Diagnosing issue with an appliance - blown fuse and tripped breaker

Discussion in 'Technical Repair' started by balfazar, Mar 28, 2018.

  1. balfazar

    Thread Starter New Member

    Mar 28, 2018
    2
    0
    Hi all,

    Hoping someone may be able to shed some light on what might be going on based on the following results:

    1. Have an appliance with universal power supply, attempting to run it on 240v mains (Australia)
    2. Upon plugging into live socket (before switching on appliance), after several seconds, fuse in appliance blows and circuit breaker trips (combined MCD and RCB breaker)
    3. Above has happened multiple times in a row with different units of same appliance, different sockets on different circuits and replacement fuses

    Manufacturer points to oversupply or poor grounding in my mains - seems unlikely as no issues with other appliances

    I point to faulty appliance - but seems unlikely as manufacturer plausibly claims that numerous units are running successfully on 240v mains elsewhere, and I've had the same issue with the original unit and a replacement.

    Who do you think is right and why?
     
  2. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    What is this mystery appliance?
     
  3. balfazar

    Thread Starter New Member

    Mar 28, 2018
    2
    0
    Food processor, essentially a DC motor and a control board.
     
  4. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    7,595
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    Make/Model?
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    17,926
    5,478
    Is the appliance fed from 2 wire or 3 wire (GND).
    If it is 3 wire, you could try it with the earth wire open circuit and see if it trips, (earth leakage).
    Just ensure you do not touch the metal parts of the appliance when testing this.
    Some SMPS power supplies have suppression components connected to earth gnd.
    Max.
     
  6. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    811
    The fact that the internal fuse blows leads me to suspect that there may be a transient suppressor, typically a metal oxide varistor (MOV), across the line and either it is rated for insufficient continuous RMS voltage or your line voltage is unusually high.

    "Accepted practice" for universal input switchers is to design for nominal AC voltage of 100 to 240 RMS with limits of 85 and 264 volts.

    As max wrote, if a switcher has a ground wire it is nearly universal that radio frequency interference suppression capacitors will be connected from each side of the line to ground, but I would expect a breaker that detected ground current to trip nearly instantly, since they are rather useless if they don't. Normally the capacitance is limited by safety regulations to a value that will not cause lethal shock if the ground connection is broken and replaced with a human.
     
  7. MisterBill2

    Distinguished Member

    Jan 23, 2018
    3,220
    631
    The obvious question that is missing is about a voltage selector switch. If there is one then it must be set correctly. Then the next question is does the motor turn freely? A bound up motor will certainly trip the protection system.
     
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