Welding a steel craft whilst afloat - electrolysis?

Discussion in 'Metal Boat Building' started by LittleVlet, May 6, 2010.

  1. jonr
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    jonr Senior Member

    A GFCI does not use the ground for anything. It makes sure that the live and neutral wires have equal currents. This is quite effective although when it comes to electrical safety I believe in belt and suspenders unless there is a good reason to make an exception.
     
  2. capt littlelegs
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    capt littlelegs New Member

    How many more times? It's an earth leakage device, a residual current device, a ground fault circuit interupter, how the hell is it going to detect an imbalance if the current can't leak somewhere back to the supply earth/neutral? Of course it uses the ground, ground fault, get it?

    http://en.wikipedia.org/wiki/Residual-current_device
     
  3. CDK
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    CDK retired engineer

    WRONG AGAIN!
    Jonr is right, you are wrong.
    You even supplied the wikipedia page that proves you wrong, the GFI works perfectly without the ground wire present.
     
  4. LyndonJ
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    LyndonJ Senior Member

    There's a difference between an RCD (Residual current device) and an ELCB (Earth leakage circuit breaker). But what we are talking about is the ELCB and as already said it trips on an imbalance in current between phase and neutral.
     
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  5. capt littlelegs
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    capt littlelegs New Member

    I never called an RCD an ELCB because ELCB are obsolete in the UK and were voltage operated and wired to earth but some people still call an RCD an ELCB as in the German info, I called it an Earth Leakage Device for clarity. It's an RCD, same thing, call it what you like it still needs a good earth/neutral connection at the supply and locally and to earthed appliances in order for the fault current to return and create the imbalance that will trip it out, no good earth return, no trip or until someone gets a shock, simple and that's if it's in working condition!

    You people are not considering the whole picture and are assuming an imbalance can occur in any circumstances even without any earth connection and that is simply not true.

    If you disconnect the earth to a boat the basic fuses or current trips will not work with an earth fault because the earth resistance through the hull will undoubtedly be too high, the RCD may work but this cannot be guaranteed on a boat due again to high resistance to the supply earth point without the earth wire so an earth fault can leave the boat live with sufficient current flowing to kill anyone. This is fully accepted by all authorities and anyone who says different is an idiot!

    http://en.wikipedia.org/wiki/Earth_leakage_circuit_breaker
     
  6. CDK
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    CDK retired engineer

    You may also get negative reps for being rude and will eventually be banned from this forum.
    For forum member who really paid attention at school it is very annoying to have someone contradicting every attempt to explain something.

    The next line is from the link you provided to prove your point. Again it does exactly the opposite:

    RCDs are current balance devices. They measure any current imbalance in the flow in and out of a circuit or appliance via its Live and Neutral conductors (or on three phase circuits, the combined sum of currents in all phases and neutral). Should the current imbalance exceed the tripping threshold for the device, it will activate and disconnect the circuit.

    The neg. points were not mine, but in my opinion you do deserve them.
     
  7. capt littlelegs
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    capt littlelegs New Member

    Let's hope so because this is a waste of time! You have not explained anything but simply pursued your daft and dangerous idea of removing a boat earth when that is against accepted practice and regulations. I know how they work, I had to know, I've used and installed them for nearly fifty years back to the old ELCB. You can't answer my previous questions but I'll try one more; Where do you think the current is going in order to create an imbalance between each conductor going through an RCD if there is no earth connected? I have to say I am not confident you can give a sensible answer but we live in hope!

    http://www.absorblearning.com/media/attachment.action?quick=rn&att=1981

    http://www.google.co.uk/imgres?imgu...E-SearchBox&rlz=1I7ADFA_en&ndsp=20&tbs=isch:1

    https://student.ee.uwa.edu.au/practical-resources/RCD Operation/image_large

    http://www.google.co.uk/imgres?imgu...E-SearchBox&rlz=1I7ADFA_en&ndsp=20&tbs=isch:1
     
    Last edited: Jun 1, 2010
  8. jonr
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    jonr Senior Member

    My concern with relying only on a GFCI for shock prevention is the failure modes of corrosion or the neutral wire getting cut. Without the latter, the GFCI has no power to activate. I agree that a double failure - a neutral wire gets cut and a live wire shorts to something (eg, equipment case) and a grounded person touches it isn't very unlikely.

