The problem and the reason

Discussion in 'Electricians' Talk' started by X, Sep 4, 2009.

  1. X

    X New Member

    Hi all

    Heres a strange one that took me a while to sort out.. just sharing the info in the hope that if you come across something similar it will save a bit of head scratching.

    2 LG HD lcd tv's in customers house

    Each one works fine on its own (using aerial or scart input).. when both switched on, then rcd trips.

    When both switched on with aerials disconnected.. working fine.

    Connect aerial to one set only .... all is still ok

    Connect aerial to other set... trip

    The tv's are class 1 psu's (with leakage to earth, not measured as I didn't want to get blamed if testing caused the psu to fail).

    So whats the aerial got to do with it ??

    It seems that the aerial connector on the tv is not at chassis potential so a few extra milliamps of leakage goes through the aerial.

    The reason.

    The chimney had 2 aerials.. (one was the neighbours) the aerial lashings were touching each other, the neighbours aerial was connected to a class 1 tv...........

    Doh !!

    The quick fix involved an isolated aerial outlet and a bit of cunning, the better fix will involve removing the existing lashing and replacing it with one that uses tape, but even better would be if LG either isolated their aerial connections or made sure that they were earthed.
     
  2. sinewave

    sinewave Screwfix Select

    Afternoon Peng!

    Long time no see! :(


    Domestic ***** eh? :'(
     
  3. JP.

    JP. Screwfix Select

    Nice one X..handy stuff M8..:)
     
  4. X

    X New Member

    Hi Guys... good to see your still about

    Yes, domestic for the quick payments...lol
     
  5. jclark

    jclark New Member

    Its to do with harmonics, I'm not certain what is happening, but if you put one of the TV sockets on an RCD socket the problem will be sorted. This Sort of thing happens normally when you have a large number of computer systems running.
     
  6. Lectrician

    Lectrician Screwfix Select

    Ok, thanks for that J.........I think.
     
  7. wally2

    wally2 New Member

    When I enquired to a TV maker about 100 v or so at the aerial socket he said it was normal, to do with the components, capacitors etc, as already mentioned, a good idea to have aerial going via an isolated outlet.
     
  8. Removed 4

    Removed 4 New Member

    How nice to see the the return of the penguin.....


    Lucia x
     
  9. Lectrician

    Lectrician Screwfix Select

    When I enquired to a TV maker about 100 v or so at
    the aerial socket he said it was normal, to do with
    the components, capacitors etc, as already mentioned,
    a good idea to have aerial going via an isolated
    outlet.

    quote^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^



    No, that is why the aerial screen is meant to be earthed, and what marshalling bars are designed for.
     
  10. JP.

    JP. Screwfix Select

    Must say always thought it was a good idea to ''whip a flyer'' from a convenient earthed socket..it goes up the AE conduit..;)

    :)
     
  11. JP.

    JP. Screwfix Select

    Must say always thought it was a good idea to ''whip a flyer'' from a convenient earthed socket..it goes up the AE conduit..;)

    :)
     
  12. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    I thought it was a no-no earthing any part of the aerial circuit??


    Earth fault on the mains could enter the aerial system in telly and blow it up(much like lightning does) :(


    Mr. HandyAndy - Really
     
  13. X

    X New Member

    aerial circuit??

    In multiple dwellings where a there is a shared aerial or satellite dish (MATV, SMATV) or IRS – Integrated Reception Systems then earthing is very important.
     
  14. wally2

    wally2 New Member

    I agree that aerial systems should be earthed but my comment reffered to cannot be earthed from the TV as this item is double insulated
     
  15. sinewave

    sinewave Screwfix Select

  16. Lectrician

    Lectrician Screwfix Select

    Isolated TV outlets are designed for older communal systems where the TV points where supplied from a central distribution amp.

    Older amps would shut down if one leg shorted or went faulty. All outlets would be lost. Fitting isolated outlets prevented someone with a dodgy TV or flylead causing the amp to shutdown and loosing reception in the hole building.

    They are not designed to prevent the floaty voltage from DI appliances on the screen of the cable.

    Most decent amps these days have an earth terminal to connect to.
     
  17. Hi X..

    Trust you are well:)



    Reggie
     
  18. X

    X New Member

    as this item is double insulated


    I also initially assumed that a new tv would be class II.
     
  19. X

    X New Member

    Reggie & Lucia

    Thank you

    Soooooooooooo now we get to the more important issue.

    We know the cause of the problem by class 1 and the ways around it,(you can blame our dear old friend sinewave for these next questions..:D)

    1. Whats going on (If for example) its a building of multiple occupancy on PME ?

    2. Is there any difference between using coax cable with copper or aluminium screen foil.


    ?
     
  20. JP.

    JP. Screwfix Select

    Hmmmm No 1 must admit have not got a clue..

    No 2..Its obviously something to do with the resistance per meter of A..Copper foil (as per PF 100 etc) and B.. Ally foil which I have never seen.

    Thus in this situation Ally foiled cable probably would have negated the fault (or the fault would not occur)?

    So the system is using PF/CT 100 or similar which is only sporadically earthed (or maybe not at all) ie the downstairs might have earthed foil but flats going upwards may or may not have the foil earthed..this causes an imbalance which can trip the RCD in the flat affected by the imbalance (pre excitation of RCD etc).

    Not saying its correct, but just a guess really.
     

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