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Unfused spur.

Zoomup
Hello All,


Could an unfused R.C.D. protected spur, from a complient ring final circuit, supplying a single outdoor socket via 1.5mm2 6242Y cable of max. length 300mm through a brick wall from an indoor socket outlet be considered compliant?


Z.
  • 0
    It is satisfactory Andy, 1 mm2 and 1kA is safe for 13 milliseconds to 70 degrees. A B32 will easily break that. This ring must have a PFC of a few hundred amps max, and the final temperature as long as it is less than about 500C, may damage things but will not fail. Old rings may have 1mm cpcs, and these do not fail, now you know why! The RCD will probably trip too in typically 20ms at such a current. It is very safe. Z can do a loop PSCC to Earth test at the new socket, the result will be interesting.
  • 0
    It is satisfactory Andy, 1 mm2 and 1kA is safe for 13 milliseconds to 70 degrees. A B32 will easily break that.

    Not according to BS EN 60898 (or BS EN 61009) as far as I can tell. It seems a 6kA rated B32 can have an energy let-through of anything up to 45,000 A²s - even a 3kA device is permitted up to 18,000 - whereas a 1mm² conductor in a PVC cable (final temperature 160 degrees, k=115) can only withstand 13,225 A²s. As I say, by the book, some devices may well be faster in practice of course, and cables won't always be at their maximum operating temperature when the fault occurs, but it's hard for the likes of us to reliably determine that.  There must have been a a good reason we stopped using 1mm² c.p.c.s in 2.5mm² T&E after all - most manufacturers don't back changes that'll increase costs for no benefit.

     
    Hi Andy, what range do you mean by moderate to high, thanks :)

    I'd say above around 575A - on the basis that above that the 40ms a 30mA RCD should open within at that current may let through (I²t) more than the 13,225 A²s the 1mm² can officially withstand.


      - Andy.
  • 0
    If the RA is about 200 Ohms an L to E fault current is restricted in magnitude. And the 30 mA  R.C.D. is still very swift in disconnecting the fault. 


    Z.
  • 0
    wasn`t the concern that someone did the sums for a BS3036 rewireable fuse using the 1.0 cpc and in some instances might be found wanting?
  • 0
    Don't worry Andy, most, if not all, Jet Washers don't have any exposed-conductive-parts.

    How to Fix a Jet Pressure Washer - AC Motor Common Fault - Bing video


    Z.
  • 0
    ebee:

    wasn`t the concern that someone did the sums for a BS3036 rewireable fuse using the 1.0 cpc and in some instances might be found wanting?


    And long before R.C.D.s were in use as well. So the earth fault could linger and cook the C.P.C. and insulation.


    Z.


  • 0
    Zoomup:

    If the Ra is about 200 Ohms an L to E fault current is restricted in magnitude. And the 30 mA  R.C.D. is still very swift in disconnecting the fault. 


    Z.


    If the earth fault current is limited then I agree there's far less of an issue. Ra alone doesn't guarantee that though - bonding to metallic water or gas supplies can re-introduce a low impedance path (via neighbouring TN installations), so potentially bring prospective earth fault currents back up to TN levels.


       - Andy.


  • 0
    AJJewsbury:
    Zoomup:

    If the Ra is about 200 Ohms an L to E fault current is restricted in magnitude. And the 30 mA  R.C.D. is still very swift in disconnecting the fault. 


    Z.


    If the earth fault current is limited then I agree there's far less of an issue. Ra alone doesn't guarantee that though - bonding to metallic water or gas supplies can re-introduce a low impedance path (via neighbouring TN installations), so potentially bring prospective earth fault currents back up to TN levels.


       - Andy.




    Yes I suppose that the water pipe, if metal, could affect things. I did not check to see if it was plastic or metal. But I know that there is no mains gas as the owner said that he used bottled gas. He could not have a big under-lawn gas tank installed as the high water table would make it float and it could not be installed.


    Z.


  • 0
    AJJewsbury:
    Zoomup:

    If the Ra is about 200 Ohms an L to E fault current is restricted in magnitude. And the 30 mA  R.C.D. is still very swift in disconnecting the fault. 


    Z.


    If the earth fault current is limited then I agree there's far less of an issue. Ra alone doesn't guarantee that though - bonding to metallic water or gas supplies can re-introduce a low impedance path (via neighbouring TN installations), so potentially bring prospective earth fault currents back up to TN levels.


       - Andy.




    So, if a substantial  current flowed L to E at the plugged in appliance, which would open first, the 13 Amp plug fuse or the 1.5 C.P.C., which if used as a fuse wire link is rated at about 50 Amps?


    Z


  • 0
    So, if a substantial  current flowed L to E at the plugged in appliance, which would open first, the 13 Amp plug fuse or the 1.5 C.P.C., which if used as a fuse wire link is rated at about 50 Amps?

    Off the top of my head, yes I'm sure a 13A fuse would fully protect a 1.5mm² c.p.c. - more than likely the 1.0mm² one in the 1.5 T&E too - but as this is an unfused spur that doesn't really help for faults before the socket.

       - Andy.

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