Tying the Common to Ground, 110V / 220V

[Clifford]

I've been wondering about tying the "Common" wire to Ground as most existing houses (and grids) are wired.  (Currently I am working off grid).

With a cheap 12V - 110V inverter, the first inverter that I had (300W) whined and complained when I had tied the White Common wire to ground.  The second inverter (700W) blew out almost immediately doing this (ok, so I've been experimenting a bit).

I've partially gotten around the issue by putting in a 110/220V transformer...  bumping up my power to 220V, and then splitting out the two 110 circuits.  However, the problem is now that I have to be quite symetrical in my usage.  If I turn on a single 10W bulb...  it won't work.  I have to turn on two 10W bulbs (separate halves of the circuit) to get it to all work.

I may be going to Compact Fluorescent DC Lighting so this may not be as big of an issue as I had thought.....

How are other people dealing with this?

Is this one of the advantages of a $3000 inverter over a $30 inveter?  Sinewave might be nice, but if modified sine runs most of my stuff, it may be ok, and I believe that it has a much simpler circuit and less overhead.

[Opera House]

Perhps you have the wrong transformer for the job.  The transformer should balance any load.  What you want is a center tapped transformer on the 220 side with the tap tied to neutral and ground.  The transformer for this is usually a universal type with two 110V windings on the primary and the secondary.  Primary windings are in parallel and secondary are in series with the center of the two connected to neutral.  Phasing of the wires is important so test the transformer with a 110V 100W lamp in series with the primary.  If in phase, there mitht be a very dull glow of the lamp.  Out of phase, the lamp will be bright!  Out of phase windings will make the transformer look like a dead short.  Once primary is done, you can phase the secondary.  In phase you get 220, out of phase you get.

[crashk6]

The main reason a home connected to the grid is bonded to earth ground is because in an electrical system that large having all of the wiring at floating voltage becomes a real safety risk. This is because you could have a fault in the system and not know it until a second fault on the opposing line occurred creating a MASSIVE electrical explosion.

It is also for lighting protection.

If you are running a system with a grid tie inverter then by nature that inverter will have bonding provisions... or if your inverter is not grid connectible but produces several KW it will most likely have bonding provisions.

If your inverter does not have a bonding provision then do NOT earth it, it was not designed to be bonded and could pose a significant safety hazard operating this way.

If you are running totally off-grid and have such an inverter then don't worry about grounding it's AC output. Just ground your battery banks negative terminal and of course the ancillary equipment such as solar panels, wind genny etc..

From the stand point of the inverter output being an electrocution hazard, it is no more so than any grid powered receptacle in a house. Which is not to say it can't kill you... it can... so can the grid. (A bonded inverter isn't any safer from the shock hazard point of view ether.) To address that concern treat it just like one would the grid... use GFCI outlets or GFCI breakers in your load center. If you go the breaker route use square D's QO line of GFCI breakers they are one of the more expensive breakers but quality wise are better and the GFCI portion functions properly even on square wave inverters. My statement is of course based on my experiences with these products and as such I'm a bit partial to the QO line for many reasons.

Although something to be aware of is that some GFCI's may be seen as a fantom load to inverters with "search mode" or "sleep" features.  

Anyway hope this answers your questions about bonding and it's purpose.

--

crashK6

[Tom in NH]

Having blown out an inverter myself, here is my take on the situation. I think you could tie your neutral to an earth ground, but if you do, DON'T connect it to the negative battery terminal as if to create a chassis ground.

In a grid powered system, think of the ac voltage rising above 0 volts at the top of the wave and dropping below it at the bottom. Zero volts would be your neutral wire, and it would be connected to an earth ground.

In an inverter system, there is also an alternating wave, but it is relative to the battery terminals. Put another way the negative terminal of your battery corresponds to the bottom of the wave, the negative voltage of your inverter's ac. The positive terminal of the battery corresponds to the top of the wave. The neutral or ground would have to lie somewhere inbetween the two. But if you mistakenly tie the negative terminal  of the battery to the neutral wire at an earth ground, you will create a fault between the negative side of the wave and the neutral. Poof.

 In your setup, was the negative battery terminal connected to the earth ground like mine was? It was ugly, not a pretty sight. --tom

[crashk6]

In my first post "It is also for lighting protection." Should have read lightning... not lighting... Darn the typos.