    In summary, my opinion is that a frequently tested GFCI (including a circuit breaker) and a galvanic isolator with alarms for failure or overvoltage would be best - or an isolation transformer where the grounding is defective.
     
  9. troy2000
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    troy2000 Senior Member

    I hate to interrupt CDK and capt littlelegs. But maybe I can squeeze in a question or two, while they're in their respective corners preparing for the next round.:)

    Did or didn't Lyndon answer the original question about electrolysis properly? It seems to me he did....suppose I'm working at a welding table. If I clip my welder's ground cable to the table, lay the workpiece on it and start welding, is anyone going to get shocked by touching the table?
     
  10. CDK
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    CDK retired engineer

    Yes, let's try once more.

    assumptions:
    230V grid, return grounded on shore, metal hull in water, earth wire disconnected, protective ground connected to hull.

    The electronic GFI needs 5 milliamps (rms) to trip, response time < 300 mSec. That condition is met when a 47 K-ohm path exists between live and ground.

    I gave a crude calculation of the hull to ground resistance already, but you didn't buy that (although you didn't come up with your own figures), so I measured the ground resistance of my own boat.
    That doesn't meet the criteria above because it has a 26' GRP hull with only a few small metal parts in the water (stern tubes, props, rudders), but it is moored in seawater with a depth of 1.80m.
    The resistance between the ground electrode on shore and protective ground in the boat is 71 ohms, including the 75 meter cable (3.5qmm) needed to do the test. It is the average of two readings with different polarity to cancel out galvanic action.

    A short between protective ground and the live wire from the shore results in more than 3 amps through the path from the metal parts to the seabed, 600 times the current needed for the GFI to disconnect te power. In that case the device trips in less than than 4 mSec.

    So to answer your question:
    I think the current is going from the contact area to the submerged metal parts of the boat and from there in a diffuse cone shaped pattern through the water to a circular area under the boat.
     
  11. CDK
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    CDK retired engineer

    The original question was answered completely.
    In your example I recommend wearing gloves, especially when changing the elctrode. There is 60V between table and electrode, not dangerous but unpleasant when your skin is wet.
     
  12. capt littlelegs
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    capt littlelegs New Member

    That's probably the most sensible answer so far, thanks.
     
  13. capt littlelegs
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    capt littlelegs New Member

    Yes that part was answered but CDK refuses to accept that the safest way to have power on board, especially in a work environment, is to have a shore earth to the appliance case and bonded to the metal boat to prevent any difference in potential that could cause electric shock. That is a health and safety requirement in boatyards in the UK without which the boatyard would be in serious trouble. An amateur is no less vulnerable and electrical safety is more important than any short term electrolyses problem.
     
  14. capt littlelegs
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    capt littlelegs New Member

    Yes wear gloves and switch off when changing rods. The maximum fault voltage that someone can be exposed to is 50 volts, but in the wrong place that is still enough to kill and welders have been killed so don't assume the welder output is safe to play with as it can be 80 volts!

    http://www.mcga.gov.uk/c4mca/mcga-mnotice.htm?textobjid=19F91F009E3ACB72

    http://www.ehow.com/video_4426766_electric-shock-protection-welding-safely.html

    I will respond to your other post later.
     

  15. troy2000
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    troy2000 Senior Member

    I assume you're talking about AC welders hooked up to an external source of electricity? It would obviously be completely impractical to shut off my old trailer-mounted, gas-powered Lincoln every time I changed electrodes. Nor have I ever seen a professional do so, including our certified pipeline welders who also use truck or trailer-mounted Lincoln DC welders.
     
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