Why is AC ground and battery negative connected on boat?

Discussion in 'OnBoard Electronics & Controls' started by sdowney717, Nov 5, 2012.

  1. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

    As it is I understand a safety issue to include the engine in the AC ground in case some wire breaks and lays on the block, which to me seems very unlikely unless it is a total rats nest of crazy wiring with AC hot wires laying over the engine. It makes me feel like, oh, place an engine block on the ground. Run an extension cord next to the engine block. Then say the engine block is a shock hazard so put a ground wire on it.

    Think of it this way. Your working on your steel car. You have electric AC power running over and around the car with loose plugs, drill, lights.
    The car should now be considered an AC shock hazard and you need to connect the green grounding wire to the engine block. This seems very silly to me.

    I do understand the real concern for grounding the fuel tanks and fill tubes for spark prevention due to static charges.

    Adding this later
    Thinking more on the fuel tank grounding. The argument is a static charge might exist between land pump nozzle and boat fill?
    However when motor boat over to fill up, you are not hooking up to any shore power so no ground connection between boat and pump will be made. So I dont see how grounding the tank will help that situation if charge potential exists between shore and boat.
    It might help if your own boat is generating a charge onto the tank to self ground electrical charges away to the water?

    Perhaps the ground the tank idea is less static charges and more foolish rat nest bare hot wires touching tank and energizing the surface of the metal. Then your own boat if tank is grounded will hopefully blow the fuse and disconnect itself from the electric charges. If not tank is hot with a potential voltage, then perhaps a circuit thru the pump nozzle (which includes insulated rubber hose) would be able to complete a circuit which sounds impossible unless it was a static charge like lightening.

    Makes me wonder why cars with rubber tires which can generate static charges dont have more fillup fires.
  2. carlos bairo
    Joined: Dec 2008
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    Location: australia

    carlos bairo Junior Member

    building on aluminium from plywood plans 19'

    i m intrested in the safe wiring installation of my electrical distribuition on my new boat.wuold be posible to have a schematic distribuition gide o squetch ,that would be a big healp,thanks.
    carlos Bairo

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  3. michael pierzga
    Joined: Dec 2008
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    michael pierzga Senior Member


    Very many diagrams on the net for small craft

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  4. larry sellers
    Joined: Sep 2012
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    Location: chico ca

    larry sellers Junior Member

    Thoughts....An old pal, now dead, was an electrician on the build of CVN65 (Enterprise). He an' I worked together for years at an Army base, doing maintenance - he being the spark-guy and me being the hammer-guy. Anyway, old Bob told me that ship AC in his day was ungrounded, and that the 110 volt ac receptacles were installed with the ground prong up to suggest this. That said, I'd follow the rules and do it by the book. Not all marine engines are chassis ground, I believe, some are made such that the block is isolated, ie the alternator has a ground lug isolated from the alternator frame and the starter also has an isolated ground lug. Over many years of liveaboard I noted that shorepower wiring was usually pretty substandard....Fairly large transformers can sometimes be found cheap, and a pair can make an isolation circuit. heavy tho. arrl website has some stuff on radio set grounding...
  5. larry sellers
    Joined: Sep 2012
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    Location: chico ca

    larry sellers Junior Member

    oh yeah - to add - when large currents run through the iron block of a non-isolated engine, the block becomes magnetized. this subsides gradually with heat and vibration, only to be rejoined each time the engine is started. if said engine is proximate to your compass... that's off topic, but might be worth keeping in mind...it was a problem back in the day, for me.
  6. Ike
    Joined: Apr 2006
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    Location: Washington

    Ike Senior Member

    Farewell to the Enterprise. I used to see her going in and out of San Francisco Bay, and was aboard once for a conference. She is one amazing ship. Too bad she's leaving service.
  7. Frosty

    Frosty Previous Member

    We always fitted the micro wave aft of the block for this reason and positive terminals on the battery facing north.

  8. mach128
    Joined: Nov 2018
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    Location: IL - USA

    mach128 New Member

    I realize this is old thread but still very valid.

    No matter how confident you are in your good wiring you should never skip your AC to DC ground bond.

    Reason for it is that you most likely have number of AC and DC appliances on board. Battery charger be an example, Inverter, dual AC - DC fridge... and list goes on. You might have done bulletproof wiring but in the era of value engineering your appliances could be another story, hot boat is a thing to avoid at all means and could save a life because awkward things do happen in the marine environment, we have all seen it.

    Even if you skip the bond wire you would still very likely suffer from galvanic corrosion as many appliances require chasis ground and join AC to DC internally (battery chargers and inverters are common culprits here) Solutions are galvanic isolator on smaller boats and ideally isolation transformers for larger vessels.

    GFCI on another hand is a very good idea but it should be installed circuit wide and not just at the outlets. My own solution is 30AMP rated Hubbell Circuit Guard at the double pole breaker where shore power enters the boat (or as close as possible)

    Back in the days that was not a norm and could be why ships had more fires and went down a much higher rate. Resulting loss of life was also significantly higher.

    Many ABYC rules could be argued about but work well for the statistics alone and most are there for good and valid reasons.

    Personally and on top of ABYC rules I like to add high amp (200 plus) MRBF fuses to all positive battery terminals, plastic covers and rubber boots on all power distribution terminals and blocks (positive and negative) In addition to individual positive battery switches I like the idea for single heavy duty NEGATIVE battery disconnect switch which gives you ability to disconnect all batteries at once. I also refuse to use automatic AC switches.

    Also best electrical systems for boats be would fully isolated ground and I believe most engine manufacturers are fully capable of doing it (some have isolations kits available as I heard such are mandated on military and government fleet in some countries) Obvious reason manufacturers are not interested in fully isolated boats is that would be very bad for business. No corrosion would result in too little wear, waay too much reliability and would be global disaster for new motor sales and spare parts business. At the end, as always, It all comes down to money!
    Last edited: Nov 12, 2018
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