Floating earth problem

Discussion in 'Automotive Electronics' started by kenread001, Oct 21, 2015.

  1. kenread001

    Thread Starter New Member

    Oct 21, 2015
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    I am presently constructing a diesel/electric narrowboat and have a knotty problem regarding earthing.

    I am constantly being advised to consult an electrician, but the advice received would only be as good as that one person's knowledge so I have turned to this blog in the hope of a great wealth of knowledge.

    The narrowboat contains the equivalent of normal domestic 230v 13amp ring mains (UK) to supply various equipment. There are 3 potential sources of supply, a mains shoreline connected through a galvanic isolator, a fixed onboard diesel generator and a 600watt inverter powered from the battery bank.

    All knowledgable sources state that when switching between these various sources a "break before make" switching must be employed. This is no problem as all supplies are carried through 16amp industrial sockets and plugs, which obviously have to be disconnected before reconnecting.

    The problem lies with how the earthing is arranged. Shorepower has its own PME and is no problem through the GI. The generator was designed with floating earth but this shows up in tests as reversed polarity because a earth / neutral link is not installed in the generator. Knowledgeable advice on the web states the solution is simple to change a generator from floating earth to "grounded"earth. "Put an earth /neutral link in the outlet plug, not in the generator and provide an earthing spike." The first one I can do, but not in the plug, it will have to be in the generator socket, because the plug is transferred to the shoreline source. The second, providing an earthing spike is impossible on a moving narrowboat.

    My question is, that if all power sources are "earthed" to the central earthing point on the boat side of the galvanic isolator will this provide a safe installation?
     
  2. GS3

    Senior Member

    Sep 21, 2007
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    I am having some difficulty following you. The boat has onboard generator and batterybank-inverter and the distribution wiring is connected to each source with plug and socket. So, if you want to use the genset you plug into the genset and if you want to use the inverter you plug into the inverter.

    The way I see it the genset frame should be connected to the earth pin and this way to other earthed items on the boat. And the inverter also probably has an earth lin which you should connect.

    Or am I not understanding the question correctly?
     
  3. kenread001

    Thread Starter New Member

    Oct 21, 2015
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  4. kenread001

    Thread Starter New Member

    Oct 21, 2015
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    The explanation did get rather complicated so I will try to simplify

    Shore power is obviously only used when moored in a marina and works fine through the GI on mains with PME.

    The boat has a 110v DC Lynch motor driven by a Lithium Ion battery bank, this battery bank is charged by a top of the range battery charger from the generator when running. The boat also has a full 230v 13amp ring main powered by the generator (or shore power) up to 5Kw, less what the battery charger requires.

    A small inverter was unfortunately, also required (0.6Kw) to power a CH/HW boiler and some 230v lights which may require power when the generator is switched off. This is powered by the battery bank

    Everything works fine from each individual source, but a simple polarity test on the generator returns reversed polarity, not because the generator is incorrectly wired, but because there is no earth/neutral link to balance the two circuits. The 13amp ring mains are protected by RCBO's so it is important that a cross connection is available to balance the circuits.

    To put an earth /neutral link in the generator is no problem but this is only advised in conjunction with a earthing pin to take any short circuit fault. What I am looking for is a suitable alternative to an earth pin, which cannot be provided on a moving boat. I believe joining all the earths together would be a suitable alternative but I have not yet found any confirmation of this.

    This joined earth would also include a hull bonding to the steel narrowboat shell.
     
    Last edited: Oct 21, 2015
  5. GS3

    Senior Member

    Sep 21, 2007
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    I am a boat owner and have dealt with this type of thing in the past. It is simple and complicated at the same time. I have had the experience in some marinas that using shore-side earth would eat away the protective zincs like crazy. In another marina if you swam you could feel the stray electricity. I guess some boat or boats were leaking electrons.

    The way I see it is: on a boat your bonded motor, shat, mast, etc. are your "earth". Your battery negative is connected to that. The frame of the genset is connected to that. Then the output of the genset and the inverter are initially floating. One side may be earthed internally in which case it is resolved. If not then you take one side and connect it to earth and that becomes *neutral* and the other one is "live".

    If the genset is permanently installed, besides having a protective earth connection in the output plug, the frame should be bonded to the rest. Same thing with the inverter.

    Basically you do not want loose, "floating" voltages, give everything a reference.
     
  6. kenread001

    Thread Starter New Member

    Oct 21, 2015
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    Thanks for your reply, I think we are both advocating a similar solution. With a UK steel hulled narrowboat it is not recommended to use the hull as a neutral return for electrics, therefore even the battery bank is isolated from the hull. All earthing requirements are done through a third wire which is bonded to the hull. Linking the neutral return to the earth bonding at source is required to balance the two circuits for the RCBO's to work properly.

    The Galvanic Isolator will take care of stray earth micro-currents and stop hull erosion or swimmer electrocution ( not that you should swim in UK canals, but people do fall in) So bonding the generator frame, inverter , 13amp ring mains and the hull to the boat side of the Galvanic Isolator should do the trick.
     
  7. GS3

    Senior Member

    Sep 21, 2007
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    No, the metal hull should not be used as a conductor at all but I would connect all the electrical earths in one place to the bonded hull, motor, etc. But definitely not as a conductor.

    In general you would not swim in an American marina either but sometimes you need to get in the water to do something. If I remember correctly the problem was with the earth connection coming from shore which is not isolated by the galvanic isolator. These things can get pretty tricky.
     
  8. kenread001

    Thread Starter New Member

    Oct 21, 2015
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  9. kenread001

    Thread Starter New Member

    Oct 21, 2015
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    Thanks for considering the problem, I feel more confident to bond everything together now.

    Talking about "floating" earths when related to boats can get a bit confusing.
     
  10. GS3

    Senior Member

    Sep 21, 2007
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    Another possibility, especially with 230 volts, is to center tap the supply so that instead of having a "neutral" and a live wire at 230 Vac you would have two live wires at 115 volts which considerably lowers the max voltage. In other words, what they do for residential supply in America. Water and high voltages are best kept separate as much as possible.

    You probably do not have that option with the genset though.

    With my boats I avoid leaving the shore power connected permanently and only use it when need it. In some marinas the zincs go like crazy. We tend to think that "earth" is all at the same voltage but this is not true and, for different reasons, natural and man-made, there can be a significant voltage gradient over distance.

    My boat is plastic but in any boat I would want everything electrically passive bonded, including the metal hull, and that would be the boat's "earth". This is, IMHO, also the best protection against lightning and sparks.
     
  11. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    I'm only going to comment. I think the issue is really shore power, however, even with a shore isolator, do you really have isolation?

    it would seem that when on "shore power", you would want to use "shore ground".

    Your generator really has to have the neutral and line tied together at whatever you decide is common.

    So, the "in water" reference has to be different and not connected to the "shore reference".
    I don;t see any reason why you can't move the reference(s) when docked.

    "On shore power" might be able to do that automatically.

    I don;t see any reason why the "on shore reference" can't be connected to the "boat hull reference", but the path has to be "different" The "boat hull" essentially can act as another "ground rod". e.g. connected to the shore reference". All of the the other system have to tie to shore reference when docked (shore power).

    Think of it as:
    Not on shore power
    Hull is reference for inverter
    Hull is reference to generator

    On shore power
    Shore reference is reference to inverter
    Shore reference is reference to generator
    Hull connected to shore reference.

    Then no ground currents. No ground loops.
     
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