Author Topic: Inverter output wiring.  (Read 10567 times)

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jumpingjackflatch

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Inverter output wiring.
« on: December 17, 2004, 02:38:52 AM »
Hi everyone, I have a question, but first some background. I live in the city and after the big blackout, and a long time interest in the subject, decided to install a couple of solar panels on my roof. I don't really need them, but it's an interesting hobby and it will hopefully provide a bit of free energy. The system consists of two 110 watt panels, six 100 A/h deep cycle AGM batteries wired series/parallel for 24 volts, a charge controller and a 3000 watt MSW inverter.

The system currently runs only the washing machine. I will also remove one small circuit (that runs several cfl lights) from the grid panel, and connect it to the inverter eventually. I would really like to wire the inverter to the furnace (oil burner, 120v@2A and water circulation pump, 120v@.75A) so if there is a grid outage I can flip a switch to change the feed from grid to inverter. I was figuring I could use something as basic as a 3-way light switch. Then I read something somewhere about generators and inverters not having neutral bonded to ground. It got me thinking that this may not be as easy as I originally anticipated. I tested the output of my inverter when off with an ohmmeter and found no continuity. A strange thing happens when I test voltage between either hot and ground or neutral and ground. When an incandescent lamp is plugged in it flickers and the green led on the inverter flickers between green and yellow.


  What is the correct way to wire this? Normally, distribution panels have neutral and ground bonded. I would imagine it would kill the inverter. I measure 55 or so volts between each conductor and ground (using a Fluke 8060a).  Right now I have

two of the inverter outlets going to a small 4 position breaker box with one circuit going to the washing machine outlet.


Comments and suggestions appreciated,

Randie

« Last Edit: December 17, 2004, 02:38:52 AM by (unknown) »

ghurd

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Re: Inverter output wiring.
« Reply #1 on: December 16, 2004, 08:26:42 PM »
Randie,


Very good question.


Search this board, at the bottom, for comments containg "floating ground".

(or inverter question, inverter overload, hot neutral) Click the bottom ones.


I can not say what is right for your inverter, just that it is an interesting subject.


And that seems like a lot of battery for the panels. Possibly you should consider another panel or 2, to make the most of what you have.


G-

« Last Edit: December 16, 2004, 08:26:42 PM by ghurd »
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ghurd

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Re: Inverter output wiring.
« Reply #2 on: December 16, 2004, 08:27:27 PM »
Does this work?


http://www.fieldlines.com/?op=search&offset=0&old_count=30&type=comment&section=&
;string=floating+ground&search=Search&count=30


G-

« Last Edit: December 16, 2004, 08:27:27 PM by ghurd »
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jumpingjackflatch

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Re: Inverter output wiring.
« Reply #3 on: December 16, 2004, 09:06:13 PM »
Well, I read those posts and I am still not clear. There is a ground wingnut on the side of the inverter. Should it be attached to a cold water pipe? I also do not have the battery system grounded in any way. Does it need to be? Will the inverter self-destruct if neutral meets ground? Or does it depend on the unit?
« Last Edit: December 16, 2004, 09:06:13 PM by jumpingjackflatch »

jimjjnn

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Re: Inverter output wiring.
« Reply #4 on: December 16, 2004, 09:28:05 PM »
Only Connect your inverter ground as the manufacturer dictates as people have blown the FETS in their inverters by connecting ground to neutral. Some inverters may allow it. Check your owners manual  first . If that doesn't give you the info then contact your manufacturer.
« Last Edit: December 16, 2004, 09:28:05 PM by jimjjnn »

jimjjnn

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Re: Inverter output wiring.
« Reply #5 on: December 16, 2004, 09:29:11 PM »
By the way, I would connect the inverter ground to the cold water pipe.
« Last Edit: December 16, 2004, 09:29:11 PM by jimjjnn »

