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SIMPLE FUSE QUESTION

Discussion in 'General Electronics Discussion' started by Mustwin351, Jan 19, 2016.

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  1. Mustwin351

    Mustwin351

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    Apr 10, 2013
    Why do Christmas light strings have two fuses? Why not just one?
     
  2. Tha fios agaibh

    Tha fios agaibh

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    707
    Aug 11, 2014
    Because there are probably 2 individual strings (branches) of lights. Something like 30 lamps on each circuit.
     
  3. Mustwin351

    Mustwin351

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    Apr 10, 2013
    No that can't be it...but maybe because the plugs are not polarized they use two fuses so the hot conductor is always fused?
     
  4. Tha fios agaibh

    Tha fios agaibh

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    707
    Aug 11, 2014
    Could be. But Christmas lights often have 3 wires which likely means they are wired series and parallel. In other words 30 or so lamps in series and another string paralleled to that, totaling 60.

    Heck, Pull one of the fuses out and see what happens.
     
  5. Mustwin351

    Mustwin351

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    Apr 10, 2013
    On the male end of the plug I have you only have two conductors coming out of it. So pulling either one of the fuses causes an open circuit for the whole string. After the first lamp you then have 3 wires for the series parallel circuit.
     
  6. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    Your probably right about fusing either side because non polarized plug.
    Course it could be wired a number of ways. The 3rd wire could also be for feed thru power to the receptacle on the opposite end.
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    But there's no guarantee that in case of a short the fuse in the hot wire melts. There is an equal chance that the fuse in the neutral wire melts. In the latter case there will still be dangerous potential on the lamp strings.

    A good design doesn't use two fuses of the same rating in series. It is common to have a staggered arrangement of fuses with the higher rating nearer to the power source, the lower rating nearer to the load.
     
  8. duke37

    duke37

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    Jan 9, 2011
    In the UK two fuses must not be used as reasoned by Harald.
    I do not know the regulations in Outer Mongolia.
     
  9. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    In US it is illegal to fuse the neutral line per the NEC. But individual products or appliances like this are governed by the UL or a similar agency, where each product is reviewed and tested in a laboratory to meet safety requirements.

    In modern Christmas lights, both L1 (hot) and L2 are both spaced an equal distance from each other and the surface, so the danger of having exposed conductors is not as severe as it would be in a lamp holder like an Edison socket.
    It should also be noted that a danger can exist by being exposed to the neutral conductor also.
     
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