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Thread: Connecting an inverter to a transfer switch

  1. #1
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    Default Connecting an inverter to a transfer switch

    I want to do something the instruction manual for my inverter says not to. I want to hook it up to a transfer switch to power some circuits so I don't have to use extension cords when I need to run off batteries when the utility power is down.

    I have a Samlex SSW-1000-12A inverter whose manual says: "WARNING: Do not connect directly to AC distribution wiring. This inverter is NOT grid interactive." That's not a surprise, I know better than to try a plug it into a live circuit, thus the transfer switch.

    It also says: "Do not connect the power inverter to any AC load circuit in which the Neutral conductor is connected to Ground (Earth) or to the Negative DC (battery) source." That's more of a problem since my house wiring bonds neutral and ground, of course.

    I'm guessing this is a fairly common problem. Is there a common solution other than "don't do that"?

    Hooking my inverter to a transfer switch isn't as odd as it may sound. The transfer switch is an APC UTS6H, an automatic transfer switch with connections for both generator and battery backed UPS. This model is a varient for Honda that accepts a L5-30 120V connection for a Honda EU generator, I have an EU 3000is. The UPS connection is probably intended for a standard APC UPS, but I figure my inverter with 380AH of battery behind it will give me a lot more time before I need to run the generator and recharge the batteries.

    I first hooked the switch to a standard 750VA UPS, which worked just fine. I then tried plugging it into my inverter. I immediately heard a pop and no power from the inverter. The GFCI outlet on the inverter tripped. The GFCI tripped even when hooked up through a switched off power switch since the neutral and ground weren't switched off. I finally used a two prong extension cord without a ground wire to connect the inverter to the switch. That works without the inverter complaining. It works, but is it a good way to handle the problem?

    Other than the problem with the GFCI on the inverter I'm pleased with the setup. If power fails the switch automatically transfers the load from up to 5 circuits to the inverter. When I get out the generator and fire it up the loads are transferred to it and I can recharge the batteries. I can now run my gas furnace when the generator is on, though that's too much for the inverter. The switch is configured to only run the furnace circuit off the generator. Depending on how long I want the batteries to last I can configure the switch to just run the fridge circuit from the inverter or to include the TV room, computer room, and bedroom.

    I'm just worried I've violated some important safety rule by hooking my inverter to the switch without a ground wire. I also haven't gotten around to making a proper ground for the battery system. Possibly once I do that I'll have the same problem since the ground on the inverter will then be connected to the same ground as the house wiring. Do I need to remove the GFCI outlets from the inverter or defeat it in some way?

  2. #2
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    Default Re: Connecting an inverter to a transfer switch

    Most Mod-sine inverters (I don't know what yours is) have an output leg, tied internally to the battery, 'cause it's cheaper to do that way. And if you connect it to a household electrical panel, 1 of 2 things can happen, the batteries get raised to 120V on one side, or the magic smoke comes out of the inverter. there may be a 3rd or 4th, beyond my imagination.

    Follow the instructions in the manual - safety third you know !
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  3. #3
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    Default Re: Connecting an inverter to a transfer switch

    The inverter has a push-pull DC-DC boost to produce about 155 vdc for a MSW inverter. This 155vdc then is fed to an H-bridge to chop the DC to +155,0,-155,0 volts for the MSW output.

    The issue is the H-bridge reverses the polarity on both neutral and hot lines creating a problem for a grounded neutral situation. Some inverters have optical isolation for the feedback loop for the 155 vdc boost DC-DC converter. This allows the whole secondary HV side of the DC-DC converter to float with respect to primary battery side. These can tolerate a grounded neutral.

    If there is not opto isolation for the 155v switcher feedback then, when you ground the neutral, the negative terminal on the battery becomes live with HV pulses with respect to output grounded neutral. Touch a terminal of the battery while standing grounded on floor and you will get zapped.