Opera House

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Re: Inverter output wiring.
« Reply #6 on: December 17, 2004, 07:52:12 AM »
To maybe help explain it, a large number of inverters have no output transformer. 12V powers an inverter transformer that produces high voltage DC. This high voltage DC has to be monitored by the inverter so it has an electrical connection to the 12V which is usually grounded.  It is done this way to save a few cents in the circuit.  The AC output is developed by a "H bridge."   This is basically like attaching jumper cables to a high voltage battery and reversing them very quickly.  If you ground one of the AC lines it either shorts an output or seriously interferes with the monitoring circuit.  If you have to ground the AC system, an isolation transformer can be used.
« Last Edit: December 17, 2004, 07:52:12 AM by Opera House »

jumpingjackflatch

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Re: Inverter output wiring.
« Reply #7 on: December 17, 2004, 08:51:03 AM »
So I should attach the inverter cabinet ground to a cold water pipe.

I have also heard of grounding the battery system. Is this important?

Will grounding the inverter ground insure the washing machine is safe

to touch?
« Last Edit: December 17, 2004, 08:51:03 AM by jumpingjackflatch »

jimjjnn

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Re: Inverter output wiring.
« Reply #8 on: December 17, 2004, 09:38:44 AM »
the wash machine should be already grounded through the 3rd(round) pin on the plug so you shouldn't have to ground it again unless the wall outlet isn't properly installed
« Last Edit: December 17, 2004, 09:38:44 AM by jimjjnn »

ghurd

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Re: Inverter output wiring.
« Reply #9 on: December 17, 2004, 09:52:22 AM »
You should probably call the inverter company to see what they say about grounding the batteries, chassis, round prong and neutral prong all at the same time.


What brand and model is the inverter?  Someone here may know what works and what smokes.


G-

« Last Edit: December 17, 2004, 09:52:22 AM by ghurd »
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Ungrounded Lightning Rod

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Re: Inverter output wiring.
« Reply #10 on: December 17, 2004, 10:14:15 AM »
I was figuring I could use something as basic as a 3-way light switch. Then I read something somewhere about generators and inverters not having neutral bonded to ground. It got me thinking that this may not be as easy as I originally anticipated.


Correct.


Unless the inverter is designed to have the neutral ground-bonded you will need to switch both the hot and neutral on the load.


Even if the inverter IS designed for ground-bonded neutral you must only bond it in one place (to prevent putting neutral return currents in your grounding system, creating a hazardous condition and possible fires).  And you'll have to consult the inverter manual or manufacturer to find out whether a ground-tie is already present inside or if you should add it outside.


But many inverters don't LIKE having neutral ground-tied.  Many inverter designs (especially the less pricey ones) switch both neutral and hot between two voltages (one of which is usually internally bonded to ground or one side of the DC supply).  Others (more high-end) put a load-sensing circuit in the neutral.


You said your inverter has a ground nut.  You bond THAT to your house protective ground - or a water pipe that's continuous copper from there to the water/electric house bond, using a wire rated for the maximum inverter output or more (15A -> #14, 20A -> #12, 30A -> #10) and (if necessary) a good UL-approved pipe clamp.  That might "do the right thing" for the inverter neutral - but check the manual or manufacturer.


Same story on grounding the battery - check the nice manual, and call the nice manufacturer if the manual doesn't explain it.

« Last Edit: December 17, 2004, 10:14:15 AM by Ungrounded Lightning Rod »

jumpingjackflatch

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Re: Inverter output wiring.
« Reply #11 on: December 17, 2004, 12:26:03 PM »
Well the manual is pretty useless and does not even mention the ground nut.

It is a cheapy inverter (powerbright). I'll send the manufacturer a note and

see what they say. Thanks
« Last Edit: December 17, 2004, 12:26:03 PM by jumpingjackflatch »

srnoth

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Re: Inverter output wiring.
« Reply #12 on: December 17, 2004, 12:38:06 PM »
Good comment about the isolation transformer - that definately adds a safety aspect, and you can then bond the ground and the neutral coming out of the transformer.


