super-basic inverter-to-subpanel-to-outlet setup gives unexpected meter readings

MajorA
MajorA Registered Users Posts: 4
Thank you energetically in advance for being willing to consider a worried beginner's questions.

I think I've connected things as shown in the Xantrex Installation guide. Basically:
Inverter Line to "top" of subpanel circuit breaker; "bottom" of circuit breaker to one side of outlet.
Inverter neutral to the other side of the outlet.
Inverter ground to subpanel ground bus. Ground bus to grounding rod outside the cabin.

NOTE: Haven't yet turned the inverter on *with the subpanel and outlet connected* -- because ... well, see item #2, below.

Unexpected meter reading #1:
Without the subpanel and outlet connected, when I read AC voltage across the Inverter's outs I get only 84 V.
Being a rank beginner, I am expecting 110-120.
What am I doing wrong?

Unexpected reading #2
With the connections as described above but with the inverter off:
With the subpanel circuit breaker ON, the outlet reads shorted. (Excuse my lame terminology. What I mean is that the screws on both sides of the outlet plug have continuity, zero-out the ohmmeter.)
With the circuit breaker OFF, the outlet reads as I expect it to: no continuity between the two sides.
In fact, everything about the subpanel and the outlet reads as I expect WHEN IT ISN"T CONNECTED TO THE TURNED-OFF INVERTER. When connected to the turned off inverter, the circuit breaker and outlet start scaring me and my multimeter.

Gives me the heebiejeebies to think about turning the inverter ON with what seems to be a shorted-out plug.
Can I be helped?

Thank you again for being here!

Maje

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: super-basic inverter-to-subpanel-to-outlet setup gives unexpected meter readings

    Maje,

    Welcome to the forum and good thing to stop when you see something that does not look "right". Can save you a lot of money by not letting out the magic smoke.

    First, what model of AC inverter do you have? MSW inverters will tend to read "lower AC voltage" than TSW types with the typical AC volt meters.

    Second, have you verified the voltage readings of your meter? Plug into AC power and get around 120 VAC (assuming 120/240 VAC power at your location)? Also try measuring DC Voltage on the car's battery (around 12.7 when turned off/light loads, around 14.2 volts when engine running/battery charging). Flaky DMM's are common enough, and those with a dying 9 volt battery, to make it worth double checking the meter.

    And the "sub panel"... Just to make sure--Are you making a backup power sub panel (have 120/240 VAC to the home, power goes to AC transfer switch, then to sub panel--or through AC Inverter if it has its own AC transfer switch internal to inverter)? Or is the sub panel completely powered by the AC inverter and there is no Utility power or AC generator connections involved?

    Regarding the breaker question... If the breaker is connected to the output of the AC inverter--It probably will read as a dead short between hot and neutral if the inverter is turned off. If this is an MSW inverter (modified square/sine wave), most do not have electrical isolation (between DC input and AC output) and you can get a short circuit through the MSW Inverter if there is a ground AC neutral and a grounded DC negative battery bank.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: super-basic inverter-to-subpanel-to-outlet setup gives unexpected meter readings

    Welcome to the forum.

    Okay, thing one: inverters have transformers on their output. As far as the Ohm meter is concerned this is a wire running from Hot to Neutral. If you use a low enough scale you could read the effective resistance of the big coil of wire that is the transformer's secondary side. This is why it reads open circuit with the breaker off (inverter disconnected) and shorted with it on (inverter connected).

    Thing two: which Xantrex inverter is this? Perhaps an MSW type? Most meters will not read the correct 120 VAC output from one of those but will instead show a very low Voltage - like 84 Volts.

    Many inverters aren't really "off" when they're "off"; they're just powered down. You have to disconnect the DC side and wait for the caps to discharge before the internal circuitry is truly "off".

    If there is a connection between ground and neutral, make sure the inverter can accept this. Otherwise things may go "poof!" and your inverter output will be zero forever more.
  • MajorA
    MajorA Registered Users Posts: 4
    Re: super-basic inverter-to-subpanel-to-outlet setup gives unexpected meter readings
    BB. wrote: »
    Maje,

    >>>what model of AC inverter do you have?

    >>>verified the voltage readings of your meter?

    >>> is the sub panel completely powered by the AC inverter and there is no Utility power or AC generator connections involved

    >>>you can get a short circuit through the MSW Inverter if there is a ground AC neutral and a grounded DC negative battery bank.

    Bill, oh so many thanks!

    I tried the meter out on everything safe I could think of, and it gave plausible readings ... until it told me that both sides of my outlet were 'on the same wire,' so to speak!

    The inverter is one of the Xantrex TR models, the 1524 I think. (I'm away from the cabin, where all the docs are; but the 1524 pictured on the Web looks right.)

    What I'm lamely calling the "subpanel" is just a little steel box with one breaker mounted in it and a "ground bus kit" screwed down on its back wall. There's nothing in it but the breaker-interrupted hot Line -- the line on its way to one side of the outlet.

