50amp/220V reverse polarity

Discussion in 'OnBoard Electronics & Controls' started by Larry Hayden, Feb 17, 2009.

  1. robherc
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    robherc Designer/Hobbyist

    I agree with Alan 98% here.

    The one (and only) thing I disagree on, is that 240V (split-phase) equipment, IMHO, should work regardless of which "hot" wire goes to which "hot" receptacle. You have 2 leads that are 110v away from ground; 60 times per second, their polarities are switching back-and-forth between the two...so which one you grab first, should make no difference. That said, the deadly important part is that both "hot" wires go to "hot" receptacles, and not to "neutral" or "ground" as both of those should be at 0v, and could produce a 120v short, or cause the same problem as is caused by reversed-polarity 120v sockets.
     
  2. alan white
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    alan white Senior Member

    There may be a misunderstanding. The test I described was for 120 volt outlets. I mentioned that aboard a boat, 240 volt equipment would likely be hard-wired. I didn't describe a test for a 240 volt recepticle. In testing any 240 volt recepticle, as you say, there are two hot legs instead of only one. Their phasing is opposite and their polarities switch at 60 hz, so they can be considered reversable in terms of wiring them up.
    A test between the two hot slots will show 240 volts on the meter. Then between either of those slots and either ground or neutral will show 120 volts.
    However, while a 120 volt outlet (or system or circuit) can be hooked up in reverse and hence be very dangerous aboard a boat, a 240 volt outlet reversed will not have any effect on operation or safety (unless the ground or neutral has been reversed with one or both of the hot legs).
    But this, I think, is different from the question originally discussed. I'm sure the polarity reversal had to do with the two slots of the 120 volt outlets, which is easy for an inexperienced owner to reverse because the equipment involved will still work perfectly. One could go for years with reversed polarity and not even know it.
     
  3. robherc
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    robherc Designer/Hobbyist

    OK, thanks for the clarification, Alan.
    ...From my first reading of your post, I thought you were implying that 240v equipment would "simply not work" if you switched the two hot leads...now I can agree with you 100%. :cool:
     
  4. alan white
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    alan white Senior Member

    Nope. The test was only for 120 volt outlets. Test a few of them to be certain, as described, and a VOM is all youi need, like the simple ones available for $10.00 at any home center.
    As said, the danger in hooking up to 120 volt outlets is the reversability of the two sides---- black is hot and neutral is white, and black is screwed to the brass screw, white to the chrome or nickel-plated screw.
    Also the prongs of a plug (especially if there's no ground or it broke off) can be an equal danger, since the item being powered still acts the same. An appliance or tool or whatever is usually wired so that the hot incoming line is immediately switched as it enters, but when reversed, the whole neutral side of the circuit (most all of the internal wiring) is still live even when the switch is turned off. That means the user is lulled into a false sense of security. The chance of accidental electricution is far greater as a result.
     
  5. TedZ
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    TedZ Junior Member

    Reverse wired 120 VAC circuits, no matter what they are, should be tripping GFI's from the boat to the power house! Really, the GFI's on the boat, the dock and even the main GFI breakers should be tripping. If not, or no GFI's, that is a bad circuit desgn or implementation. Nuisance tripping of GFI's with even the slightest leak to ground on anything with a grounded case is a very common occurance as we all know too well.

    If no GFI's you are usually certain to fry TV's, stereos etc. when you put 120 VAC on their neutral/ground. Smoke usually ensues.

    Ted
     
  6. alan white
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    alan white Senior Member

    The moral of the story is you should check to make sure your boat has GFIs even if the marina or power source is supposed to have them. Also do a polarity check (voltage and continuity tests as described).
    The marina, in this case, is requiring a polarity test the manufacturer says is not necessary.
    The marina, or many marinas in the past, have had their share of problems due to badly wired boats, and they obviously have a blanket policy whether a boat is new or not to make damned sure.
     
  7. CDK
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    CDK retired engineer

    A GFI will only trip if there is an unbalance between the power leads. It doesn't matter whether there are 2 live wires or just one and a neutral; also the potential between the power leads and ground is of no importance. If you 'lift' the mains voltage say 1000 volts above ground, maintaining 120 vac between them, the GFI will not trip as long as the installation has been properly done, using 3 separate wires for phase, neutral and ground.

    Only if more leakage to ground exists than the threshold of the GFI (5 or 25 mA), the device opens.
     
  8. TedZ
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    TedZ Junior Member

    Maybe i am not understanding the above correctly. A 240 VAC GFI will not trip if there is a difference between the power leads i.e. each hot lead. If that were the case they would trip all of the time as we do not draw on those legs equally. In each case, 240 VAC or 120 VAC they trip when there is an unbalance in the conductors. What does that mean? Simply if you are powering something any short including you, will change the ampacity in the conductors hot and neutral, briefly. The GFI measures that difference in milleamps in the conductors with a solid state comparator and as pointed out at 5 milleamp DIFFERENTIAL in the conductors - caused by the short, it trips. The comparison is done in milleseconds as does the trip. You can feel a shock but it is 99.99% of the time non-lethal.

    Hope this helps.

    Ted
     

  9. Jack Daniels Eq
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    Jack Daniels Eq Shockwave

    Tell the marina wonk to sort it out - then call ya lawyers
    BR>Jack
     
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