As far as I understand, with grid power, the voltage difference is as follows:


Live to Neutral --> 120V

Live to Ground --> 120V

Neutral to Ground --> 0V


If you have 240V power, then it is:


Live1 to Neutral --> 120V

Live2 to Neutral --> 120V


Live1 to Ground --> 120V

Live2 to Ground --> 120V


Live1 to Live2 --> 240V


Neutral to Ground --> 0V


With a configuration like that, since there is no voltage between ground and neutral, joining the two does not cause any problems.


The problem with inverters is that there is voltage between the neutral and ground, so joining them causes a huge amount of current to flow from neutral to ground, blowing something.


So that is the basic theory, but there are some interesting things I have noticed:


I just checked my cheep 400W MSW 'Chinese' inverters, and they don't even have a ground! There are no wires connected to the ground on the sockets, and no voltage between live and ground or neutral and ground! So in this case, they simply did not bother with a ground. Interesting.


This means that you could probably just use the live and neutral from your inverter, and leave the house ground connected to the circuit. The only problem with this is if the negative terminal on your battery bank is grounded, you don't exactly want the high voltage neutral on your inverter directly connected to the batteries!! Shouldn't be a problem though since you say that your battery bank is not grounded. I don't see why there is any point grounding your battery bank. Anyone?


Basically, the safest thing to do is to use an isoltation transformer, and bond the neutral output of the transformer to the house ground. So you can use one of those 3-way light switches, without worrying about the ground and all that, if you use an isolation transformer.

« Last Edit: December 17, 2004, 12:38:06 PM by srnoth »

jumpingjackflatch

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Re: Inverter output wiring.
« Reply #13 on: December 17, 2004, 12:59:36 PM »
An isolation transformer sounds like a great idea except from what i've seen, the prices are very high. Does anyone know of a source of reasonably priced units with a decent power rating?
« Last Edit: December 17, 2004, 12:59:36 PM by jumpingjackflatch »

jumpingjackflatch

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Re: Inverter output wiring.
« Reply #14 on: December 17, 2004, 01:48:37 PM »
Here is my question to the supplier, and their fantastic response!


The following is the information you requested:


MESSAGE:


Can you please provide the information regarding the proper grounding

required by the inverter and what battery grounding is correct for the

AED300024V if any. Also, does this inverter have a neutral to ground

bond internally? May ground be bonded to neutral externally?

The manual provides none of this info.

Thanks,

Randie


ANSWER:

The positive and negative wires are connected directly to the battery

and the grounding is done by a nut bolt at the rear of the inverter.

You would have to connect a wire from that grounding bolt to the

vehicle's chassis.


Thank you,


 This, believe it or not, is more info than the manual provides.

« Last Edit: December 17, 2004, 01:48:37 PM by jumpingjackflatch »

Ungrounded Lightning Rod

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Re: Inverter output wiring.
« Reply #15 on: December 17, 2004, 08:00:38 PM »
They may have isolated the output from the input.


Try this:


 - Ground the grounding bolt.

 - With power off use an ohmmeter to check for continuity between the neutral and the grounding bolt.

 - With power on check for voltage between the neutral and the grounding volt.  (Use both AC and DC settings on the meter.)

 - Repeat the voltage check with power on AND a load on the inverter.  (A 100 Watt lightbulb will do.)


If there is a very low resistance between the grounding bolt and the neutral, and there is well under a half volt in all four voltage tests (AC no load, AC load, DC no load, then try this:


 - Hook a load (a lamp again) between the hot and the grounding lug.  Measure the voltage (AC and DC) between the grounding lug and neutral (and be alert for smoke.  B-) ).


Continued operation this way and again well under a half volt on the neutral implies that the inverter might be usable with the neutral tied to the building neutral.


Or open it up and check wither they're tied together by a heavy wire inside.  B-)

« Last Edit: December 17, 2004, 08:00:38 PM by Ungrounded Lightning Rod »

scoraigwind

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Re: Inverter output wiring.
« Reply #16 on: December 18, 2004, 03:03:54 PM »
For safety the most imortant thing is to connect all the grounds together, be it inverter or load wiring.  That way no things you can touch will have a voltage between them.  They will be connected together.  This grounding system should also be connectd to a spike in the ground.


