Shore power grounding

Discussion in 'Electrical Systems' started by Gould1, Apr 20, 2010.

  1. Gould1
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    Gould1 Junior Member

    Hello, I am new here so please bare with me I just finished reading an article on marine AC wiring and it was very interesting. I am currently in the process of installing a shore power system and I have a grounding question. If I connect the green AC ground wire to the negative battery terminal, does it leave an oportunity for stray current to find it's way into the DC grounding system and if so would this cause a situation where it would be dangerous for onboard ECU computers? I know that when we weld on a new vehicle we have to unplug the battery in order not to fry the engine's computer. Wouldn't this be the same? The last thing I would want to do is destroy two engine computers at $2,000.00 each. Thanks in advance for any help.
     
  2. pistnbroke
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    Is it a metal hulled boat ? are we talking 110v with an earth tap at 55v.
    Also consider if you do not take steps you will get bad corrosion on any earthed metal parts in the water,,,,
    I dont see your problem with the grounds of the engines being connected to earth ..the engine ecu will be running on 12v ...you wuld need to get spurious voltages between 12v pos and earth do damage .. modern engine ecus are well protected....
     
  3. CDK
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    CDK retired engineer

    A boat can be considered as an autonomous object, grounded by the hull itself or metal objects in the water.
    Not only is there no reason to connect it to shore ground, but doing so can only introduce electrical noise and galvanic processes.
    So snip off the green wire.....

    Stray currents killing an ECU or other electronics is quite another matter.
     
  4. Gould1
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    Gould1 Junior Member

    The boat is a fiberglass hull. What I did was hook up the green AC wire to the Ground side of the distributoin panel. Then I ran a green wire lead to the back of the boat to the neg terminal of a battery. So what you are saying is I should not have done this and just left the panel alone and not run the green wire to the Neg terminal of the battery? Would this leave anyone in danger of being hurt in any way? Sorry for all the questions but I just want to be safe and not hurt any equipment. Especially my ECU.
     
  5. capt littlelegs
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    capt littlelegs New Member

    Firstly don't take everything you read on marine AC electrics as gospel, there are varying opinions about earthing and most marine writers are not experts and don't take into account the wide range of situations encountered or the finer points that can affect what you do for the best.

    There are electrical and boat regulations to be followed that should form the basis of what you do but in general the earth wire goes to a dedicated earth point in the hull depending on what material it's made of. This may not be the same as any low voltage earth. A proper low voltage DC system is fully isolated.

    The earth is an essential safety component without which you risk your vessel or parts of it becoming live in case of a fault with full voltage between it and the surrounding water and ground. There are a number of cases of swimmers being electrocuted because of this.

    Many argue that the ground fault interrupter will protect... only if the resistance through the ground, water and any swimmers back to an earth point is sufficient to do the tripping and the interruptor actually works or is actually there! A good earth will also protect to a lesser degree with fuses or overload trips.

    The safest way to provide shore power if there are galvanic issues is through an onshore isolation transformer.

    I value engine reliability highly so I wouldn't have any electronics on my engines but I understand there may not be a choice with new engines.
     
  6. Gould1
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    Gould1 Junior Member

    Ok...If I'm understanding correctly, I need to hook up the green "earth" wire to a dedicated ground "Neg battery terminal". But will my ECU be in danger in ANY way?
     
  7. capt littlelegs
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    capt littlelegs New Member

    No, if it's fiberglass the earth point can be completely isolated from the boat and machinery but if metal hull parts can be touched at the same time as AC electric equipment then all must be bonded at one point such as bolted to the engine to prevent potential differences. Do not connect directly to the battery as this will at some time be disturbed and affect the earth integrity.

    In any event positive and negative should ideally be isolated from the engine and floating so that a first fault does not cause an immediate electrical failure, this isn't always the case of course.
     
  8. Gould1
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    Gould1 Junior Member

    So what I should do is hook up the green ground wire to one of the engines directly or a transome bolt that holds the drive transome? This is an Inboard/outboard boat with twin engines.
     
