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Thread: Should DC and AC circuit have common ground ?

  1. #1

    Bean's Avatar
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    Default Should DC and AC circuit have common ground ?

    I'm working on a project that uses a power inverter, it converts 12VDC into 120VAC (you know the kind you can plug into a cigarette lighter to power a laptop).

    So my question is, should the 120VAC ground wire(not the neutal, the green·ground wire)·be connected to the 12VDC ground wire ? I know in house wiring you always tie the grounds together, but I'm not sure in this situation if the the DC and AC should have a common ground or not ?

    Can anyone help ?

    Bean.


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    Post Edited (Bean (Hitt Consulting)) : 9/24/2007 5:07:25 PM GMT
    Last edited by ForumTools; 10-02-2010 at 09:44 PM. Reason: Forum Migration

  2. #2

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    Bean,
    It depends on the inverter and the application. The green ground wire is usually connected to the inverter's case. It should be connected to chassis / case / frame of your project. It may or may not be connected to the 12VDC ground wire. The 12VDC input terminals may be completely isolated from the green ground terminal. In any event, connect the green ground wire to the chassis / case / frame.
    Last edited by ForumTools; 10-02-2010 at 09:44 PM. Reason: Forum Migration

  3. #3

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    If a house is properly wired, the green ground gets connected to the bare wire inside the Romex running through the house.· All of those grounds have a path to an earth ground which is a copper-clad steel bar driven into the ground somewhere near the main power panel (circuit breaker panel).·

    It typically doesn't matter if the DC ground is at the same potential as an earth ground.· It's just a reference for the DC circuit.
    Last edited by ForumTools; 10-02-2010 at 09:44 PM. Reason: Forum Migration

  4. #4

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    The only downside I can see is Noise interference from a "dirty" earth connection as most electrical systems have. probably would need good filtering?
    Last edited by ForumTools; 10-02-2010 at 09:44 PM. Reason: Forum Migration

  5. #5

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    The green wire is household AC goes directly into a 10 foot copper rod that is driven into the soil to divert any direct shorting to a genuine ground. Often it is also connected to the white or neutral wire in 110 AC or the 'middle wire' in 220AC at the service box for the building. But in any event, it has no fuses on it and goes directly into the soil.

    The problem is really about two questions: isolation and safety. When AC goes through a transformer, the far side is isolated. That is to say you don't complete a circuit by merely standing bare foot on the ground. For instance, a wall wart with a transformer will not provide you with a lethal shook, but one that is transformerless may actually kill you if you happen to touch a water pipe and the wire at the same time as the 120VAC has a possilbe·pathway.· And that is why you should look for the UL stamp·on wall warts or test them for isolation.

    Since 12VDC is not really a safety hazard in the same way as 120v AC or DC, I would just provide an option for the green 3rd wire of the 120VAC to be connected to a cold water pipe [usually that goes eventually into the ground]. In that way, any 120VAC applicance that might need the proper safety of a ground would have good access. I might also tie the external chassic of the converter into the same ground system in case of a failure where shorting is occuring on the 120VAC side.

    But I really wouldn't worry at all about the 12volt DC to ground as a safety feature.

    Though in automotive, the negative is generally the equivalent of ground and all questions of polarity should follow the same. But I would skip tying it directly into the chassis ground in common with the AC if it would be on the 'other' side of the transformer that is creating isolation. In sum, keep the isolation and use it for safety. There is always the remote. possiblity of lightning or another AC source entering the 120VAC side and eventually reaching the DC side via a shared, but not properly grounded system.

    Also, it is always easier to have a non-conducting, fully insulated chassis rather than a steel box for safety.

    I hope that makes sense to you.

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    Post Edited (Kramer) : 9/24/2007 6:48:13 PM GMT
    Last edited by ForumTools; 10-02-2010 at 09:44 PM. Reason: Forum Migration

  6. #6

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    Terry,

    My understanding is that if the inverter has a metal chassis, that chassis should be connected to your AC ground connection. (You already know that.) I think it is typically advantageous to connect your DC section to the AC ground as well, though technically this may not be absolutely required.

