Mains AC Voltage Polarity Poser

Discussion in 'The Projects Forum' started by GARYN, Apr 24, 2010.

  1. GARYN

    Thread Starter Member

    Sep 4, 2009
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    Hello i am an electrical/mechanical engineer in a food production factory with limited electronics knowledge.....
    I just fixed a fault on an external lighting circuit (sodium fittings) which when one of the fittings (fitted by previous elec.) was reconnected would pop the circuit fuse. I expected to find water in the fitting or a blown choke, but all was good. The only problem i found was the mains was connected the wrong way i.e L to N and vice versa, (both cables were black).
    This is where your electronic knowledge would be helpful, i know that if you connect a heater coil for example the polarity does not effect the function. I was wondering on AC circuits in general not just the above fittings what electronic components when added to the circuit can pop the fuse if the polarity is wrong and why. For example is it the igniter module causing the fuse to blow in the lightfitting and why, as i cant see the polarity having any effect on the choke or lamp.....
    I am sure this is obvious to you experts but any explanations would be appreciated..... Thanks in advance..:confused:
     
  2. Bychon

    Member

    Mar 12, 2010
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    All components (or none of them) will pop the circuit breaker when the hot line (L) is connected to neutral. Neutral is ground. Connecting the hot wire to ground is a short circuit.

    The lamp doesn't care about polarity (as you correctly said). The short is not going through the lamp circuit.
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
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    If there is gfci on the breaker, when it sees more than ~4ma in neutral, it will trip. You may be getting this after the igniter cuts off.
     
  4. GARYN

    Thread Starter Member

    Sep 4, 2009
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    Thanks for the quik response but there is no Neutral to Ground path in the fitting, this is only at the soure so if the Live wire is connected to the N terminal and vice versa as in a heater for example the it would be fine so in the situation with the lamp fitting it must be something to do with the electronics in the igniter..... or am i missing somthing obvious? Many Thanks
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
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    The neutral is connected to the ground in the breaker box, not in the light.
    Are these lights 208v by chance? If so, you will have 2 hot wires.

    If you can switch this fixture with a working one to see if it is the fixture. I diode may have blown closed.

    If the power was backwards and the breakdown voltage of a diode was exceed, If may have failed shorted.
     
  6. Bychon

    Member

    Mar 12, 2010
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    Lets see now...the only thing you did to correct the problem sucessfully was to reverse the way the power wires were connected. Try putting the power wires back the way they were and disconnecting the igniter and ballast. That will remove the lamp circuit. If it does not pop the breaker, you have found a sodium fixture that cares about polarity.
     
  7. retched

    AAC Fanatic!

    Dec 5, 2009
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    Metal arc needs proper polarity.
     
  8. retched

    AAC Fanatic!

    Dec 5, 2009
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    There are cathode and anode in the arc chamber. The mercury sits on bottom, unless there mounted on side, then there should be a mercury hold divot. They need proper polarity.
     
  9. GARYN

    Thread Starter Member

    Sep 4, 2009
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    Hello i am sorry if my previous post was confusing but i was trying to say as you stated the Neutral to Earth connection is in the breaker box not the fitting. That is why as i said if the mains polarity was wrong at the fitting why does the lamp blow fuses unlike a piece of eqipment not containing electronic components i.e as stated before, a heater coil, which would work fine.

    The supply is a domestic UK mains 240v live and neutral.

    The fitting works fine when mains polarity connected correctly and only blows a fuse when reveresed, so what electronic components would cause this? thanks again
     
  10. retched

    AAC Fanatic!

    Dec 5, 2009
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    The igniter would.
     
  11. GARYN

    Thread Starter Member

    Sep 4, 2009
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    Thank you Retched, out of interest could you possiby give me a brief explanation as to why, Many Thanks
     
  12. retched

    AAC Fanatic!

    Dec 5, 2009
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    The igniter charges and draws alot of current to arc the bulb. It is trying to draw the juice from the neutral, not hot. The breaker senses voltage on the neutral side and trips.
     
  13. Bychon

    Member

    Mar 12, 2010
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    Here's a drawing.
     
  14. Bychon

    Member

    Mar 12, 2010
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    Let me know how this turns out.
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    I'd say the igniter was failing; internally shorting.

    You might get by with it for awhile since you swapped hot and neutral, but the problem will return after awhile.
     
  16. Bychon

    Member

    Mar 12, 2010
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    I'd say I've owned 2 of these, and maintained them for 30 years, and the ballast inductor is used to isolate the igniter from the mains. Otherwise, the high speed igniter pulse would just be swallowd up by the low impedance of the mains. If the igniter completely shorts out, the only thing that happens is that the starting (igniting) pulse isn't produced and the lamp doesn't start. The mains power just keeps happily flowing through the ballast coil at the rated current.

    Of course, this is only from my personal experience.
     
  17. eblc1388

    Senior Member

    Nov 28, 2008
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    I agree with Sgt there are problems with the lamp. You should have it check out properly by a certified electrician.

    If swapping live & neutral would make such a difference then it will be the case where there are a third wire connection to the lamp fixture, e.g. earth. that plays a part in this setup and current is flowing in this wire for some reason.

    Very likely that you have an insulation failure to earth somewhere in the circuit or inside the ballast winding.
     
  18. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    your ignitor is designed around a certain voltage. Hooking the lines in reverse applies incorrect voltages to the ignitor via the choke. It may or may not injure the ignitor, but be prepared to replace.
     
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