  9. CDK
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    CDK retired engineer

    You are sending confusing messages!

    The ground point in a boat is the engine block: it is always connected to the battery negative terminal because the starter motor and alternator are in contact with it.
    From the engine block, ground straps must lead to the submerged metal parts, including rudders and sacrificial anodes. Mast, railing, winches and other on-deck metal parts must all be bonded to ground and so do all ground terminals from AC wall sockets and permanently connected equipment.
    Wired like that, people and equipment are fully protected against electricity.

    The shore power should be connected to a ground fault interrupter and nothing else, with 2 or 3 wires, depending on the cable used. From the output of that device, only the neutral and live wire are used to connect to the ship's circuit. That will immediately severe the supply in case of a current leak.

    Only when the boat is placed on land with shore power connected, the shore power ground can be temporarily connected to the ship's ground.

    Anybody who has doubts about this should use a multimeter and measure the differential between shore ground and ship's ground, which should of course be zero, but never is.
     
  10. capt littlelegs
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    capt littlelegs New Member

    There should be one point where all AC earth/ground wires are connected together, it can be at or in your incoming switchgear if that is more convenient in your case but if you need your engines earthing, and galvanic action on expensive stern drives will be a consideration, second to safety, then both need bonding together anyway and then connected by green wire to the earth point. Stern drive engines will be DC negative earth I'm sure.

    In your case with the sterndrives you may be better off having no AC earth connection to the engines with the AC system isolated but without seeing it and any potential problems I can't be definite on that. What you are trying to do is avoid any voltage between metal cased AC appliances and the boat machinery or ancilleries under fault conditions, if you can't touch between the two then no problem.
     
  11. capt littlelegs
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    capt littlelegs New Member

    No I'm trying to explain a lot in a short post! There is no standard way of wiring a boat as all are different as we see above with a plastic boat but in general you are correct up to the point you say only use two wires.

    It is extremely dangerous to ommit the earth connection unless you know what you are doing and the earth fault leakage current has been checked in that location for sufficient operating resistance from your boat if you only rely on an interrupter, otherwise your boat can be live in the water!

    Using a multimeter to check resistance and without knowing what you are looking for is futile!
     
  12. KnottyBuoyz
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    KnottyBuoyz Provocateur & Raconteur

    Maybe this will help. From ABYC e-11 which is out of date. Sorry I don't have the current version.

    [​IMG]
     
  13. capt littlelegs
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    capt littlelegs New Member

    Just to add a bit more to this. If galvanic erosion is a problem and I don't believe it always is, then what can or needs to be done depends on how far away the boat is from the nearest ground plate on shore.

    If it is close as my barge is then provided enough current can flow to trip an interrupter in all conditions after the proper checks then disconnecting the earth will solve most galvanic problems provided I'm not using computers etc. which can dump current to earth, creating the same problem.

    If my boat is too far away from that ground plate to give sufficient tripping current then usually it will also be too far away to have any galvanic problems so the normally earth can be used. If not then if a fault to earth occurs on the boat the current flow can still be enough to paralyse a swimmer into drowning.

    The problem for mere mortals is they've no idea where the earth plate is or if sufficient tripping current will flow, so encouraging boaters to ommit the earth as a general rule is very bad advice. The best way is an isolation transformer.
     
  14. capt littlelegs
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    capt littlelegs New Member

    That's the way to do it if the trany is onboard provided the shore cable is connected directly to the transformer without onboard plug/sockets or connectors. The AC earth should really have it's own earth terminal adjacent to the DC terminal so that disconnecting or working on one terminal won't affect the other. Should also have onboard interrupter as well.
     

  15. CDK
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    CDK retired engineer

    Yes KnottyBuoyz, that's the way to do it.
    A newer version of this document probably shows a ground fault interrupter instead of a circuit breaker, but that doesn't change the principle.
    If shore power is only fed to an automatic battery charger, the transformer is already there.
     
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