Shore power cord?

Discussion in 'OnBoard Electronics & Controls' started by michigangeorge, Dec 27, 2012.

  1. Don H
    Joined: Apr 2012
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    Location: Queensland Australia

    Don H Junior Member

    Jon, the charger already has a 3 wire lead on it. The company building them looks very proffesional and as i said i would be very surprised if the earth is connected to the negative (boat ground). I dont have access to the charger so i cannot be 100% sure. there should be no problems whatsoever with stray earth currents.
    Switchmode supply's can be very electrically noisy depending on the circuit, If as i suspect the earth is being used to suppress conducted noise it must have a 3 wire lead.You dont want to be plugged into the same mains supply if a 3 wire lead was not used, depending on the switching frequency it could be very annoying.
    It should also be noted that cutting the earth pin off is illegal and if that lead is then used on an appliance that is meant to be earthed there will be a safety problem

    Thanks Don
     
  2. jonr
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    jonr Senior Member

    And if that 3 wire setup is then used with an appliance that does connect shore ground to boat ground, you may be missing a prop. Not the case initially, but things change (which is why some experienced, alive and un-incarcerated boaters say not to bring the shore ground on-board at all - see above).
     
  3. Don H
    Joined: Apr 2012
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    Don H Junior Member

    ok you're correct but in this case George has said it is a small boat and all he wants to do is charge his batteries from shore power. He doesn't have a complicated setup.The only appliance he has is the charger mentioned so provided there is not an earth to boat ground connection he is fine. If he plans to fit inverters with mains changeover he might have problems but the simple charger should not cause any problems.
    Something to remember here is that the earth wire is fitted to power leads (and mains wiring in general) to provide a low impedance path should an earth fault occur that either blows a fuse or trips a breaker. Electrical safety should never be compromised in an attempt to stop electrolysis.

    Thanks Don
     
  4. jonr
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    jonr Senior Member

    It would be interesting to see safety statistics comparing a 3 wire grounded circuit to a two wire, GFCI protected circuit (which didn't exist when they first specified ground wires). My somewhat educated guess is that the latter is safer but I have no proof of that. Some codes do support the use of a GFCI in place of a ground wire (but such sockets are supposed to be so labeled).
     
  5. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    I fully agree with you. Just measuring with an Ohmmeter will tell you all.
    a) less than 5 Ohm, he has to ensure to add some galvanic separation between his boat earth and the secundair of his charger, Either in the form of 2 diodes opposite to each other or a mulktiple of them, to increase the barrier. But I doubt that this professional charger made such a mistake.
    b) less than 2000 Ohm (2 KOhm) the charger has galvanic separtation build in.
    c) open circuit in both directions. i.e. one swop the leads and measue again.
    The manufacturer feels there is no need to have the secundair connected to shore. It means, the ring transformer core, in their switch mode powersupply, has their primary and secundair windings so far apart from each other, never a short circuit could be created and a dangerous situation.
    Also any feedback from secundair for the switchmode circuit is safe.

    George in Michigan, you bought yourself a nice charger.
    Bert
     
  6. CDK
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    CDK retired engineer

    There is no doubt in my mind that a GFCI is safer, certainly when it is tested on a regular basis. Fortunately there is a small red button on such a device to check its function, a feature not present on 3 wire circuits.
     
  7. BertKu
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    BertKu Senior Member

    (I am back to normal, power supply problems !!) I assume that with GFCI, any difference between outgoing life and return wire back by more than 10 mA? , 15 mA, 20 mA?? means tripping the circuit. This principle is the same for a current sensing earth leakage switch. (Not voltage sensing earth leakage switch) We have a special button to simulate and test the earth leakage switch here in SA (some of them, not all). However, GFCI has to be more safer, because with earth leakage current switches, there are situations, that the switch does not trip. Per example, ground current compensation the current flow through a human body. This cannot be happen with a GFCI.

    Bert
     
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  8. CDK
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    CDK retired engineer

    I checked the nearest Siemens one, the test button isn't red but gray. It puts a bypass resistor over one side of the current measuring circuit, tripping the contacts. It doesn't matter whether current flows through a person or just leaks to ground somewhere, as soon as there is a difference between current through live and return, the device trips.
    The only drawback is that equipment with input filtering sometimes trip a sensitive GFCI when there is nothing wrong.

    There have been safety devices in the past that measured only current through the ground wire. They detected only isolation leaks, not current flowing through a person, so they do not provide safety. I do not know if such devices still exist!
     
  9. Don H
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    Don H Junior Member

    It’s not really a legitimate comparison as both systems protect in a different way. In Australia GFCI or RCD (residual current devices as they are called here) have been mandatory for 20 years or so for all mains installations house, boat, caravan etc. They are great devices and no doubt have saved many lives but they do not replace the need for an earth wire.
    Let’s say you have your shiny toaster on the boat and the toast sticks. In a senior moment you poke a fork in to get it out. Click, the GFCI senses more current on the active wire than the neutral and shuts the power off giving you a fright and a wake up call. In this case the earth wire would do nothing. Now let’s say that shiny toaster has been around for a while and the active wire is fatigued, comes loose from its terminal and rests against the metal case of the toaster. Without the earth wire the GFCI will do nothing as there is no imbalance until someone touches the live toaster.
    If the earth wire is fitted the GFCI will trip as soon as the active wire touches the case.
    Before GFCI devices were developed the current through the earth wire would have tripped the breaker or blown the fuse. With the GFCI it will probably trip before a breaker will trip/

    Personally I would prefer the earth wire to be connected and trip instantly rather than wait for me to grab the live toaster to complete the circuit.


    Thanks Don
     
  10. BertKu
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    BertKu Senior Member

    Don, at what current trips your RCD? Here it is 20 mA (Although I would prefer 10 mA) Do you know at what current differentiation is the GFCI tripping?
    Bert
     
  11. BertKu
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    BertKu Senior Member

    Yes, you are right. Interference suppression capacitors are often the reason why one wonders why the flipping earth leakage switch trips for no reason. However , every system has it's own advantages and disadvantages. I think, GFCI combined with a good earth leakage system may be an option. Bert
     
  12. Submarine Tom

    Submarine Tom Previous Member

    George,

    Check your Personal Messages.
     
  13. CDK
    Joined: Aug 2007
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    CDK retired engineer

    That's right Don, for exactly that reason appliances with a metal case must be connected to ground or better: water. The GFCI is at the entrance point of the (2 wire) shore power cable.
     
  14. Frosty

    Frosty Previous Member

    This is a conversation between Africa, Australia and USA ---all have different set ups. A US boat trips the marina everytime.

    If your going to connect to water im gonna make damn sure its not through my sea cocks.

    If you earth to you grounding system ie your sea cocks and engine then your the earth for you neigbour and every one else.

    KISS dont buy metal bodied appliances . Infact what is these days?


    Oh and if you need to be told not to stick a fork in the toaster then there is no help here for you. Neither would an earth or ground save you.
     

  15. jonr
    Joined: Sep 2008
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    jonr Senior Member

    My understanding is that the only plausible ways to get electrocuted with a GFCI in the circuit is:

    1) connect yourself to live and neutral or
    2) be startled and do something (like fall overboard)
     
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