Tom in NH,

"Having blown out an inverter myself, here is my take on the situation. I think you could tie your neutral to an earth ground, but if you do, DON'T connect it to the negative battery terminal as if to create a chassis ground."

If you do not ground your battery bank, and you receive an induction or direct lightning strike or even just stray voltage from some other source, you risk a battery explosion, electrical/chemical fire, or at the very least internal plate fusing.

You may actually ground a battery bank on ether the positive or negative terminal... the point of grounding ANY electrical system AC or DC is to bring one line to the same electrical potential as planet earth, and effectively you as well.

The standard in the RE industry is to bond negative to ground. This is largely because of the likelihood a piece of equipment with a negatively bonded chaise/ground plane might get connected to the system, a prime example would be a CB, Ham radio, or even a DC television connected to their respective antenna.

The telephone company grounds there system on the positive side to help reduce feedback and detect line faults... but phone systems and there electronics are made to be positively bonded. I don't recommend it for your home system.

Did someone order magic smoke?

"In a grid powered system, think of the ac voltage rising above 0 volts at the top of the wave and dropping below it at the bottom. Zero volts would be your neutral wire, and it would be connected to an earth ground."

This is not how the "neutral" wire in an AC system works, in fact both hot and neutral wires carry full voltage. What makes neutral.. well.. neutral is the fact you are at the same electrical potential it is. Yes the AC wave hits zero volts but this is an independent happening.

For example in a three phase alternator connected in delta there isn't a "neutral" per se, Now on the other hand in a three phase alternator connected in Wye pattern you may tap the common connective point and earth bond that as a neutral. In both cases the AC wave still goes to zero independent of the path the energy flows. The later is how most of the powers company's systems work.

When I first learned about electrical potential it took me a day or two to fully understand it, and if you only have partial information it's very easy to misunderstand, especially in an AC system.

To clarify some of my earlier statements in my original post, the inverters I spoke of as not having a bonding provision; the reason for this is quite simply cost to manufacture and/or intended use.

Most low cost inverters in question are made with the intention of powering devices directly not an entire household electrical bus. In addition being that these are mobile in nature bonding is unnecessary.

When an inverter is intended for hardwireing in most instances it has isolation electronics and/or an isolation transformer on the output side that keeps the AC output and the DC input electrically separated. Thus preventing the AC from back feeding thru the grounded lines to the battery; which of course to the DC side would look like a dead short thru the inverter electronic!

If an inverter without such isolation has it's output bonded to ground the results can be... shall we say... spectacular! Although most just die a whimpering smokey death.

So Clifford... to do this after-market you've got the right idea with the transformer. Opera House is right on.. you've got the wrong transformer for the job. Unless you need 220-240 volt for anything specific I would suggest a 1:1 isolation transformer.

The thing to be aware of it that with any transformer you lose a bit of power so only go this route if you have a large electrical system and are intent on bonding... but with figures of 300-700 watts I would recommend you not complicate the system and leave your AC side un-bonded.

--

crashK6

[Experimental]

     Hi Clifford,

    I see you have made the same mistake, I did, some time back !!

    If you read the manual on many of the less expensive inverters -- they warn you, not to use them in a bonded system..(neutral bonded to ground)

    Most of these inverters, see this as a short and I have found that most have very poor, short protection -- resulting in a release of "magic smoke" --

    After loseing two inverters -- one to an internally shorted plug receptical, and one to being plugged into a trailer, with a bonded electrical box..

   (most RV,s, are not bonded, because of the use of inverters)

    Zantrex inverters, manual -- is very specific about this caution -- but if you bought it at Costco -- they will exchange it....

   Homes are required to bond the box, by many (if not all) local codes -- but RV,s and remote cabins, that are not tied to the grid -- use unbonded systems, if useing these cheeper inverters -- but be sure you have an active ground system..  Bill H..

[Clifford]

Thanks for the comments.

I assumed that I wasn't alone with this dilemma, and most of your thoughts were in line with mine (except for the center tapped transformer which makes sense, but I need a bit more study of electronic circuits).

I did accidentally acquire a 1:1 isolation transformer at the local electronics junk store.  It might be worth considering testing on my system (when I have another inverter that I don't mind experimenting with).  I believe it was 110V, 500W, and they had a few more if anybody needs one.