    If the inverter is installed in something like a car or an RV then the vehicle chassis ground and the negative terminal of the battery are typically connected together. If the inverter is installed in a building I think it can be “acceptable” to have the 12V DC section ungrounded if designed properly. However, I think it is most often advantageous to ground the DC side as well. I will attempt to explain this below.

    I can not speak with any real authority on the interpretation of the National Electric Code. So feel free to check it out yourself. However, my understanding of what I have read in the past is summarized in the following statements.
    • 1.) Technically, circuits running at less than 50V do not require grounding.
    • 2.) However, all current carrying conductors in an ungrounded circuit need to have over-current protection devices and disconnects installed. (More on this later.)
    • 3.) Equipment-grounding conductors should be used with appliances to ground exposed metal surfaces. (I think you know this.)
    • 4.) Prior to 1996 ground wires needed to be the same size as the largest conductor in a DC system. This is still a good guideline. However, with the release of the 1996 National Electric Code several exceptions were made that now allow smaller conductors to be used in certain cases.
    • 5.) If the requirements for the AC and DC grounding electrode conductors are different, the larger of the two should be used for any common conductor. Also the common grounding point should be associated with the largest required grounding electrode conductor.
    • 6.) Be sure to keep in mind that the connection between one of the current-carrying conductors and the grounding electrode conductor should be made only at one point in the system!
    You also need to keep in mind that these statements may not be completely accurate! They represent my understanding of what the code requires.

    I think it is because of statement 2.) that grounding a DC system sometime has an advantage. It seems to require only one over-current protection device (possibly a fuse) rather than two (one for each current carrying conductor) for an ungrounded system.

    Of course, it is really late! (Or is it actually early now?) So I may not be making much sense with my attempted answer. If I am wrong I hope someone will gently correct me.

    - Sparks
    Last edited by ForumTools; 10-02-2010 at 09:44 PM. Reason: Forum Migration

  7. #7

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    Bean,

    This is cheating, but I ohm'ed a 12VDC to 115VAC Power Inverter that I have (Radio Shack 22-132B), and it shows a short between the ground of the 12VDC connector, and the ground (center) plug of the 115VAC connector.

    Slowly getting back into it!

    Rodney
    Last edited by ForumTools; 10-02-2010 at 09:44 PM. Reason: Forum Migration

  8. #8

    Bean's Avatar
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    Thanks to everyone for the help.
    I put a wide trace to connect the two. I can cut it if I need to.

    Bean.'

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    ·
    Last edited by ForumTools; 10-02-2010 at 09:44 PM. Reason: Forum Migration

  9. #9

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    If this inverter is intended only for a Laptop computer, I'd probably keep it isolated. Also, I'd provide an all plastic outer casing and use a two prong AC plug.

    If you share ground, you have to be 100% you are not bypassing the transformer's function. And you might pick up noise from odd places. Also you have to think about all the other silly safety possiblities of touching the car, getting hit by lighting, stepping in a puddle, etc.

    At the end of the day you are going from DC to AC to Isolated {?} AC battery charger to DC. The end user device has very little shock hazard. The Battery charger is probably equally well insulated and safe.

    But if someone is going to use it to drive an aluminum cased power saw in the rain or around salt water, it is a whole different issue.·'AC side' grounding should be genuine. I've been knocked on my butt by a wet saw with faulty ground many times, multiple times in one day.

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    "Everything in the world is purchased by labour; and our passions are the only causes of labor." -- David·Hume (1711-76)········
    ···················· Tropically,····· G. Herzog [·黃鶴 ]·in Taiwan
    Post Edited (Kramer) : 9/26/2007 7:09:05 PM GMT
    Last edited by ForumTools; 10-02-2010 at 09:44 PM. Reason: Forum Migration

  10. #10

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    Hey, Bean…

    Are you by any chance building an inverter that can be paralleled? The ones currently on the market with this capability are very expensive. If you are building one I would love to hear about it!

    Just wondering…

    - Sparks
    Last edited by ForumTools; 10-02-2010 at 09:44 PM. Reason: Forum Migration

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