    When an inverter is running asyncronously to the grid it is not a good idea to make an immediate relay switchover. If you have any AC induction motors, like a refrig compressor (which we don't recommend running from a MSW inverter), an immediate switchover may occur when the inverter happens to be out of phase with the grid. To an induction motor this creates a large surge current that can damage the motor. MSW UPS units syncronize their inverter to grid to allow a more immediate crossover.
    Last edited by RCinFLA; December 9th, 2011 at 20:20 PST.

  4. #4
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    Default Re: Connecting an inverter to a transfer switch

    The inverter is a pure sine wave 1000 watt unit capable of surges to 2000 watts.

    http://www.samlexamerica.com/product...roductsID=4002

    I've successfully run my fridge and sump pump on it.

    I believe the automatic transfer switch does take a couple of seconds to switch from one power source to another. I certainly noticed a pause a couple times during testing, just not sure there was always a gap. Hopefully the won't be a problem from too quick a cut over.

    My goal was to put together a system much better than a standard MSW UPS. I've found those useful but limited in past outages. I ran across the transfer switch after I had the inverter. What sort of specs would I be looking for in an inverter that could act as either a stand alone unit or power household circuits through a transfer switch?

    Is it the GFCI outlet on the inverter that is the problem or is it something more basic?

  5. #5
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    Default Re: Connecting an inverter to a transfer switch

    Most small inverters have a floating neutral, and if you wire them to a grounded neutral buss, through the transfer switch, you will find the inverter going up in smoke very quickly,,instantly in fact!

    I know this for a fact, as I. Ironed up two of them in rapid succession before I figured it out!


    So unless you can isolate the neutrals on the circuits that you intend to energize my guess is that you are going to have real trouble doing this. Some small inverters permit a grounded neutral, (Suresine 300 for example) Check the specs on your inverter.

    Tony
    Please note, being a moderator does not add any weight to my opinions 300 watts Siemens/BP panels,plus a Sun 90,, making ~400. ~30 amps into Rogue MPT-3024, 450 ah of Trojan T-105, Morningstar ts300 inverter, a Tri-Metric meter.a collection of antique generators, plus 2 Honda eu-1000i's (also a BS2512 IX controller) and assorted other stuff!

  6. #6
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    Default Re: Connecting an inverter to a transfer switch

    I just Installed a Magnum MS-AE-4000watt 24 volt inverter. It isn,t advertised as a U)PS inverter but transfers at 16 hundredths of a second with the grid power switched off. My computer doesn.t go down. It doesn,t flicker the lights. Don,t know if you want to go that big but I can say I am very happy with it. I picked up mine used but if I was going to buy new I would get the pae model that suports AC coupling. solarvic
    16 KC 158G & 3 KD185GX-LPU panels on Fronius IG PLUS 3.0-1 inverter and 14 SHARP NDU3A & 1 KD185GXU panel on FRONIUS IG-3000 inverter. All mounted on pole top racks. Retired and enjoying it!!

  7. #7
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    Default Re: Connecting an inverter to a transfer switch

    Typically, TSW inverters are isolated and MSW inverters are not isolated between battery and AC output.

    See word "typically".

    Pick a couple inverters and read the manuals closely to see if they meet your needs (or not).

    The one problem with common neutrals in branch circuits is that many homes are wired with Romex (Black/Red/White + ground)... If you use 120/240 VAC split phase, then a 120 VAC load on Black/White will balance the current on the White lead if an equal load is placed on Red/White.

    If, instead you use a 120 VAC inverter, then the Black and Red load currents will ad up on the White lead:

    • 120/240 VAC split phase; 15 amps on Black and 15 amps on Red (120 VAC) will add up to zero amps on White
    • 120 VAC output with 15 amps on Black and another 15 amps on Red will add up to 30 amps on the White return wire--Not good.

    So, if your home is wired with 2 hots and a return, you are correct, a 120 VAC only inverter can overheat the white/return cables if A) The inverter is large enough and B) the loads are on both Black and White, at 50% or greater of rated maximum current.


