Question about a blown fuse

Discussion in 'General Electronics Chat' started by fredz0003, Jun 15, 2012.

  1. fredz0003

    Thread Starter New Member

    Mar 22, 2012
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    I have a circuit where a transformer takes 480v and then the secondary is 120V
    from there two circuits branch out, one is a control circuit bunch of relays and buttons that control two motors with a 1A fuse, the other circuit is a florescent light and a fan which I just installed that had a 5A fuse for the circuit, problem is the specification sheet said to run the fan in a 15A circuit I advised my manager and he said it would be ok... I turn it on and the 1A on the other branched circuit was blown, he instructed me to put two 6A fuses, which I didn't agreed to but I did anyways and made a note on my work order so if anything burns later down the road I cover my arse. Question is why is the fuse getting blown, on the other 1A circuit, and not the 5A on the fan circuit? After disconnecting the fan and installing another 1A everything seems to work fine. Any suggestions? :)
     
  2. gerty

    AAC Fanatic!

    Aug 30, 2007
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    What is the transformer rated for (how many kva).What does the nameplate on the fan say for amps?
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    And any of the other hundred variables we have no idea about...
     
  4. fredz0003

    Thread Starter New Member

    Mar 22, 2012
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    Transformer 750 Kva
    Fan 2.9 Amps sorry for not including this earlier.
     
  5. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    I'm going to assume your trans is a 750VA, and not a KVA. Your load is pulling down your voltage, which increase current through inductive loads. Your control side with 'a bunch of relays' is probably border line already.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    1A does seem a little low for whatever a "bunch of relays and buttons" is..
    I'd start with a fuse size roughly 125% of maximum required current for that branch.
     
  7. fredz0003

    Thread Starter New Member

    Mar 22, 2012
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    Yeah 750VA, I will check voltage drop then... Thanks to all for the input
     
  8. #12

    Expert

    Nov 30, 2010
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    I tend to agree with the manager. I install air conditioners with 40 VA transformers in them (24 V, 1.6666 A) and they come with a 5 amp fuse! One amp is a very stingy fuse, considering you can't possibly be using wire that needs to be protected at 1 amp.

    Your 750VA transformer can carry 5 amps so I would tend toward a pair of 5's.

    The only reason the spec sheet said, "15 amp circuit" is that is usually the smallest residential circuit for 120V.
     
  9. fredz0003

    Thread Starter New Member

    Mar 22, 2012
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    Well these are industrial fans, but funny thing is that there's a voltage drop I measured from 120V to 111V when starting the fan, but I got 4 fans running right now with 1A fuse on the control circuit and 6A on the fan circuit. Only one electric box when I turn on the fan the control circuit 1A blows, the 1A fuse it was designed like that by the conveyor systems company, but reading the schematic they only intended to put a 1A FL fan, not 2.9A fan. Anyways I will try to upload schematic when I get home to get a better sense of the circuit. Thanks again.

    On a side note we had a the first fan installed a year ago, and it lasted that long without any problems, until it blew the 5A fuse, that's why my manager and the master tech opted for the 6A, but I had the understanding that you can't go higher than 5A on that transformer.
     
  10. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    Protecting and loading your trans are different things. Continous loads must fall within the VA rating, protection is typically 1.25 times rating. Imagine the load on your control branch. It is unlikely that all the relays pull in together, so your current draw doesn't see the inrush of all relays together. However, if you overdraw the trans with additional loading to the point where a voltage drop allows your relays to drop out, then at that point, you are in effect attempting to pull in all your relays at once, resulting in higher than designed currents. It's not much different in a motor circuit, where, unless multiple motors are started together, I can design the circuit based on the largest start current only, adding only running current for the others. A line voltage drop however would have all motors struggling to maintain speed, resulting in much higher currents on the branch.
     
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