THE 17TH A 18TH REGS RULES WRONG WRONG

Discussion in 'Tech Talk' started by sean griffin, Nov 9, 2018.

  1. sean griffin

    sean griffin New Member

    HELLO IM SEAN GRIFFIN A NAPIT MEMBER
    PLEASE CAN ANYONE ANSWER THIS QUESTION ON ZE TESTING
    THE RULES SAY 0.8 OHMS ON TNS EARTHING SYSTEM TRUE.
    THE VOLT IS AT 230V TRUE.
    MAIN FUSE IN A HOUSE IS 100AMP FUSE TRUE.
    THE RULES STAT THE DISCONNECTION TIME ON A 100 AMP FUSE IS 5 SEC. TRUE
    TO BLOW A FUSE IS 5 X THE FUSE SIZE AND PLUS 100AMPS = 600 AMPS OF FAULT CURRENT. TRUE
    WELL AT 230V / 0.8OHMS = 287.5 AMPS OF FAULT CURRENT. TRUE
    SO THE FAULT CURRENT ON 0.8 OHMS ON A TNS SYSTEM ISNT LARGE ENOUGH TO BLOW THE FUSE IN
    ITS CORRECT TIME.
     
  2. Coloumb

    Coloumb Well-Known Member

    It looks like your using a "typical" value. In reality Ze would usually be much lower so the fuse would blow at less than 5s.

    It's really up to the supplier to get this bit right, but yea, as you say, they are under no legal obligation to maintain the Ze at any time, so it can vary. I would imagine that if the bonding is done correctly all the touch voltages would be about the same for the length of the fault. From the charts a bs1361 would take about 15 seconds to pop at 287v.
     
  3. chippie244

    chippie244 Well-Known Member

    Turn your caps lock off :)
     
  4. sean griffin

    sean griffin New Member

    Sorry dude
     
    chippie244 likes this.
  5. sean griffin

    sean griffin New Member

    had reading at 0.4 ohms the pfc is 575 amp on ze read will pass but disconnection
    on main fuse wont blow within its time, if the installation for water was in plastic piping
    there wont be much bonding done ... and if the phase line got cut into the problem still stands. tns should be lowered to 0.35ohms ish to correct this . correct ?
    18th edition page 364
     
    Last edited: Nov 11, 2018
  6. sean griffin

    sean griffin New Member

    how do you remove induction on in cables ???
     
  7. HappyHacker

    HappyHacker Member

    The supplier does not work to BS7671 they work to ESQR and the main fuse is to protect their cables not the installation.
     

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