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Fuse in neutral blows when machine turned off

Discussion in 'Electrics UK' started by vsynth, 9 Apr 2011.

  1. vsynth

    vsynth

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    Hi all, a strange fault for you I hope, something to get those little grey cells working ;)

    A PLC power supply that has a 2A fuse in the live and a 2A fuse in the neutral...an Italian design.

    Sometimes when the machine is turned off the 2A fuse in the neutral will blow.

    Anyone come across this before?
     
  2. plugwash

    plugwash

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    IF

    the switch is double pole and
    the switch is turning off stuff other than the PLC PSU and
    the switch happens to sometimes break neutral before live and
    there is a neutral to earth fault in the PLC PSU

    then it would explain the neutral fuse to the PLC PSU blowing.

    That is the only explanation I could come up with for why it's always the neutral fuse...
     
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  4. KingboyD

    KingboyD

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    When the Italians design electrics this happens!

    [​IMG]


    Draw your own conclusions :p
     
  5. bernardgreen

    bernardgreen

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    Two possible causes

    [1] current flowing in or out of the filter capacitor between neutral and earth / ground as the PSU shuts down. Neutral and earth being connected together in the mains supply network means there is a circuit for the capacitors to discharge

    If there is pre-regulator in the PSU ( 300 + volts DC ) then this may dump via the neutral when the mains live is removed and the regulator is shutting down the outputs in a controlled way.

    [2] The neutral in the PSU is connected to or has a low impedance path to the 0 volt of the field wiring thus creating a low impedance path between supply neutral and earth / ground. This could be intentional or a wring fault which does not noticably affect operation of the equipment. That connection would then be a route for current from a potential on the neutral of an un-balanced 3 phase supply to ground.
     
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