Inverter Grounding Question

Discussion in 'OnBoard Electronics & Controls' started by fireman_bob, Mar 3, 2015.

  1. groper
    Joined: Jun 2011
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    Location: australia

    groper Senior Member

    There is nothing inaccurate in what you've said, but you're missing the point.

    When someone is electrocuted under fault conditions, it is inevitably because they have touched a metal part which has become live. The wiring rules here in Australia, and no doubt throughout the rest of the first world, stipulate what requirements need to be met so that a person cannot be exposed to live metal parts under normal operating conditions expected within the electrical installation. A protective earthing system is absolutely mandatory, along with many other requirements totalling 452 pages in just the basic wiring rules AS3000.

    If there is no protective ground wire, then the now faulty metal part will remain live forever without an easy path for current to flow. So my stove top for example, just sits in my galley with the entire metal frame at raised potential waiting for someone to touch it - not good.

    Now consider your reasoning ... Your happy for that stove top to sit there at raised potential, and wait for the gfci to trip only AFTER someone touches it and becomes part of the faulty circuit! No thanks, prevention is better than a cure...
    With a protective earth wire intact, and then an earth neutral link, the fault current goes back to the inverter on the neutral after it goes around the gfci... so the gfci will trip the moment the fault occurs and not after a person is electrocuted and then hopefully saved... In our wiring rules, it is referred to as fault loop impedance and it must be measured as part of the installations commissioning by licensed electricians.

    The reason I could feel a tingle, is because my gfci will only trip at its rated 30ma current, I was obviously getting less than that standing on my fiberglass floor... Now that I have the earth and neutral bonded at my inverter, the fault current from the faulty stovetop goes back on the low impedance earth wire and my gfci trips before I get a chance to touch anything. Once it trips, I know I have to clean out the salt buildup in my appliance terminal block- which was a cheap peice of rubbish made in China btw...
     
  2. sdowney717
    Joined: Nov 2010
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    Location: Newport News VA

    sdowney717 Senior Member

    30 ma is too high just for convenience sake I think, if you think that will keep you living?
    30 MA sounds like ELCI circuit protection.?

    Currents as low as 30 mA AC or 300-500 mA DC applied to the body surface can cause fibrillation.
    http://en.wikipedia.org/wiki/Electrocution

    I still have all my ground wires connected.
    I only have GFCI 5 ma on all outlets and another GFCI dedicated to the microwave. What I found was no significant nuisance trips so far.

    My electric heaters, water heater, stove-oven, fridge are just green wire grounded with no GFCI. I have thought about adding 30 amp double pole GFCI 5ma breakers for the entire boat. When I ask, people tell me it will constantly nuisance trip, so I dont even bother with GFCI or ELCI.

    I run a couple outlets to easily potentially wetter places, and those extensions have a plug into a duplex socket, so if there was an issue, I could unplug AC runs and reset the GFCI.
     
  3. groper
    Joined: Jun 2011
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    Location: australia

    groper Senior Member

    If its on wiki it must be true :rolleyes:

    30ma is the minimum requirement here in Australia.

    Nuisance tripping you will probably never get, because you don't have your neutral and earth bonded - this is my point! You will also never get full protection either, nor will you become aware that an appliance has become faulty. ..

    It's better to have some nuisance tripping than annoying tingles which unnerve your friends...

    Do you have a through hull bonded to earth as well?
     
  4. sdowney717
    Joined: Nov 2010
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    Location: Newport News VA

    sdowney717 Senior Member

    When the boat is on shore power, ground and neutral is bonded.
    When the boat is on gen, ground and neutral are bonded.
    I suppose when on inverter ground and neutral are not bonded.

    I read that inverters actually alternate current flow 60 volts up on hot wire then 60 volts down on neutral wire, so if you bond ground to neutral it shorts out the inverter.

    I have yet to have ever felt a shock from the appliances in the boat.

    I DID once get a jolt when I felt the water flow coming from the Heatpump throughhull. Opening up the wiring box on the cruiseair, rain water had gotten in to it. The hot connection for the water pump was in contact with the metal case, sitting on the bottom in water. Because it was soaking wet, current was flowing from hot to the heat pump chassis and then out into the bay by way of the raw water flow. I redid the connection and raised it so it could not get wet again. I had it wrapped with friction electric tape, that tape I thought would be ok.

