Inverter pulling power when conected to house ground

[WindriderNM]

When I try to connect my 1500W mod. signwave inverter to my homes service box it pulls about 400w. I am not connected to the grid. This happens when I connect the nut./ground wire of the house the ether 110v side even with all the breakers off It also happens when I connect to the house Nut/ground with ether 110v wire alone. I have a wind/solar 12v system with 12 6v batteries PLEASE HELP
 ~~~Keep those electrons flowing~~~
              WindriderNM

[bob g]

the modified sine inverters cannot be connect to the house ground, because
the house white wire is bonded to the ground at the fuse/breaker box (at least here in the states)
therefore the white wire being supplied by the inverter will the shorted to the inverter case ground as well.

this is a common problem, and i don't know how you get around it legally.

the only way to make it function is to not bond the case of the inverter to the house ground, keep it isolated

this is likely counter to code, so it probably is not legal, but it will generally solve the draw problem
just keep the inverter somewhere that is protected from incidental contact to reduce shock hazards.

you will also need to keep the inverter battery supply negative isolated from either the house ground or earth ground as well.
otherwise you will find the inverter will find ground internally to the AC side and you will end up with the same issue.

bob g

[dnix71]

You are lucky you didn't hurt yourself or fry the inverter. You absolutely cannot legally or safely connect that inverter to your house mains.

It has a floating neutral. Homes have neutral bonded to ground. Generators under 5KW in the US are wired the same way as your inverter and are also never to be hooked up to house wiring.

On 120v house wiring, the wider prong is neutral, the narrower one is hot. Neutral and ground go the the same place. Ground is really just a redundant neutral in case the neutral on the appliance or outlet goes open, the current has a safer, easier place to go than through your body.

On your inverter and small generators neither prong is hot or neutral. When there is a complete circuit across the two, a sensor in the inverter switches the inverter on.

When you connect the inverter to the house ground you are shorting out one side of the power circuit in the inverter. It senses a "load" and switches on, but the return path is not proper. Normally that would cause the inverter to blow majic smoke or give you a nasty shock. You were fortunate.

[crashk6]

As the other two respondents state, you can not bond your modified sinewave inverter's "neutral" to ground as it is floating. There are however two ways to safely connect it to your panel and preserve the safety ground for the structure.

Method one, you may use a 1to1 isolation transformer between the inverter and the connection to your panel then neutral bond the panel as usual, provide a main breaker of course. There are caveats to this approach, your inverter will waste power in the transformer, but, it will be electrically isolated from the bonded neutral. In this configuration there are some concerns about grounding of the DC side of the the system.. Depending on the inverters topology, not bonding the DC side to ground may be desirable. This method would have the sole benefit of keeping you compliant with electrical codes for neutral bonding. Making it "legal".

Method two, which I favor, is to bond the battery bank to ground, the ground wiring in the AC panel should connect to this same ground system.

DO NOT bond the "neutral". The inverter output then passes through a residual current device or RCD (GFCI in the US) before connecting to your main breaker in the panel. Thus in this configuration if any power leaves the floating potential AC system and finds ground, power from the inverter is disconnected automatically. It makes for a usable system with a cheep modified sinewave inverter and does not waste energy in a transformer. Plus you have a safety ground on the whole system.

It does however have caveats. It is not code compliant. Some motor loads may cause nuisance tripping of the RCD, such loads would need capacitors but most capacitors do not like modified sinewave.. if it's a universal motor you may install a rectifier followed with a smoothing capacitor. However speed controls are incompatible with this modification. Most devices will function without problem or need for modification.


I will remind you that an inverter such as yours and mains power should absolutely never meet, my examples are for isolated stand alone applications only.

Another point to think about that may be important to you is from the standpoint of insurance.. If you like many people participate in things like homeowners insurance, companies may invalidate your coverage if you use a non standards approved inverter in your power system.

I will however say I have seen instances where some electrical inspectors familiar with RE systems have approved systems in the configurations I speak of.

What is safe in not always legal and what is legal is not always safe.. it's nice when the two find resolution in an educated electrical inspector.

Truthfully I feel I have glossed over allot, I can get more in depth if need be. I simply wanted to illustrate that although not an ideal circumstance electrically or legally it is not impossible to overcome the technical issues.

~S~

[WindriderNM]

Thanks this has been very helpful. I have a few large isolation transformers I will try and see what the losses are. I am very carefull when when hooking things up and only do it momentarily . I tried a power conditioner to see what it would do to the modified sign wave, it make a lot of noise (humming) and didn't affect the output much when checked with on O-Scope.  I may have to get a sign wave inverter. $$ouch. So far I only have about $2000 in the system. (a lot of surplus parts and good luck) 
Thanks again
                WindriderNM
~~~keep those electrons flowing~~~

