Neutral ground bonding relay????

Discussion in 'OnBoard Electronics & Controls' started by cgoodwin, Feb 20, 2015.

  1. cgoodwin
    Joined: Mar 2011
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    cgoodwin Junior Member

    Phil,
    The issue is that the relays are not in the box. There is one single relay and it is not enough. The issue is that you can not have multiple grounds in the system, so since you already have to have a single ground point and when on inverter or gen the neutral and ground are tied, you need to eliminate that when on shore.
     
  2. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    You are mistaken here. Breaking the neutral to the genset in the inverter is sufficient when running on shore power. Breaking the neutral to the shore in the inverter is sufficient when running the genset. Plus, you always have a shore ground, even if all breakers are off or a component is removed for maintenance.

    Trying to break the ground is a bad idea unless you are trying to run isolated multiplexed circuits (ie half the boat on genset and half on shore power dial-a-source fashion). The inverter you have is auto syncing and runs as a booster to either AC source if necessary (and so programmed). So you don't multiplex these, you just let it boost when needed. The main thing to be aware of is that the maximum current is potentially the sum of the genset plus the inverter, subject to some hardware and software (user selectable) limits.

    There seems to be a problem in several posters heads that don't quite get the fact that breaking the neutral, just the neutral, breaks the neutral to ground bond. On the source side of the open genset contacts, the neutral is at ground. On the distribution side of the open genset contacts, the neutral has to find a different way to get to ground, such as at the pedestal via the closed shore contacts. If you break the ground at those contacts, your chassis are not grounded when switching or when performing maintenance, and that is a bad thing.
     
    Last edited: Mar 6, 2015
  3. cgoodwin
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    cgoodwin Junior Member

    Neutral and ground are bonded in both the gen set and inverter, if you are on shore power then you create a ground loop, will blow all the GFI's and cause galvanic corrosion....
     
  4. cgoodwin
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    cgoodwin Junior Member

    In addition the neutrals of the inputs of the SW4024 are common since it wants only to switch the hots. As such regardless of which hot is switched, the neutral always connects to ground both in the inverter and in the gen as well as to the shore power ground, which is also therefor connected to the neutral... This is why a switching relay is needed, an external transfer switch.
     
  5. groper
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    groper Senior Member

    PHIL, the earthing system remains intact no matter what source you are using. The earths all remain bonded together and they are never broken. It's the neutral to earth bond which is the problem, and when both hot and particularly the neautral are both switched over to the new source, you remove the problem.

    If you had a house , with a generator changeover switch in case of grid failure, then you only need to switch over the hot. All the neutrals can be junctioned on the neutral bar and the earth to neutral link is also intact on the neutral bar. So you only need a single pole to change over and there is no problems because there is no other earth to neutral bond Anywhere downstream of your GFI s in your home switchboard.

    This all changes on your boat because you can only ever have 1 earth to neutral bond at 1 time. This exists in the marina switchboard when on shore power, so you can't have another on your boat as your boat is downstream of the marina pillar box GFIs. If you do, then you will see that the GFI or RCD on the marina pillar will trip as soon as you connect to shore power. Don't beleive me, try it and see...

    You must note, that we are talking about the neutral to earth link, and not general earthing, nor AC earth to DC negative bonding.
     
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  6. cgoodwin
    Joined: Mar 2011
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    cgoodwin Junior Member

    Page 19 of the SW4024 manual:

    Note: The three neutral terminals are common to each other and can be used in any combination or order. In a residential application, it is often easier to only connect one AC neutral wire to the inverter and make the other neutral connections at a central point such as in the AC load center, etc. In mobile
    installations, the AC system must have the neutral physically isolated from the ground throughout the load distribution powered by the inverter. The SW Series Inverter/Charger does not include neutral to ground switching for the AC electrical system. This must be done externally from the inverter.
     
  7. sdowney717
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    sdowney717 Senior Member

    Yes, the inverter must not have neutral and ground joined, is what I think that says.

    When the neutral wire is open circuited going to generator, the neutral to ground bond is open. Where is the ground connected anymore by way of the neutral?

    Ground wire runs to hard wired gen-neutral bond, but if the follow on wires running through the neutral wiring is open circuit, then there will not be any current flowing through the neutral wiring, I mean where is the current going to go?
     
  8. philSweet
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    philSweet Senior Member

    Okay, as a group, I think we are at least converging on a definition of the problem as it applys to cgoodwin. The AC neutrals are common and unswitched in the inverter and that causes a problem. The manual doesn't show a specific solution for a generator + shore system.

    There are seemingly two ways to handle this. One is with a manual switch to select shore or genset. This presumes manual genset start (not inverter controlled) and slow manual switching (> .1 sec dead between sources to allow inverter disconnect). This is probably the simplest solution for most small boats and RVs. The switch feeds AC 1 input on the inverter. It switches hot and neutral.

    The second way would be to leave the genset hooked up to AC 2, which enables a lot of additional functions in the inverter; and conditionally break the neutral feeds. The inverter is biased to AC 1. If AC 1 is hot, it will be used. So there is a fairly simple logic. If inverter has selected AC 1 then connect only AC 1 neutral. Else if AC 2 is selected, connect only AC 2 neutral. The lame part here is that there is no way to actually tie the neutral switching to the actual inverter switching. You have to use some proxy such as volts on AC 1 hot and AC 2 hot. This makes life awkward. But it ought to be good enough most of time. This amounts a circuit like pg 27, except that instead of switching between AC 1 neutral and Inverter Ground, you switch between AC 1 neutral and AC 2 neutral at the relay. Panel neutral runs to inverter neutral, inverter neutral to relay Common. AC 1 hot to coil. AC 1 neut to NO. AC 2 neut to NC. One 60 amp relay.

    How does that sound to everyone?

    <<edit>> It should be a make-before-break 60 amp relay.
     

  9. groper
    Joined: Jun 2011
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    groper Senior Member

    Cgoodwin, I admit to not reading the manual for the sw4024 inverter until now, I just had a quick flick through it. I didn't realise you were wanting to use some of its functionality.


    Can you please define what exactly , the functionality of the system you wish to setup. For example, do you want uninterrupted power to certain loads. Do you want auto generator starting? Please be specific.

    PHIL, you can automatically set this up with external relays. With electrics, anything is possible. For example, you use the auto generator start terminals which are actuated by the inverter to energise a start circuit on the generator. You also tie this same control circuit to another contactor which closes the main neutral to the genset, and thus removes the earth to neutral problem when the generator is not being used. Or you could simply do it from the supply side of the generator, take the ac supply from the genset and run it through a contactor so that it closes its own neutral to the inverter.

    Either way, the inverter prevents both genset and shore power from being used simultaneously, for short circuit reasons. if there is shore available it ignores the genset. If the genset somehow starts, perhaps manually, and the neutral to the inverter is closed, it will trip the shore GFI due to the earth neutral bond in the genset becoming a path for unbalanced current sensed by the shore GFI . An override switch in the neutral path may be used so that generator can be started for maintenance purposes without interrupting shore power whilst at the marina. Simply opens the neutral to the genset when thrown. If you want to load test the genset, then you must isolate from shore power...
     
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