Battery Backup Inverter System Question

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  #1  
Old 03-01-19, 11:14 PM
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Battery Backup Inverter System Question

Hi all, to start I have a protran/2 6 circuit transfer switch wired into my main panel. It allows me to plug in a generator and legally switch over my 6 critical load circuits in case of a power outage.

I wanted the ability to operate some of the loads (like the boiler transformer) for a while without going outside and hooking up the generator, especially if during a really bad storm so I setup a small battery backup system.

3 x 12v 50ah agm deep cycle vmax batteries in parrallel. this is connected to an inexpensive but decent quality xantrex prosine 600watt true sine wave inverter. i have it setup so instead of the generator (which has a floating neutral) i plug in the inverter to the transfer switch and power it up and then i can switch over my 6 critical circuits and run them off the inverter. (probably not 100% kosher due to the inverter only being ul listed for marine and rv use, not for home service use, i am aware of this).

so since of course my neutral and ground are bonded at my main panel, i cant have the inverter neutral and ground bonded otherwise i get an ac ground loop. the inverter does in fact bond neutral to ground be cause its meant for marine and rv use where this would be desired. I was curious so I did test it this way and it does work fine, i did notice 16v ac between the dc battery terminals and the equipment ground on the panel which i assume has something to do with the ground loop. now of course in this case i measure 120v AC between the hot and neutral coming out of the inverter, and 120v ac between hot and ground as the neutral is bonded to the ground on the inverter.

so what I did was open up the inverter and find that although its undocumented, they make it so 1 single screw bonds the ground and neutral and removing it, removes the bonding. at this point I turned the inverter into a floating neutral and i believe this alleviates my issue of the ground loop. the inverter functions normally and works normally in my transfer switch. with mains on, i was able to run 1 circuit off the inverter via the transfer switch. I did notice 60V AC between the hot conductor and ground and 60v between what would be the ungrounded conductor and ground coming out of the inverter socket. I believe this is NORMAL though for an unbonded 120v inverter and I believe this is the same behavior as the honda inverter generator with the floating neutral.

this all works well after testing, no stray ac voltage was detected on the dc side or between dc and ac however I want to make sure I am not missing something that one of the experts here might pick up (besides the already mentioned fact that this is not 100% kosher due to the inverter not having the correct UL listing)

Thanks for your help.

Mark
 
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Old 03-02-19, 10:06 AM
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"ground coming out of inverter socket"

I read this twice and am left slightly confused. To be called "safe", the inverter chassis must be connected to house ground. Is it? You don't want the chassis to be 60V off of local ground, that is definitely dangerous. Same with the battery posts.

Also, I don't understand the obsession with ground loops. Is this causing an inverter fault?

ps: most every service has two neutral to Protected earth connections. The first is at the overhead or pad mount transformer, the second is a single place in the dwelling, usually at the load center. Yes, its technically a "ground loop", but they are there for important purposes, and don't have a significant down side.
 
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Old 03-02-19, 02:21 PM
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With an inverter..... the neutral and the ground should remain connected.
The only reason for un-bonding the neutral is to keep a GFI from tripping on the generator.
 
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Old 03-03-19, 06:52 AM
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"ground coming out of inverter socket"

I read this twice and am left slightly confused. To be called "safe", the inverter chassis must be connected to house ground. Is it? You don't want the chassis to be 60V off of local ground, that is definitely dangerous. Same with the battery posts.
I'm sorry I wrote this in a confusing way. The inverter chassis is bonded to the ground pin on the inverter output outlet, when connected to my transfer switch with a 3-pin cord the inverter becomes grounded to my house ground. The inverter also has a chassis ground connection that I will connect to the house ground as a backup for permenant installation but since the inverter cant be used without the 3 prong cord, its by default grounded.

When I say ground coming out of the inverter socket, I am talking about the ground pin on the outlet on the inverter.

So basically if the inverter is not connected to the house, just powered up from the batteries, I measure 60v from the "hot" pin to the ground pin, and 60v from the "neutral" pin to the ground pin. I believe this is how a 120v inverter or inverter generator works when it has a floating neutral. It makes sense in my head since it techinically doesnt have a neutral so I assume this is normal. Similar to the way a 240v ac circuit works, you have half voltage on both pins in reference to ground.

Now once I connect the inverter to the transfer switch, the neutral is now bonded to ground at my ac panel so this behavior of 60v between neutral to ground and hot to ground disappears since the neutral is now grounded.

Also, I don't understand the obsession with ground loops. Is this causing an inverter fault?
Well its my understanding that inside a house, there should only be one single bonding of neutral to ground. Its my understanding, ground loops can damage inverters, at least that is what I read about the AIMS inverters. They tell customers to disconnect the neutral to ground bond when installing in a setup like mine.
 
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