Earth ground for battery/inverter system

OneSock

I have a 24VDC battery system with a Xantrex Freedom X inverter feeding an AC breaker box. The batteries are charged solely by solar, with no "shore power" of any kind.

The inverter manual says to ground the inverter's frame to the DC negative bus, which I have done. Other than that, however, the manual is not very clear about grounding. It says, "The neutral conductor of the Freedom X's AC output circuit (that is, AC Output Neutral) is automatically connected to the safety ground during inverter operation." This seems to indicate that the AC grounding occurs within the inverter, and that no ground (to a rod in the dirt) is necessary at the AC breaker panel. Is this correct?

If that is the case, do I need the DC negative bus to be grounded to a rod in the dirt?

BB.

Welcome to the forum OneSock,

Grounding is a relatively complex subject... I will try to break it down to help make sense.

1. In North America, we have 120/240 VAC Split Phase power with a grounded neutral.

More or less, think of a transformer with the center tap as "neutral" and L1 & L2 as the 120 VAC neutral and L1 to L2 as 240 VAC.

By having a ground reference neutral (connected to cold water pipes, now hot water, natural gas pipes, vehicle/chassis metal grounds, etc...) that means that the Neutral will never get above zero volts (roughly) with respect to "local grounds"--So there is no need for fuses/breakers between Neutral output (transformer, inverter, genset) and the loads... Only L1 or L2 need a breaker/fuse... Or double pole breakers for L1-L2 240 VAC circuits (if you get a short from L1 or L2, then both breakers are tripped and make the branch circuit "safe").

Note: The grounded neutral only works with (mostly) Pure/True Sine Wave AC inverter (PSW/TSW--Same thing--Always refer to the mfg's manual for instructions). For MSW (modified sine/square wave inverter), they do not (usually) have an isolated transformer output. If you try neutral grounding and DC negative bus grounding, it will let the "magic smoke" out of the inverter.

For a typical home, the Neutral+Safety Ground bonding is done in the main circuit breaker panel (one location). For smaller AC gensets and AC inverters (call it ~3,500 Watts or less), typically have "floating" AC output. For larger inverters and gensets, they typically have N+G bonding inside the genset/inverter... If you have N+G bonding in the AC source, and in the AC main panel, it can do several things... One is if there is a GFI (ground fault interrupter) outlet in the genset, the two N+G bonds will trip the GFI (looks like a ground fault). Also, you will have parallel current paths for the Neutral (normal loads) and in the AC main panel, having current flow in the Green Wire safety circuit for normal loads is "against code" and other "weird things" can happen (overheating the sheet metal in the AC panel, etc.).

For "mobile applications" such as RVs, trailers, boats--They can have internal power sources (AC inverter, genset) and external power input from "shore power" (such as trailer park AC power). The AC power from the trailer park will have N+G bonding, and possibly a GFI to the RV--Having a N+G ground bond is not correct. And when you are on internal power, you want N+G bonding for safety (the ideal being that N+G needs to be "bonded" somewhere for safety--With any AC power source.

With some inverters designed for "mobile" applications, they will have their internal transfer switch with an extra connection. That is to connect N+G on internal power. And "float" N+G with shore power.

Above addresses the "grounding" to plumbing/RV chassis, etc... How and why. This also applies to grounding your DC power system (typically negative grounding) to allow the use on fuses+breakers on the + Branch circuit leads. With floating power circuits--Technically you need fuses/breakers on neutrals (for AC power) and return leads (such as the negative DC power leads)... Can discuss more if you wish--But that is the basics there.

2. Now--The "earth" side of grounding. For 99% of loads out there (lights, TV, washer, computers, etc.), they will work fine if never connected to "earth ground".

The main reasons for earth grounding (ground rod into ground, metal cold water pipe from street, buried ground plates, etc.) are:

• Lightning: The earth grounding is there to direct path to "ground" from lightning strikes (keep from injuring people, limit equipment damage in home, etc.). You can limit injury and damage from lightning, but a direct strike will ruin equipment.
• Static Discharge: Turns out that things like radio/TV antennas, radio towers, and even solar panels/etc. can pickup static electric charge from the earth's electric field. This can range from 100 volts per meter (roughly), to 1,000s to much more volts per meter (such as under a thunderstorm). Connecting to earth ground rod prevents static electric build up.
• Other stuff: Such things as spark ignition systems for gas/propane need a N+G connection to detect if the flame is burning or out--Flame rectification". Also some fluorescent tube fixtures "start better" with N+G bonded connections...
• Radio systems (HAM, other transmitters, etc.): Ground rods are also used to create a better/more stable and efficient antenna radiation pattern and reduce RF noise (pops and clicks, etc.).

If you are in a "fixed location" (mobile home on "solar/off grid power", cabin, etc.) then having an 8-10 foot ground rod on the edge of the building with a 6 AWG cable to the N+G bonding strip.

If you choose to ground bond the negative bus on the DC power system (what is normally recommended for TSW/PSW inverters... Then usually connect from negative bus to the "same ground rod" as your AC and AC N+G bonding-- This helps ensure there is a solid electrical connection from an AC or DC short circuit to any "grounded metal/plumbing fixture" in the home/RV/etc... You don't want an AC short to your DC LED fixture over your sink becoming "hot" with 120 VAC (as an example) and electrocute yourself with one hand in water, and the other turning the DC LED fixture on/off.

Ground rods are typically local to what they are protecting (from lightning). You have one local to your home/main panel, Possibly another ground rod to your shop panel sheet metal (not N+G bond), etc... But for DC / AC power safety (trip breaker in case of short circuit to metal), you want all ground rods tied together/back to the main home ground rod. The 6 AWG wire provides path for AC&DC short circuits. The local ground rod provides grounding path for lightning to earth (lighting will typically only follow 6 AWG wires for something like 10-20 feet before they find an "alternative path" to earth).

In summary, you will always have an N+G bond in your Freedom inverter (default) as there are no other AC power source, no shore/utility power, and at this point, no generator power--If adding a genset--Then need to look at genset N+G options).

If you are are in a fixed location, and especially if you have lightning and/or external AC circuits (patio lighting, power tools, etc.) and you have possible lightning--Then connecting from chassis/electrical ground with 6 AWG to an 8-10 foot copper ground rod is a very good idea.

And if you are in a "high likelihood of lightning region", then you may also want to look at good quality surge supressors. For exampl where your solar power enters the unit. Your AC inverter's output or your main AC panel, etc.).

https://www.solar-electric.com/search/?q=+surge+protection

The above is a very quick/roughly overview of grounding--How and why--As alway with electrical, the details matter. It is not a complete/exact answer by any measure. I will stop here to avoid "excessive" word salad... 

A lot more information and discussions here:

https://forum.solar-electric.com/discussion/comment/84242#Comment_84242

Your questions/updates to my guesses, etc...

-Bill