Gen - floating earth

Discussion in 'Motorhome Chat' started by Glynn, Jan 27, 2012.

  1. Glynn

    Glynn Read Only Funster

    Joined:
    May 2, 2010
    Messages:
    163
    Likes Received:
    18
    Location:
    Lancashire
    I have a small generator & if it's connected via the EHU point the reverse polarity light is on
    I believe it's to do with the generator having a "floating earth " rather than a bonder neutral/earth. What does it really all mean i.e. is it OK to use in this manner for limited use as I would prefer to connect through the EHU rather than clip to the batteries for charging .
    The Autotrail book says this does happen with generators but doesn't say if it's a bad thing...
     
  2. pappajohn

    pappajohn Funster Life Member

    Joined:
    Aug 26, 2007
    Messages:
    29,440
    Likes Received:
    16,867
    Location:
    YO11 2BD
    bit complicated to explain....

    many generators have a centre tap earth in the windings which means the earth is actually connected to the 0v position of the wave form.....ie: 120v+ on one side of the 0v and 120v- on the other side of the 0v

    imagine a figure S with a centre line through the middle like so... [STRIKE] S [/STRIKE]

    the centre line has no volts so the charger sees the wrong polarity 50% of the time....120v+ 0v 120v-.
    normally it would see 120v+ and 120v- for each half rotation of the rotor with 120v- and 120v+ on the next half rotation, with the earth being seperate from the windings.

    not really much you can do about it except possible driving in an earth spike to the ground and connecting the gennie casing to that with a thick (10mmsq) wire which may or may not do anything constructive (never had the urge to try it) but reversed polarity shouldn't cause a problem.

    sorry you asked now ? :Laughing:
     
  3. Gooney

    Gooney Funster

    Joined:
    Jun 2, 2010
    Messages:
    4,758
    Likes Received:
    7,771
    Location:
    Flintshire
    Thanks for tha papps, you're definately wasted, you should be a lecturer!!:thumb:
     
  4. wivvy's dad

    wivvy's dad Read Only Funster

    Joined:
    Jun 22, 2010
    Messages:
    3,218
    Likes Received:
    1,807
    Location:
    48.066895,12.862673
    I don't get reversed polarity - I get the NO EARTH light from my plug-in polarity tester when my Honda 1k genny is patched into the EHU.

    I just make sure I'm wearing my Crocs................:BigGrin:
     
  5. pappajohn

    pappajohn Funster Life Member

    Joined:
    Aug 26, 2007
    Messages:
    29,440
    Likes Received:
    16,867
    Location:
    YO11 2BD
    that's because the earth in a gennie is actually connected to neutral internally.....0v

    the earth in your house is also connected to neutral, but at the power/sub station, and gets back to the station via the ground
     
  6. Stephen & Jeannie

    Stephen & Jeannie Read Only Funster

    Joined:
    Aug 27, 2008
    Messages:
    4,198
    Likes Received:
    3,261
    Location:
    Gobowen near Oswestry !!
    I mentioned this to Dave Newell (the fount of all motor home knowledge !) He said not to worry it's perfectly normal !!:thumb::thumb:
     
  7. jonandshell

    jonandshell Funster

    Joined:
    Dec 12, 2010
    Messages:
    5,251
    Likes Received:
    7,858
    Location:
    West Norfolk
    You get the same sort of indications from a socket tester when the sockets are being run from an inverter too, if anyone is interested!:BigGrin:
     
  8. ubuntu1

    ubuntu1 Read Only Funster

    Joined:
    Feb 13, 2008
    Messages:
    155
    Likes Received:
    44
    Location:
    Derbyshire
    no earth

    Its just the difference in the way a gennie is wired compared to a powerstation! There are ways around the problem and most service engineers that carry generators have a "special" hook up lead which simulates what happens in a powerstation. I'm not going to tell you how to wire it as I don't want to be responsible for electrocuting members of the site!!!!

    But don't worry what your seeing is normal.
     
  9. Glynn

    Glynn Read Only Funster

    Joined:
    May 2, 2010
    Messages:
    163
    Likes Received:
    18
    Location:
    Lancashire
    Thanks for that folks, I will just use it via the EHU inlet then.
    It's really only a just in case scenario anyway.
     
Loading...
Similar Threads
  1. Hughman
    Replies:
    7
    Views:
    365

Share This Page