[bob g]

in my opinion using an isolation transformer is going to be a losing proposition
it is very likely the losses will be high because of all the added harmonics from the modified square wave inverter.

a very good transformer fed with very clean AC power has a hard time being much better than 90% efficient
in smaller sizes (appropriate for 1500watt use) and maybe as good as 93% in larger sizes (10kva and larger)

i wouldn't be surprised if the end result would be something on the order of 80% efficiency for a transformer being
fed modified square wave power, and that is likely unacceptable on its own, let alone the added expense of the transformer
to start with.

first cost is higher with a pure sine wave inverter, but after you factor in all the extra effort and cost of things like isolation transformers
and the net losses over time, the pure sine wave will likely pay for the extra cost in a short time.

i think you would be better off with an APC ups system personally, they are often in the 85% efficiency range and are code approved
have lots of safeties and are well contained and not likely to catch anything on fire or shock anyone.

bob g

[dnix71]

Waverider. It's not sine vs modified sine. It's bonded neutral vs floating neutral. I have a 1000/2000 watt true sine inverter that runs a computerized washing machine but I can't hook it up to the house mains because it's floating neutral. Almost everything under 5kw is floating neutral because that size is considered "portable." The US gov't requires every thing over 5 kw to be bonded neutral so it can be hooked up to house mains as backup power. A genset or inverter that size is not considered "portable" it's considered stationary.

http://members.rennlist.org/warren/badinstallations.html   The "bad" examples here apply to inverters as well as gas generators.

The only proper safe way to house connect is to buy an inverter/genset that is wired to operate like your home is wired.

[Basil]

Why not install a small second breaker box next to the main.
Hook ONLY your inverter to the new small breaker box.
Then remove the wire ( All 3 hot, ground, neutral ) from the main.
Put it in the small breaker panel.
May not be code but will work ( IF )( in side the house )that circuit is isolated from the others.
That's a 50 / 50 chance.

I'm not telling you do it but that's what I had planed.
Just haven't got around to it.

[Ungrounded Lightning Rod]

When feeding through an RCD / GFCI, do NOT feed the power through it backward.

Unlike ordinary AC circuit breakers (which work just fine no matter which way the energy is flowing), the GFCI uses power from the "hot" side to operate the mechanism that breaks the circuit.  If you try to backfeed it, when it tries to blow it disconnects the power before it gets fully open.  Then it chatters rather than opening - failing to protect - and finally burns out the trip mechanism, typically with the circuit still connected.  Once this has happened your expensive RCD / GFCI breaker has become an ordinary overcurrent-only circuit breaker.

Happened to me once during a power outage.  Cut off the main breaker, backfed the genny through the 30A GFCI-protected trailer outlet into the panel, and bumped the "test" switch on the expensive GFCI panel breaker when adjusting the loads that I wanted to power.  So I had to buy another special-order 30A breaker the next week - for another $75 or so.  B-(

So set up your feed so the GFCI / RCD is being fed with the generator hooked to the "line" side and the house to the "load" terminals.