    Thinking about that short, it might be the reason my galvanic isolator burnt up. Since the Cruise Air heat pump is grounded by way of the copper pipes back to the control panel in the air handler, a lot of current would have been flowing in the ground wire. I had noticed the GI was burned-overheated, so I got rid of it. So for a while I must have had a nonexistent ground for shore power. I had been wondering what happened to it.


    The underwater metals that are bonded are both rudders, struts, shafts.
    None of the through hulls are bonded, I disconnected the bonding wires to those.
     
  5. groper
    Joined: Jun 2011
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    Location: australia

    groper Senior Member

    This is a concern... we must all realize that just because we have GFCI`s installed, doesnt mean we are protected. They must be installed as part of a complete wiring system in order to be effective. The earthing system is an integral part of it, which is why i am making a point of this. You CAN NOT expect GFCI`s to operate as a preventative measure without an effective earth wiring conductor back to the power supply - the inverter in this instance, AND the earth to neutral bond at the inverter output. This earth to neutral bond is upstream of the GFCI, so any leakage to the earth wire is detected the moment a fault occurs, well before a person comes into contact with anything of raised potential. You should not rely on leakage current "into the ocean" as an effective earth pathway to trip these devices. The effective current path needs to go back to the supply source, not simply to the planet itself.

    In australia, the standard is no more than 0.5ohms of resistance between any GFCI protected circuit earth wire of a final sub circuit, to the earth neutral bond. There is also stipulations on the total earth fault loop impedance back to the supply source which accounts for utility network impedance back to the neutral point on the street transformer, not just the end users installation impedance. This ensures effective and rapid trip times when faults develop and never relies upon current flowing into the planet, but rather back to the neutral point of the supply... sure we have earth stakes driven into the ground, and the earth wiring is also connected to this, but a far more effective pathway for fault current exists in the form of the earth to neutral link and then the neutral conductor back to the supply neutral.

    You will also note that under these conditions of a correctly wired system, the neutral wires are always at the same potential as the earth wires... they are permanently bonded in the switchboard of nearly every home and business and also in the switchboard of your local marina...
     
  6. groper
    Joined: Jun 2011
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    Location: australia

    groper Senior Member

    Consider another scenario...

    Lets say you have another AC circuit - which has only a circuit breaker protecting it, not a GFCI or RCDMCB combo... just a plain old 20amp circuit breaker...

    A fault develops from active to earth somewhere...

    If there is an effective earth to neutral link back to the supply neutral, this will STILL trip the device if an active wire comes into contact with an earthed chassis component as it ensures large currents flow with very rapid trip times. So you and your wiring is still well protected.

    If you rely on current flowing into the ocean, it is much more likely that only small currents will flow - such as in your heat pump example, and none of your protection devices operate despite this small current flowing without your knowledge... Its a more dangerous situation compared to one which ensures high current flow under fault conditions...
     

  7. sdowney717
    Joined: Nov 2010
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    Location: Newport News VA

    sdowney717 Senior Member

    I dont have GFCI on the big stuff, GFCI for me is only on the outlets. I dont use an inverter for the big items on the boat. The big 3000 watt 6000 surge inverter cant run the heat pump. It can run the microwave and the fridge and the stove top. But the battery drain is not worth it. Much better to run the gen.

    So I got an old UPS PC type inverter, likely about 400 watts. And I take the power from the AC breaker panel run to a DPDT on-off-on 20 amp switch. This way I can switch between little inverter and the power from the panel to feed all the boat outlet plugs. the little inverter cant put out much current so it is self limiting, it just shuts down on overloads. that little inverter can run all the AC lights, the builtin core 2 duo linux nav computer, little phone chargers, the TV and DVD player. If I flip the on-off-on switch fast, the PC wont shut down. So I can power up the nav PC in the slip, later turn on the little inverter, then flip on-off-on switch up to inverter.

    I got rid of the dead battery in the little inverter and added an internal PC fan. The circuit board has a warning beeper telling you you're on battery power which I covered with some tape. So you still hear it a little.
    I also added a remote on switch with LED on indicator. It uses a momentary switch.
    I am rather surprised how well it works. The boat has an AC powered anchor winch, it cant run that. So someday I might get a bigger little inverter that can, maybe a 1000 watts.
     
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