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Time Lag Fuse Blowing

Discussion in 'General Electronics Discussion' started by Scot Hartley, Mar 27, 2015.

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  1. Scot Hartley

    Scot Hartley

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    Mar 17, 2015
    Hello.
    I have a problem on a controller circuit board.
    The power supply is 220-240v. On the primary side the supply goes through a fuse rated at 160mA, time lag, and this has 112mA running through it. There is an Inrush Current Limiter on the circuit in front of the fuse. After the fuse the power splits, 113mA go to the next board and 46mA goes to the transformer. The fuse is blowing at completely random intervals. We have had them blow after 1 hour, 1 month, and some that have never blown.
    The controller powers a compressor, condenser, fan motor and a pump. We have been advised that no component should effect this fuse, as it is on a different circuit to the components.
    I hope I have made the circuit clear!
    Any advise as to why the fuse blows so randomly and any solutions would be appreciated.
    Thanks
     
  2. Gryd3

    Gryd3

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    Jun 25, 2014
    Can you please double-check the current through the fuse?
    On one hand you say you have 112mA through it, but then you say it splits after the fuse. 46mA and 113mA is greater than the 112mA stated previously.
    Fuses are typically sized with some play room. If a device is measured to pull 160mA you should not put a 160mA fuse on it or you can get some funny results.
    Remember a fuse is just a piece of wire sized to break when handling more than the rated current. Wire is not perfect so the rated current and the real life current required to blow the fuse will always vary slightly.

    Details about the controller can help as well. ie, does it do the switching with relays. Have you measured the current draw during a switching event to see if the current varies?
     
  3. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    If it is a slo-blo fuse, it will have a dob of solder connecting the two wires. You have to work out if the solder has melted or the wire has evaporated.
     
    Arouse1973 and Gryd3 like this.
  4. Gryd3

    Gryd3

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    Jun 25, 2014
    Colin, what would each result indicate? (I've never focused on 'how' they fail... just that they fail)
     
  5. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    How it blows is the MOST IMPORTANT point.
     
  6. Gryd3

    Gryd3

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    Jun 25, 2014
    Well, if the inside of the glass is black and charred something went horribly wrong... but most I see are simply broken somewhere in the middle.
    What would the difference be between failed solder and the fuse's wire failling?
     
  7. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    I ONLY want the OP to answer my questions.
     
  8. Gryd3

    Gryd3

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    Jun 25, 2014
    This is one of the reasons people get pissed off you.
     
    garublador and (*steve*) like this.
  9. Scot Hartley

    Scot Hartley

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    Mar 17, 2015
    Thank you both for your help.
    To Gryd3. We have checked the current through the fuse and it is 112mA. it does split a fraction different though. it splits 46mA, 103mA and 8mA. we have checked these figures with the RMS calculation and they appear right.
    it does do switching through a relay. The relay switch is a zero crossing current switch and is an octo isolation triac switch.
    To Colin Mitchell. The fuses are ceramic and not clear, so a visual check is not possible. If I pull the metal end off the fuse will I be able to see the fault or will I just destroy the fuse?
    Thank you again..
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Yeah, pull the failed fuse apart, you can't destroy it more than it already is! I would probably (carefully) break the ceramic part and examine the contents.

    My bet is that you will find that there are pieces of intact wire inside. This is based on your description of how often they blow.

    You describe the current as 113mA plus 43mA -- that's pretty close to 160mA. I would have thought a higher rated fuse would have been used here. I would think 250mA or even 375mA.
     
  11. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    Get yourself a box (1,000) of glass fuses with a fine wire and dob of solder in the middle.
    I don't think you can get 160mA versions but maybe 200mA or 250mA.
    Watch the fuse as you turn on the supply after it has been off for 1 hour..
    If the wire "dips"you know you have a surge that is lasting for a long time.
    When the fuse "blows," see if the wires have separated at the solder or if the glass has "silvered."
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I think 1000 may be overkill.
     
    Tha fios agaibh likes this.
  13. Tha fios agaibh

    Tha fios agaibh

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    Aug 11, 2014
    How about using a dvm, set to peak hold on current, in line with one of your loads?
    This should catch which part of the circuit is overloading.
    I'd also check that the relay has good suppression parallel to it.
     
    Gryd3 likes this.
  14. Scot Hartley

    Scot Hartley

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    Mar 17, 2015
    Hello there, thank you the help.
    I have managed to pull a fuse apart without damaging the main parts.
    In the fuse I pulled apart I could only find some strand of soft fibres left, so I am guessing that the wire has melted completely.
    I hope this helps.
     
  15. Gryd3

    Gryd3

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    Jun 25, 2014
    If it's completely melted and not merely broken it would seem that the fuze is very under-rated.
    It's slow blow, so the measurement is off, or the peak current draw is much higher.
     
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