    120/240 VAC split phase power source is (more or less) just a 240 VAC transformer with a Center Tap to provide a "neutral lead" for 120 VAC loads.



    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  8. #8
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    Default Re: Connecting an inverter to a transfer switch

    Quote Originally Posted by icarus View Post
    Most small inverters have a floating neutral, and if you wire them to a grounded neutral buss, through the transfer switch, you will find the inverter going up in smoke very quickly,,instantly in fact!

    I know this for a fact, as I. Ironed up two of them in rapid succession before I figured it out!


    So unless you can isolate the neutrals on the circuits that you intend to energize my guess is that you are going to have real trouble doing this. Some small inverters permit a grounded neutral, (Suresine 300 for example) Check the specs on your inverter.

    Tony
    A floating neutral isn't necessarily a bad thing, right? My Honda EU generator doesn't bond the neutral to ground, so it has a floating neutral as is required for connecting to house wiring with a bonded neutral. The transfer switch manual cautions that the generator shouldn't bond ground and neutral but doesn't mention anything about the UPS connection. They probably assume you'll be using a standard APC UPS but don't mention it as a requirement.

    Elsewhere I read that: "The term floating neutral is often used to describe a system where the neutral is not bonded to ground. Others also use it to describe two hot leads and a ground, with no neutral."

    That sounds like a 60/120 split phase system with hot and neutral each carrying 60V with respect to ground. Sort of a half voltage version of a standard 240 volt circuit. If that's the way my TSW inverter works I can see I'm going to have trouble.

    The good news is that so far I haven't released the blue smoke from my inverter though I have already rashly connected the hot and neutral to my transfer switch. Possibly this only works because my battery negative isn't connected to ground (I'd meant to get around to that). When I test with a voltmeter while there is a moderate load on the inverter I find a 120V AC potential between battery negative and a grounded pipe. That sounds like bad news unless it's some sort of minor induced voltage. Is there anyway to test if this is a meaningful problem?

    If my inverter neutral is indeed hot with respect to the battery negative is there anyway to solve that? Would using an isolation transformer between my inverter and the house wiring solve the problem? It would be less expensive for me to buy something like a Tripp-Lite IS1000 http://www.tripplite.com/en/products...txtModelID=228 than to get a new inverter meant for a hard wired connection like http://www.solar-electric.com/sa2wa12vosiw.html or http://www.solar-electric.com/gtfx2524.html

    Bill, I do understand about black/red/white branch circuits with common neutrals. I have a couple of those in my house wiring. I made sure to put only one side of any of those on my 120V transfer switch. I suppose that as the utility power returns the black wire could be powered by utility power and the red wire side by my backup source. The phase between the two sources would be random thus the two currents could add on the return neutral. But the condition would momentary, a few seconds at worse, until the automatic transfer switch noticed the return of utility power and disconnected the backup power and reconnected to line power.

  9. #9
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    Default Re: Connecting an inverter to a transfer switch

    It also says: "Do not connect the power inverter to any AC load circuit in which the Neutral conductor is connected to Ground (Earth) or to the Negative DC (battery) source." That's more of a problem since my house wiring bonds neutral and ground, of course.
    I don't mean to be overly cheeky, but what part of this don't we get?

    Tony
    Please note, being a moderator does not add any weight to my opinions 300 watts Siemens/BP panels,plus a Sun 90,, making ~400. ~30 amps into Rogue MPT-3024, 450 ah of Trojan T-105, Morningstar ts300 inverter, a Tri-Metric meter.a collection of antique generators, plus 2 Honda eu-1000i's (also a BS2512 IX controller) and assorted other stuff!

  10. #10
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    Default Re: Connecting an inverter to a transfer switch

    on the inverter I find a 120V AC potential between battery negative and a grounded pipe. That sounds like bad news unless it's some sort of minor induced voltage. Is there anyway to test if this is a meaningful problem?
    That's GOING to be a problem, if you had tied your battery - to ground, you would have fried the inverter.
    READ THE MANUAL for the inverter, and follow it's recommendations.

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