Obviously there is the need for the symmetric circuit of positives equaling negatives (which most residential houses get from using 220V).  However, a low power system would likely be better off with a symmetric 110 circuit than trying to bump it to 220V and attempting to maintain the symmetry.

What is also becoming obvious is that I way overestimated the power generation capabilities of my solar panels (now 4 x 48W), especially on wintery partially overcast days.  So, a few 5-10 watt losses here and there might not sound too bad in general, but they add up very quickly.  I have to get my last 2 panels on the roof ASAP.

I will probably try to convert my lights to 12V...  so that I can have power on all the time, but only turn on the inverter when I actually need to use something that requires AC.

As far as grounding the batteries...  I can see benefits of not having a HUGE electrical explosion in my basement.  However, I also see benefits of touching ground and the positive side of the circuit and not completing the circuit (not touching negative).   If the Negative is grounded, then there is the risk of completing the circuit by touching Positive and ANYTHING ELSE.

If one decides to run dual grounding rods for 12V and 120V, is there a recommended distance apart?

Thanks,

Clifford

[nickelbender]

I would very seriously recommend you read up on grounding and bonding in the NEC if you live in the United States and perhaps consult with your local electrical authority isnpector. Many of the code requirements in Canada differ so I won't give any advice but this.

People have been killed by capacitive coupling and suprise faults on ungrounded ac systems even at 120 vac, so I would be sure to ground and bond in a safe manner. I'm a big fan of GFI and Arc fault systems I would recomend you spend the money. Improper grounding of Generators and Inverters can damage them, assume nothing and be sure your commons are internaly grounded or floating before you start.

Don't use two grounds, I could spend all night coming up with reasons not.

[Clifford]

Ok....

So, I pulled up my copy of NFPA 70A / NEC (2002)

Here is what I gleaned in a few minutes.

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Section 690 (1 through 80) deals specifically with the solar systems, and pretty much just says the same thing (lots of redundancy in the code).

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Now, things seem to be different for > 50VDC...., and according to 690.71.E(2)



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So...  it is recommending Grounding the negative pole of the battery, but one can leave a 110v derivative circuit ungrounded (I think).  However, if the 110V system is ungrounded, all light swiches must be bipolar.

A 12V DC Only circuit may be ungrounded as long as it doesn't have an inverter.

There is more on the GFCI's and Arc Fault stuff.

OOOOOOHHHHHHHHHHHHHHHHHHHHHHHH

I found some stuff on NEC 2005.  Note, my references and quotes to 2005 codes are secondary references, and not relating to a primary source evaluation.

They've added a new section:  690.35, recognizing Ungrounded systems, but I am a little confused as to where it applies...  

There is an interesting note about "Safety" (I think).



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However, Section 690.47 further clarifies the grounding:



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REF:

Code Corner by John Wiles (with some new NEC 2005 notes)

http://www.sandia.gov/pv/docs/PDF/CodeCorner105.pdf

http://www.nmsu.edu/Research/tdi/public_html/pdf-resources/IAEI-3to4-05.pdf

System Design and Workshop Notes:

http://www.energy.ca.gov/renewables/notices/2005-10-24_PV_WORKSHOP.PDF

Ok...

So, reading a bit more....

The Transformer, by design, provides the circuit isolation between the 12V and 110V.  Thus, the 12V negative and 110V positive CAN be tied together on the same ground (if one has a transformer).  One can have a non-transformer circuit where one needs to keep the two isolated.

Thus, I think my problem wasn't the lack of a center tapped transformer, but rather the presence of a center tapped transformer.

110. V...   Tie one leg to ground, DON'T NEED "center tapped transformer", but must have an inverter designed for this.
     
220. V...   Tie "neutral" to ground, NEEDS "center tapped transformer"
     
     

OK.

Wow...  I think I've come into a complete circle in my thoughts.

With a "cheap inverter" as one might use to connect to an auto battery, one must have the circuit isolated from the case (to avoid accidental shocking by touching the case).  Thus, the two hot legs are isolated from ground, and the transformer would be "center tapped" to ground.  A more expensive 110 inverter would be built anticipating a built-in installation where the "white leg" is tied to ground, and at 110V, would NOT be "center tapped"  (I think).  But, a 220V tranformer/circuit, on the other hand, ties the center leg to the ground, and thus uses the center tapped transformer.

I guess I am rambling, but there is a lot of good stuff to read.

----- Clifford -----