isn't
    Ze...

    Yes it is.

    You'd do I = V/R,

    which is the same as PFC = V/Ze.

    I know R is resistance and Z is impedence, but that is the formlae and it's the calculation a tester makes when taking a PFC reading.
     
  11. ban-all-sheds

    ban-all-sheds New Member

    Funny - I always thought it measured the P-N loop resistance, and calculated PFC from that.

    Difficult to see how with a P-E fault, i.e. through a loop with a value of Ze, you can get much more than 1kA, let alone the several '000, or even 10's of '000s that are typically seen as PFC values.

    I wonder why they make breakers rated at 6kA, 10kA etc....
     
  12. Squarewave

    Squarewave New Member

    Look at the guys origional post, he's not talking about P-N.

    He just wants PFC from a Ze value, so, he's obviously aware that in his case, the PEFC shall be higher than PSCC, hence PFC.
     
  13. wklivesvtime

    wklivesvtime New Member

    Bas
    I thought pfc was impedance in the p-e loop and pssc was the p-n loop.
     
  14. Squarewave

    Squarewave New Member

    Difficult to see how with a P-E fault, i.e. through a
    loop with a value of Ze, you can get much more than
    1kA, let alone the several '000, or even 10's of
    '000s that are typically seen as PFC values.

    I wonder why they make breakers rated at 6kA, 10kA
    etc....

    Oh, and you've obviously not done much testing to have said the above also. Most cases, P-E is higher than P-N, giving PFC of over 1KA.

    Try doing a Ze and seeing what real life values are. Most TNS come out at 0.2 or 0.3 even though the max is 0.8, it very rarely is that high.

    So 0.2 or 0.3 over your 230v or 240.....

    Now can you see how you can get more than 1KA?
     
  15. Lectrician

    Lectrician Screwfix Select

    Talking PSC, prospective short circuit, on a TP install, you should test between phases as this is likely to be the highest reading. If your meter is not capable of working at 415v, double your P to N reading.

    You should always test PSC and PFC and record the highest. You may find that there is a higher reading from L to N (PSC) than there is from L to E (PFC) or visa versa.
     
  16. Joelp1

    Joelp1 New Member

    Ze as i usually measure it is normally 0.15 to 0.2, giving a fault current of 1.5kA max. Hence 6kA is a nice margin to the max fault current of the breakers.

    Must say i rarely bother measuring PSC or P-N resistance, my bad :( I would expect it to be a lower resistance/higher PSC current though, what with the only real difference being the N or E cable size inside the house?
     
  17. Squarewave

    Squarewave New Member

    PSCC is usually lower.
     
  18. Pompey Sparks

    Pompey Sparks New Member

    'PSCC is usually lower'

    Not on a three-phase system...
     
  19. Squarewave

    Squarewave New Member

    Did the OP say it is three phase?
     
  20. Pompey Sparks

    Pompey Sparks New Member

    Sorry, thought it was a general statement
     

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