AC wiring

Discussion in 'OnBoard Electronics & Controls' started by bcervelo, Jul 9, 2012.

  1. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    In this particular case there are two power sources of which one is floating, the other has its neutral tied to ground at an unknown distance from the vessel.

    The generator has floating outputs, so its neutral is not near ground but carries half the amplitude. As I wrote, I don't mind because I will not touch either terminal, but it is your prerogative to have a different view, hence my proposal to pull one terminal to ground through an RC combination. Something like 100K and .05 uF will accomplish that.

    The external source has unknown properties but it is certainly not floating. The neutral is only near ground if there are enough ground electrodes, the earth is damp and the load on each of the three transformer windings is equal. If any of these prerequisites is not guaranteed, there will be an unknown voltage offset on the neutral, both AC and DC. Because the only part of the electrical system I am prepared to touch is the protective ground, I will not tie that to a neutral that may be very much alive.
    Only a few years ago a tourist in the house next to ours ran screaming from the shower because they had (and have) a two wire system with protective ground tied to neutral and that neutral was 35 V above ground! The safety device didn't trigger because that particular one needed 1 Amp to do that.

    Instead of a switch and a warning light I would install an AC power relay that switches from shore power to the generator as soon as the output voltage is present. I use the same at home, with a second relay that ties/unties the 3 phases used in different parts of the building.
     
  2. pistnbroke
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    does not matter how you wire it if there is a big corroded hole in the hull you are wasting your time .......
     
  3. BertKu
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    BertKu Senior Member

    Pistnbroke, let philosophise .That big hole in the boat is not because I made sure that the earth/ground coming into my boat is earth/grounded. But, because some neighbour, is creating those stray currents. How could we measure our neighbour's stray current?.
    a) If I have a wooden boat, I could just take a meter and measure between my earth/ground and the water, and then remove the connection, if I have a current and voltage potential.
    b) If I have a metal boat, I can't.
    c) If I have a glass boat, I also could measure.

    My question to you is. Will the voltage drop over the diodes in ALL situations be enough??? If your answer is yes. Fine, but you may find that you only cover 80% of the situations. Could one measure and try to find the culprit? What do you think.
     
  4. pistnbroke
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    pistnbroke I try

    Whaterver system you adopt you need to take into account the corrosion and using the diodes is the standard way on english narrowboats that are of course made of steel.

    Just making a point that up till now you had not addressed.
     
  5. BertKu
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    BertKu Senior Member

    That is a very good answer, but my question was, is there a way to find the culprits who are creating stray currents. Preventing is better than curing. but you totally right, that a couple of 5 Amps diodes can be very useful.
     
  6. BertKu
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    BertKu Senior Member

    The circuit diagram does have a galvanic isolation. It could be varistors, zenerdiodes, diodes. the last two back to back.
    But still I love to know whether there is a way to locate the culprits who creates stray currents. Is there a way to pin point them?
     
  7. pistnbroke
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    pistnbroke I try

    by taking my point you have answered your question yes you do but with 4 diodes back to back....

    I dont think anyone uses a transformer as these narrowboats need 3 kw for the washing maching ...leky kettle etc ....
     
  8. BertKu
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    Location: South Africa Little Brak River

    BertKu Senior Member

    Pistnbroke, why don't you answer my question? I agree with you about the isolation transformer, although with the modern ring trafo, it is not that big and expensive anymore for 3 - 5 kW. The permeability they using in the ring trafo is substantial higher than a normal soft iron plate trafo.

    Listen, Pistnbroke, I also don't know how to catch the culprits, but maybe you have view on this subject. I thought maybe measuring with 2 metal sticks a meter away from each othert and then turn the sticks to see in what direction the peak microvolts is showing on the meter. That should be the direction. But I have no idea whether it will work.
     
  9. pistnbroke
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    You cannot control the *** holes on the other docks so stop making them an issue and keep your own boat safe .........KISS
     
  10. BertKu
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    BertKu Senior Member

    You disappoint me.

    1) A wooden boat, I would connect the neutral to earth, and make that connection, whatever you say.
    2) If I have a wooden boat with the ground to an engine and the engine is connected via the shaft to the seawater, I would use galvanic protection
    3) If I have lightning protection, which is connected to a metal plate or keel, and is somehow connected to ground , engine etc. I would have galvainc protection, like a low voltage varistor, diodes back to back, Power zenerdiodes etc.
    4) If I have a glass boat, I would connect the neutral to earth.
    5) iF I have a glass boat with the ground to an engine and shaft connected to seawater, I would usae galvanic protection.
    6) If I have a glass boat with ligthning protection to a metal plate and earth and they are somewhere connected to ground, I would also use glavanic protection.
    7) If I have a metal boat, I would hang myself. To many stray currents.
     
    Last edited: Jul 14, 2012
  11. BertKu
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    BertKu Senior Member

    I see that different. If I could identify the boat who has a problem and create stray currents, I could help him and myself.
     
  12. sdowney717
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    sdowney717 Senior Member

    You could unplug the suspect boat to see it it stops the stray currents. That creates its own potential problems if someone thinks your a bad guy messing with their stuff. How far do these stray currents travel? A few slips or the entire marina?
    Most owners if approached with care shown would want to make any defficiency corrected. Some owners would not and get offended.

    Could you measure the potentials at each slip in the water and get an idea where the problem is?
     

  13. BertKu
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    BertKu Senior Member

    Well, I assume that the highest potential difference between two points will create a reasonable straight line of current flow. Thus if you have a boat next door, which is "dirty", I thought, maybe it can be measured the same principle that if you have an direction areal for finding animals with a radio around their necks. Radio waves are in principle also to be converted into electrical energy, thus maybe with two points measured in the water between two boats and turning the “aerial” to find the peak micro volts, should give you an indication. Whether it works, I have no idée, maybe one day I will try it out.
     
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