Generator/dc grounding issues

Re: Generator/dc grounding issues

It is a tough call--many times because the actual bonding/requirements are not clear between all of the various pieces of equipment. So--you are left either trying something (and replacing scorched parts), over designing the system, or taking things about and trying to see what is actually happening.

I am not the expert here--but from what I have read and seen.... Modified square wave inverters normally cannot have a bonded neutral and a grounded (+ or -) DC input because there is no electrical isolation on the AC output stage (would require an extra transformer and more money).

Generally, a full sine wave inverter can support a grounded neutral because the output Hot/Neutrals are floating with respect to ground.

Generator with Inverter output--I have read that folks with a eu2000i or a eu3000i can support bonded neutral applications--but it is not clear in the eu2000i manual if this is supported or not.

Generators with normal alternator outputs should be able to support a grounded neutral output.

To, possibly, fix your first problem, you could put an isolation transformer on the generator output and support the local earthed neutral, and the remote earthed neutral at the building. Look at something like the PSX-240 isolation transformer from Outback.

Generally, where one gets into problems between AC and DC grounding systems is that when because of code, or other issues, that you end up with multiple grounds (one at the generator, another at the building, and incidental grounds like the earthed neutral, grounded cases on 12 VDC appliances, etc.).

The issue, that I see, with multiple DC grounds (non-capacitive, metallic connections, grounding) is the accidental (for example) grounding tied between a DC chassis ground and an AC neutral bonded ground. And what can happen is (for example) 1/2 the DC current flows through the - DC harness, and the other 1/2 of the current flows through the white bonded neutrals and/or green safety grounds.

And DC circuits, typically are an order of magnitude (or larger) that the AC Hot/Neutral/Ground... A 120 VAC circuit is typically 15-20 amps rated. A typical DC circuit (for an equivalent device) would be 150-200 amps (at 12 VDC)--So if there is an accidental alternative path through the AC safety ground for the DC load currents--one can easily blow the AC safety ground circuits and their related components.

-Bill

Re: Generator/dc grounding issues

I did a quick read-thru on the Outback PSX-240 transformer manuals (scroll down to find two -240 manuals here)--and this transformer cannot be used as a general purpose 120/240 VAC isolation transformer... It has a lot of neat uses as an auto-transformer--but general isolation is not one of them.

Here is an example of (what appears to be) a very good isolation transformer that is designed for shore power to boat connections (PDF File) (isolation of shore ground/line/neutral from boat ground/line/neutral).

I have no idea about this company or their product--but their installation manual does a pretty good job of describing the various methods of connections and the reasons why for each.

These isolation transformers are not small (this 30/16 amp 120/240 VAC version is roughly a 10" cube and weighs ~60lbs) and probably not cheap. You might find some that are more applicable (and less money) to a home/business at your local electrical contractor supply house.

Using two ground rods (one for AC the other for DC)--in my mind is a "cheat". You could just end up with two energized ground rods (could still get a shock if both grounding systems were touched/shorted together accidentally or on purpose).

You are better off to make sure that you have no unintended ground loops and that all ground runs are "home run" back to a single point. What you don't want is an AC/DC return connection (- battery or white/neutral) that have two (or more ways) back to the power source (the second way being through the safety ground or alternate ground connection... i.e.; some DC - current flowing through AC white or AC green wire, or AC neutral going through the DC - or AC green/safety ground wire)... The only time you would have "cross circuit DC or AC" current flow is if there is an accidental short (and the AC safety ground it to trip the breaker--there is really no DC safety ground in most systems--like grounded metal automobile chassis serves both as the "-" lead and safety ground).

I am sorry I don't have better answers... The little Honda's (eu1000i and eu2000i) are not large and will not smoke safety grounds like a big short to your battery would (and, frankly they are not that expensive to replace if something goes terribly wrong)--so I would tend to, in your case, pick one place to put the ground rod (probably at your distribution panel--just like any normal home A/C wiring) and carry the three wires back to the generator (Hot/Neutral/Safety-Ground) and plug it in there. And don't bother to use the ground lug on the generator chassis.

I would guess that it is possible that a battery charger could have grounded their DC "-" output to the Green Wire (or Neutral which is grounded back at the panel). You could confirm that with any ohm meter (Look for anything less than ~1,000 ohms as a real potential problem--Should be >100,000 ohms resistance between output and any AC line plug connection).

I can't imagine any decent model of a modern battery charger that would ground reference their DC output to the AC line cord (anything is possible--so I can't say it would never ever happen). For example, charging an old positive ground automobile would give some very fine fireworks with a negative grounded battery charger...

The other issue is battery chargers can use a Diode Bridge rectifier that would take the majority of the current near the peak of voltage sine wave--this wave form can over heat transformers, generator coils, and electrical wiring because of these very high (and short) peak currents. This is what your generator manufacturer was warning you about. If your battery charger is 1/10 of the generator capacity, then it will probably never be a problem. If your charger is 50%+ of your generator capacity--then it may be an issue with the charger.

Basically, any good brand of battery charger (like Iota), is going to be a power factor corrected switch mode power supply--the old days of a transformer and full wave rectifier is long gone (I looked through the Iota website--but I did not find their Power Factor rating--but they need to meet the new FCC/Utility specifications).

I am sorry I can't really do more--but over the web is not the easiest to diagnose strange failures...

One thing you can do is break the ground wire to the generator and put (for example) a 100 watt 120 VAC filament light bulb in series and measure the voltage drop. If you get any significant voltage across the light bulb (measure both AC and DC--they should both be near zero volts or at least below 2-5 volts--by the way, different meters will measure AC+DC wave forms differently--so I cannot be too exact here) then you may have some unwanted AC or DC connections to ground through the generator ground.

You do have to be kind of careful with DC batteries, battery chargers, and grounding... As an example, I had a large telecom system that was powered by -48 VDC -- but we were reading +48 VDC--turned out that the -48 VDC power supply/battery charger output was not ground referenced--but when the customer accidentally touched a wire (or tool) to the +48 VDC lead (which was supposed to be ground) he "saw the sun" in front of him. Turned out the DC power supply has some pretty big DC caps referenced to AC ground--enough storage that they supplied a good sized arc to a short to ground on what was a floating supply--shorting a by-pass capacitor used to keep the output voltage smooth and stable.

The normal trick to check ground connections was to use a 120 VAC filament light bulb (~40-60 watt) to see if something "floating" or suppling a voltage with lots of current behind it (in the above case, the light would have quickly and safely discharged the output capacitors when the bulb was connected from the positive lead to ground).

I probably have typed too much--you can ignore the stories and why too detailed discussions above if they (or my typing) are confusing.

-Bill