Why is AC ground and battery negative connected on boat?

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

  1. sdowney717
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    sdowney717 Senior Member

    Is this in case a hot wire from touches engine etc... and leaving it energized?

    I have recently rewired boat in last few years and was questioning this ground joining recently.

    As long as the AC system is not damaged and fully isolated then there would be no reason to do this?
     
  2. michael pierzga
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    michael pierzga Senior Member

    Sure...if an isolated ac electrical motor, component or circuit .... develops an internal fault it must be able to discharge it to earth...If not when the operator touches the motor or chassis or component box, the operator will become the conductor to earth. ZAP

    The chassis of every piece of electrical equipment needs to connected to earth.
     
  3. gonzo
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    gonzo Senior Member

    The ground only serves its purpose when the AC system is damaged in some way. Otherwise, it is passive. It is a safeguard.
     
  4. sdowney717
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    sdowney717 Senior Member

    Since the internal combustion engine, bilge pumps and the copper water pipes and other metal bits and pieces on the boat are not 120vac systems, why bother joining the AC ground to DC negative?

    I dont see how my AC hot wires could ever possibly energize the surface of those items unless it was deliberate sabotage or some kind of severe trauma or maybe fire runs thru the boat. I certainly dont have AC wires laying across engines, maybe some people do that.. Not even sure how a fire could do it seeing the hull structure would have to collapse to even allow Hot Ac to touch an engine block and I certainly wont be in there if it does and live to tell the tale..

    I figure 120vac items should be grounded and that is good enough?
     
  5. gonzo
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    gonzo Senior Member

    If there is a fire and the wires melt together, the ground will protect people from electrocution. Salt water can also make a good conductor for stray AC and electrocute. The copper water pipes are connected to the AC through the water system. If the water heater has a heat exchanger to the engine that connects them too.
     
  6. michael pierzga
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    michael pierzga Senior Member

    Your on a metal boat. The entire hull is earth. Engines are grounded to it, the ac system is grounded to it. Im not sure what your question is ?
     
  7. sdowney717
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    sdowney717 Senior Member

    The boat is wooden boat.
    If there is a fire and the wires melt, the green ground wire will conduct current back to shore. Since there are no unbundled AC wires (all wires white-black-green in the cable sheath), fire wont make any difference if the engine block is grounded.

    The water heater is AC powered and it is grounded with AC system, although
    I wonder if the copper pipes are , so can check it. I could see some extremely slim chance of a water heater fault to ground. But of course it is a 120vac appliance so must be grounded. Water heater is not connected to the engine water system. even if it was, it is grounded by the green wire. I do not think my copper water pipes are connected the DC negative by way of a deliberately attached wire.

    Found this here which is just parroting everything I have already known.
     
  8. michael pierzga
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    michael pierzga Senior Member

    The best shorepower solution is an isolation transformer. Review its installation.

    How confident are you that the shorepower point is wired and earthed correctly ? Seawater always works.



    The chassis of the hot water heater must be grounded. If a conductor touched it you would be electrocuted.

    Lightning strike survivability requires all major metal component like ss water tanks to be grounded.

    Wooden boats present a special challenge. You should do more research.

    Nigel Calder writes good books on marine installations.
     
  9. gonzo
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    gonzo Senior Member

    The water in the pipes is a conductor and will carry AC in case of a fault.
     
  10. sdowney717
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    sdowney717 Senior Member

    My water tank is grounded and I suppose the pipes are since they are directly connected to the metal tank. I could additionally add a ground to the hot and cold pipes with little trouble.
     
  11. jonr
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    jonr Senior Member

    If you are concerned about safety add a GFCI.
     
  12. sdowney717
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    sdowney717 Senior Member

    Yes, already have that, all outlets are on GFCI circuit.
     
  13. Ike
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    Ike Senior Member

  14. michael pierzga
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    michael pierzga Senior Member



    Thats an excellent introduction to electric systems and terminology.

    The OP should do a bit more research on the special problems with wooden boats and electricity. Particularly galvanic action.
     

  15. sdowney717
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    sdowney717 Senior Member

    Nice article however

    Not sure about that bolded statement at all. Current in salt water will flow back to its source better than in fresh. In freshwater the current flow is inhibited, which means less current flow but higher potential shock hazard cause it is staying in the water being more inhibited. It sends out feeler current potentials seeking the better paths to ground. A swimmer is a better path to ground because the body is saltier, so the feelers if it can find the person will preferentially flow thru the body killing the person. the person then becomes like a wire in the water conducting the current.

    Fresh water has high resistance so there is less current flow occurring UNTIL a body or a conductor lets it flow. then once the conductive path is established, much of the potential will flow thru the better conductor (person), but not all of it. I think of it like lightening. Air has high resistance and there is also very high current potential. But the air has to ionize for the current to flow, otherwise no current flows.
    Fresh water has less resistance than air so some very small amounts will be flowing all the time. Swimmers report tingling electricity which feels paralyzing and have hard time getting away. Closer they get worse off the chances.
     
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