explain why inverter instructions say use separate ground?

Discussion in 'OnBoard Electronics & Controls' started by sdowney717, Jun 21, 2013.

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

    sdowney717 Senior Member

    Reading something about it reminded me that I have no ground wire running from inverter to another ground.
    Of course it already has a massive negative ground back to the battery.
    And the negative on the battery is grounded back to the boats AC ground.

    So what is the point of it??
    I checked and the inverter neg terminal is internally grounded to its metal case.
     
  2. Mark Cat
    Joined: Jun 2011
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    Location: Michigan

    Mark Cat Senior Member

    For USA 120VAC 60 Hz metal case inverters, the case is bonded to the AC output grounding conductor (green wire). This is done to clear an AC ground fault at the inverter.

    The Marine inverters I use the DC ground yellow (DC -, return) is not bonded internally to the inverter metal case.

    If the boat follows ABYC grounding and bonding there will be a connection (conductor) between the AC grounding terminal strip and the DC ground terminal strip.

    If there is an inverter DC + input (red wire) fault to the inverter case, this DC fault will over current the AC grounding conductor (it has a return path through the ground buses back to battery neg).

    This over current will damage the AC grounding conductor. One failure mode is the conductor burns open and we lose the protection of the AC grounding conductor.

    To prevent damaging the AC grounding conductor, a large DC grounding conductor is connected to the case. So if there is a DC + fault to the case the majority of current will flow through the DC case ground blowing the inverter DC + fuse or CB. Thus, clearing the fault.

    The size of the DC case grounding conductor is as follows from ABYC:

    ABYC A-31 Inverters. The DC grounding conductor shall be of an ampacity equal to that of the DC positive conductor.

    Also, the term grounding conductor for AC and DC is applied to conductors that only conduct fault current.

    All for now,

    Mark Cat
     
  3. sdowney717
    Joined: Nov 2010
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    Location: Newport News VA

    sdowney717 Senior Member

    Ok, I accept that is true for marine inverters.

    My inverter is not marine, it is a 3000-6000 MSW made by Peak with built in overload protection. So on these you must keep the AC ground of the inverter never touching the white or black wire. Typically it can blow MSW inverter, on mine inverter has internal protection so it just shuts down if that occurred.
    Since my Onan gen set bonds ground with neutral, that is still not a problem as my transfer switch switch 4 poles simultaneously. I have the inverter hard wired into the boat's AC system.

    So far it has worked very well, flawlessly, better than the older MSW inverter which was same wattage, which I destroyed. I can tell because the microwave runs quietly, the other inverter it was bad on buzzing the microwave. They were both Peak, the good one has a squared off case, the bad one is long rectangle. Peak must have improved the circuit to create a better output.
    It can run my 3 burner Princess stove, fridge, lights, computer, electric heaters, electric grills, tv, circular saw etc... Everything will work except, it wont start my Cruisair 16k heat pump. Too much surge? I am using 2/0 not 4/0 wires in a 9 foot circular path. I will live with it as I dont care. I can always run the gen set.
     
  4. Mark Cat
    Joined: Jun 2011
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    Location: Michigan

    Mark Cat Senior Member

    sdowney717,

    Many newer inverters use an output stage + PWM/Micro that creates a representative AC sine wave, as opposed to a square wave output. Xantrex uses the term True Sine Wave, which is a good way to describe the output.

    True sine wave output helps in appliance and GFCI operation (compatibility). Some GFCIs have difficulties with a square (ish) output waveform.

    All for now,

    Mark Cat
     

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

    sdowney717 Senior Member

    Interesting perhaps circuit has been improved on mine also.

    I have 2 GFCI
    They are working fine with the inverter.
    I am really pleased with this inverter.
    I had read that some inverters might cause a GFCI to buzz.
    My inverter has a remote switch mounted near my control switches for AC power selection to easily turn it on and off.

    One is a 20 amp Leviton outlet style
    Other is a QO breaker GFCI inside the small QO breaker box.

    I keep the fridge off the GFCI power, all other outlets are GFCI and have not had any nuisance tripping.

    This QO box is quite nice, original to the 1970 boat. It has 8 breakers no main.
    The screws don't bear down on any of the wires directly, It was supplied with some small c shaped clips for ground and neutral buss bar screws which I have never seen anywhere else.

    Anyone know or have a source for those?

    Since boat has dual 30 amp, Twin neutrals must be split so I have 3 neutral loads ganged up using a copper bolt on one side and the other side just uses the internal neutral bus bar.

    I have a separate breaker box located 6 feet from the shore power receptacles for the main disconnect. Your allowed up to 10 feet away for this. Both poles break and make, so breaking and making 4 poles, 2 neutrals 2 hots all in the one GE box. It actually has twin 30 amp GE breakers. I though about getting GFCI for that and then the entire system would be GFCI, BUT I read then you may have nuisance tripping, seeing likely the GE breakers would be set to 5 milliamp sensitivity, so it is just on outlets.

    Can you buy 30 milliamp ELCI GE style standard dual pole breakers?

    And yes it is all multi strand wire with solder tinned ends.

    I recently added an outlet that runs out to the aft stern and is mounted under the teak for plugging our electric grill. It is totally out of the way, invisible unless you bent down to look. This way don't have to run extension cord. I was concerned it might bring down the GFCI system, seeing it is more exposed to weather and seas. So the other end has a plug plugged into an outlet in a closed area under the deck behind our coach. If it caused an issue, I could quickly disconnect that run from the circuit.

    The Leviton GFCI runs most outlets, about 10 strewn about the boat.
    The QO GFCI runs the microwave and computer circuit. I have a built in PC for Nav, internet, etc...

    Really cool to use the verizon phone as a hotspot out on the bay.

    Since the boat is a 37 foot egg harbor sedan cruiser, mostly all the wiring is well protected from water and it is all dry never get wet.

    Years ago, before GFCI was in the boat, I remember being in some heavy seas and water was coming in the side window and splashing, dripping into an outlet. It caused it to smoke a little. Salt water is very conductive. Not sure how to protect against that. It is inside the wood cabin. Only happened that one time in many years. With GFCI that will take all the outlets off line except the fridge, microwave and computer.

    Microwave and computer are on the separate 20 amp GFCI QO breaker with well protected outlets. They could only get wet if the boat sinks.
     
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