Whether the neutral is grounded or not is less important for safety although in theory you should use double pole switches if the neutral is not grounded.  In countries like the USA and the UK where neutral is normally grounded this could also confuse electricans and that could make things unsafe.  It's best to think of both poles as live.


It is true that cheap inverters sometimes connect the battery first to one live pole and then the other, so you cannot ground the battery and the neutral without shorting it out and destroying it.  A transformer would cure it but so would a better quality inverter.  A heavy one.  With the cheap lightweight ones you are probably best off treating both wires with respect and making sure your grouding is complete on the wiring, chassis and spike.

« Last Edit: December 18, 2004, 03:03:54 PM by scoraigwind »
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jumpingjackflatch

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Re: Inverter output wiring.
« Reply #17 on: December 18, 2004, 09:09:30 PM »
After all these great replies, I have decided that the safest thing would be to use a DPDT switch to switch the feed for the furnace from grid to inverter. I will attach the inverter ground bolt to the cold water pipe before the meter, and run only two wires leaving the ground behind. This way the inverter is completely isolated from the grid panel, neutral is not bonded to ground, and ground from the grid panel and the inverter are connected at the same point. Does this sound like a good bet?  (The inverter manufacturer finally replied that the inverter is not internally bonded ground to neutral, and should not be externally connected that way either).


Randie

« Last Edit: December 18, 2004, 09:09:30 PM by jumpingjackflatch »

nothing to lose

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Re: Inverter output wiring.
« Reply #18 on: December 19, 2004, 12:00:41 AM »
"I will attach the inverter ground bolt to the cold water pipe before the meter, and run only two wires leaving the ground behind. This way the inverter is completely isolated from the grid panel, neutral is not bonded to ground, and ground from the grid panel and the inverter are connected at the same point. Does this sound like a good bet?  (The inverter manufacturer finally replied that the inverter is not internally bonded ground to neutral, and should not be externally connected that way either)."


Not sure if your still doing the same thing or not?

Your going to ground the inverter case to the pipe, and run 2 wires? I missed where those wires go I think. Then your grid panel ground (which is tied to the neutral right) goes to the ground pipe the inverter is grounded to.


Are you just making a loop to tie the ground and neutral together instead of doing it directly when you do that?? I could be wrong since I missed where the 2 wires from the inverter are actually going to be connected. But if they connect to the same wires as the grid for a neutral at any point then it seems they end up tied to the ground again anyway.


 I really don't know on that so I'll bring up the question and let someone more smarterer than me answer it. :)


Electronics was not my strong point, spelin was :)

« Last Edit: December 19, 2004, 12:00:41 AM by nothing to lose »

ghurd

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Re: Inverter output wiring.
« Reply #19 on: December 19, 2004, 12:54:49 AM »
I can't say about the safety of any of it.

It would be prudent to do the tests above.


Most Chi-Com inverters I have used will blow something if the neutral and ground are connected together (like grid or house).  Some could blow if the battery negative (ground) is connected to the neutral OR ground.


G-

« Last Edit: December 19, 2004, 12:54:49 AM by ghurd »
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jumpingjackflatch

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Re: Inverter output wiring.
« Reply #20 on: December 19, 2004, 09:48:55 AM »
The two wires, hot and neutral would go to the existing furnace disconnect box.

I would install a DPDT switch to select either the feed from the inverter, or the feed from the griid panel. This way, my inverter neutral would be isolated from the neutral that goes to the grid panel eliminating the ground-neutral connection in the panel. This way, the ground to the panel remains intact.
« Last Edit: December 19, 2004, 09:48:55 AM by jumpingjackflatch »

jumpingjackflatch

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Re: Inverter output wiring.
« Reply #21 on: December 19, 2004, 09:06:29 PM »
Well, tried it wired this way and the inverter didn't complain. No sparks,

no pops, no people running and screaming. I guess if you keep ground away from

neutral on a cheap MSW inverter, it can be used in a permanently installed domestic environment. Thanks for your input. It probably saved my inverter!


Randie

« Last Edit: December 19, 2004, 09:06:29 PM by jumpingjackflatch »