    I wish like crazy I understood the third statement of yours that I quoted above but I don't. Still, it seems you're trying to reassure me that the "shorted" reading I'm getting when the "turned off" inverted is connected to the outlet via the ON breaker is understandable -- is not ready to blow everything up if we turn the inverter on, leave the breaker ON, and plug a lamp into the outlet?
  • MajorA
    MajorA Registered Users Posts: 4
    Re: super-basic inverter-to-subpanel-to-outlet setup gives unexpected meter readings
    Welcome to the forum.

    >>>As far as the Ohm meter is concerned this is a wire running from Hot to Neutral.

    >>>which Xantrex inverter is this? ... Most meters will not read the correct 120 VAC output from [a MSW inverter]

    >>>If there is a connection between ground and neutral, make sure the inverter can accept this. Otherwise things may go "poof!"

    Cariboocoot, thank you for your help! You seem to see good [safe?] reasons for the readings that are worrying me. The inverter is, I'm nearly certain, the TR1524, which is a MSW model -- so my 84V may not doom me.

    Of course it's your last quoted comment that rings in my head, mostly because I don't quite understand it. Where might this connection between ground and neutral be? The way I've ignorantly done things, the white wire goes straight from the Inverter Neutral AC Out to the non-hot side of the outlet. There is a ground bus "kit" screwed against the back wall of the little subpanel box, where the circuit breaker and the hot black wire are, but that ground bus is not yet connected to the ground rod outside the cabin.

    Is "poof!" still a possibility?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: super-basic inverter-to-subpanel-to-outlet setup gives unexpected meter readings

    Right: AC has three wires to it; Hot, neutral (the two flat prongs), and ground (the round one).

    In fact the two flat prongs are both hot until one of them is connected to ground. That takes its Voltage potential to zero in respect to ground, because they're the same thing after that. This is known as the Neutral-Ground bond. It is normal to see this inside a breaker panel in a house. Your comment about "sub-panel" made me concerned there might be one in the box.

    Most MSW inverters can't take this connection. Some can. You need to check the manual on it specifically to see if it is allowed. When in doubt, leave it out. It complicates the AC wiring in terms of safety: it is then considered "floating neutral" and should have a dual breaker that will disconnect both lines in the event of an over-current problem. Otherwise the "neutral" is still hot in respect to ground.

    Something of a minor consideration, really.

    You can fire up the inverter with no breaker/wire/outlet connected to the output and check the AC Voltage then. if it's going to blow, it's going to blow; you've got to try it sometime. If there's proper circuit protection on the DC side the effects should be minimal.

    So your AC output from the inverter should look like this:

    Hot ---> Breaker ---> Outlet
    Neutral ----> Outlet
    Ground ----> Outlet

    No connection between Neutral and Ground. Ground can be connected to Earth.

    I almost think the TR series is one of a few that can have the N-G bond and that this came up in another thread just a couple of days ago. Gawd my memory is getting sludgy! :blush:
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: super-basic inverter-to-subpanel-to-outlet setup gives unexpected meter readings

    Aha! Found it!

    The installation manual link: http://lib.store.yahoo.net/lib/wind-sun/Trace-installmanual.pdf
    Thread about the TR2412 wiring: http://forum.solar-electric.com/showthread.php?18992-Off-Grid-Cabin-Wiring-with-my-Xantrex-TR2412
  • MajorA
    MajorA Registered Users Posts: 4
    Re: super-basic inverter-to-subpanel-to-outlet setup gives unexpected meter readings
    >>>Most MSW inverters can't take this [neutral to ground]connection.

    >>>Hot ---> Breaker ---> Outlet; Neutral ----> Outlet; Ground ----> Outlet
    No connection between Neutral and Ground. Ground can be connected to Earth.

    >>>You can fire up the inverter with no breaker/wire/outlet connected to the output and check the AC Voltage then.

    Superb to have you looking at that durn manual page with me. I had been puzzling over it madly, apparently not quite getting the graphical conventions. I see the Line coming in to the top of a breaker-graphic ... but not emerging out the bottom. And I saw the assertion of a Neutral to Ground connection. Cariboocoot, I'm concluding from your discussion that with my TR1512 I can use *either* approach. I can skip the N-G bond or I can use it. But since my little "load center" box has no neutral bus, I'm'unna skip it. Yes?

    Your quick "diagram" is exactly what I've currently got ("no connection N to G] ... and I'll take it a step further by hooking the ground bus to the rod outside.

    When I detached the breaker and outlet from the inverter altogether and fired the inverter up, that's when I got the 84 V AC reading with my digital multimeter ... which you guys have suggested may simply be a response of the meter to the MSW nature of the inverter.

    If I now boldly plug a lamp and a small TV up to my setup, the voltage I need should be present.

    Grateful applause to youse!

    Maje