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grounding neutral

Discussion in 'Home Power and Microgeneration' started by Q, Jul 8, 2003.

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  1. Q

    Q Guest

    I have a 1000 watt Xantrex inverter that I would like to use as a backup to
    my 3000 watt Vector. The Vector works fine in my house with standard wiring
    which includes a grounded neutral. The Xantrex on the other hand says the
    neutral cannot be grounded. I let the smoke out of a smaller inverter when I
    plugged it into the house wiring with a grounded neutral.

    My question is:

    Is it safe to disconnect the neutral from ground?

    Q
     
  2. Dale Farmer

    Dale Farmer Guest

    The answer is, it depends. The manufacturers data with the thing should
    have
    some sort of diagram telling you how to connect it up to make a safe and
    legal system. Get and follow their directions. The telling you that the
    neutral
    cannot be grounded makes me deeply suspicious about the quality and safety
    of the unit, and I'd have someone who knows their way around inverters better
    than I take a look at it.

    --Dale
     
  3. Mark or Sue

    Mark or Sue Guest

    I don't know if we have enough information to tell. Does this unit have a
    grounding pin on its AC power outlets and if so how do they connect
    internally to the inverter?. If the output is totally isolated from earth
    ground, it is shock safe from ground faults but could be susceptable to
    lightning strikes if connected to long wire runs. Since house distribution
    systems are grounded (or more correctly must have their neutral and ground
    wires bonded together), you will have limited use of this inverter if you
    can't determine a way to ground the thing. I don't know why there would be a
    difference between ground and neutral in an inverter...just where does
    ground go that neutral can't?
     
  4. I know that the Xantrex Portawattz line can not have the neutral grounded. I
    smoked one last week. There is a hard to notice warning in the user
    manual that the inverter is not to be connected to a wiring system with
    a grounded neutral (grounded neutral is required by the USA National
    Electrical Code). I thought that they were trying to avoid a situation with
    dual neutral-ground bonds. Turns out that the AC is not isolated from the
    DC input and that grounding the neutral when the dc is grounded causes
    internal non-warranty damage. Send the unit in with $US 65 and they will
    replace it. This is also the reason the Portawattz line is not UL listed.
    Non-listed components should not be used in systems where the NEC
    applies.

    After calling/e-mailing all the smaller inverter manufacturers I found that
    except for the Coleman Powermate (PMP800), all other small inverters
    have this problem. Even the Coleman is not UL listed.

    I checked inside the non-functioning unit and there are no obvious burnt
    components. The basic dc/dc up converter (to 145 vdc) does not work
    and the integrated circuit associated with this conversion is totally
    unmarked. Unit is made in China.

    Therefore, I do not think you can use a low cost inverter as backup, but
    investigate the Coleman. Outpost.com has the 800W for about $US 70,
    but the first unit I bought was dead on arrival.


    Bill Kaszeta
    Photovoltaic Resources Int'l
    Tempe Arizona USA
     
  5. Are you talking about the wiring in your house? (I assume so, since no
    inverter has an internal connection between AC output and ground.)

    Are you entirely off the grid?

    If so, you could disconnect neutral from ground at the AC distribution
    panel in a pinch, hopefully temporarily. After all, being connected to
    ground at the panel is what makes it "neutral" in the first place, and
    much of the safety strategy is built around the assumption that this
    connection exists. It is not a recipe for instant death or anything
    (if it was, all those little car inverters would be slaughtering
    people regularly) but I wouldn't leave things that way.

    Using your Xantrex (or any non-isolated inverter) with the neutral to
    ground connection broken and the DC negative input grounded, the
    (former) hot and neutral AC terminals will both be at 60VAC with
    respect to ground. So, e.g., the threaded connection of a lamp socket,
    which should normally be at ground potential, would have 60VAC on it.
    Equipment and circuits that are supposed to be turned off (switches or
    panel breakers thrown) would still have 60VAC on their neutral
    terminals. Like I said, not exactly instant death scenarios, but
    certainly a lot less safe than normal.

    FYI the real problem with non-isolated inverters isn't anything to do
    with ground per se, the problem is connecting one of the AC output
    terminals to one of the DC terminals by any means. It happens when you
    ground both DC negative and AC "neutral" as in your house, but even if
    you had a battery and your Xantrex inverter sitting apart from the
    rest of the world on a rubber mat, and you connected "neutral" to DC
    negative (or positive for that matter) with a wire, it would still let
    the smoke out. Without isolation (transformer) between input and
    output, any connection between the two is a no-no.

    So back to the house - another option is to leave the neutral grounded
    and lift the DC negative-to-ground connection instead. Then you have a
    new twist - AC voltage with respect to ground on the DC terminals, but
    hey, only when the inverter is actually running. The DC loads
    shouldn't notice. I don't think.

    Either solution is suboptimal IMHO, but in a pinch they will work. For
    a temporary fix, I'd choose the former, i.e. leave the batts grounded
    and lift the neutral.

    Last thought - you could buy a honking isolation transformer that will
    take the full output of the Xantrex (and then some - it will probably
    get a little warm with all those unexpected harmonics) so you can
    ground both ends of the inverter/transformer combination properly.
    Then you've built yourself a fake Trace DR, with no charger. :)

    Electrically it makes sense, economically it may not. Unless you get a
    damn good deal on the iso-trans (not impossible at surplus, ham swap
    meets etc.) that solution would be pretty silly compared to just
    getting a better inverter with isolation built in.

    -=s
     
  6. There ought to be a ground on the system _womewhere_ and any system that
    floats is trash.

    ;B
     
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