Floating ground and ground faults - Theory questions

Discussion in 'OnBoard Electronics & Controls' started by DarylH, Oct 12, 2012.

  1. philSweet
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    philSweet Senior Member

    Go find those old Simspon Meters. Get them calibrated and use them. It IS going to be a ***** to find. I spent three weeks with a crew of five looking for one on a missile silo. We basically moved in. The Coke truck delivered to us. Girlfriends brought us food. You're going to get one heck of a practical education. You do need somebody along who likes physics and can actually interpret what you find. It is possible to zone in on the problem by making very accurate measurements with calibrated equipment. Once most of the electronics are out of the way, you may be able to inject an AC signal on to the dc side, say by hooking up a signal generator to a ballsy amp, and track the signal's attenuation on the hull. Find an old amp that can operate indefinitely near rated power with the output shorted. Where there is one, there are usually several.:mad:
     
  2. jonr
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    jonr Senior Member

    Or a welder. Apply enough voltage and current and any connection bridge (metal, moisture or whatever) will vaporize.
     
  3. philSweet
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Actually, I just realized we are probably talking about a small boat. If you disconnect the batteries and hook up a cheap auto battery charger, it should be plenty noisy to find the fault with an Oscope. Of course, you need to make sure the battery charger isn't causing an additional fault. Run it off an isolated portable genset. Make sure ALL the dc electronics are isolated if you do this.
     
  4. Frosty

    Frosty Previous Member


    He has not said. I think its big as he once mentioned --crew--
     
  5. bcervelo
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    bcervelo Junior Member

    Can anybody give me an idea how a ground fault indicator would be wired into a boat?
     
  6. jonr
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    jonr Senior Member

    You can wire it anywhere in series with the live and neutral wires.
     
  7. bcervelo
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    bcervelo Junior Member

    Does anyone have a wiring diagram of a ground fault indicator?
     
  8. CDK
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    CDK retired engineer

    The device has two input and output terminals, that is all. There usually also is a ground terminal on both sides but that it just a mechanical feature.

    A basic ground fault detector has two windings around an iron core, one for the phase or DC+, the other for the return or DC-, inversely connected.
    In a healthy circuit, the magnetic fields of the windings nullify each other because the currents in and out are exactly the same. If a ground fault exists, part of the current flows elsewhere, so the magnetic fields are not equal; the resultant field is used to activate a relay contact.

    Because the resultant field is weak, this type of circuit only works is the difference in current is quite large (> 0.5 Amps). Devices that can detect smaller differences ( 0.05 Amps) have an electronic amplifier circuit.
     
  9. bcervelo
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    bcervelo Junior Member

    Would you be able to do a wiring diagram of the circuit CDK?
     
  10. Frosty

    Frosty Previous Member

    The thread is called floating ground and ground faults. I don't think he is talking about AC ground faults but faults of a DC system of the Alluminium boat he is having trouble with.

    As gonzo has said it could be the VHF antenna, probably not but its not what he is asking.

    I think a more clear question is needed. I am under the impression he is talking about floating voltage in the hull. If not my apologies.
     
  11. CDK
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    CDK retired engineer

  12. FishStretcher
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    FishStretcher Junior Member

    As for a few common culprits- most security cameras have case tied to batt negative, as do a few other things that have serial ports, and RF antennae, as has been noted. I don't know if this is a big enough boat for lots of stuff like that. You can get a few volts with salt water, aluminum and other metals, too.
     

  13. gonzo
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    gonzo Senior Member

    Marine electrical systems are unique on their complication. There is a mix of AC and DC. Also, there are a variety of voltages, with the added problem that the hull is a galvanic cell producing a random electrical potential too. RF will create voltage spikes depending on variable condition and are hard to identify. To make matters worse, the whole thing is floating in a conductor. Troubleshooting is easier if you isolate sections of the system. For example, disconnect the DC power and see if the problem remains.
     
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