Grounding in 2 locations

mish

Gentlemen,

I built an off-grid battery based system using a 48v Magnum ms-pae 120/240v inverter, Midnite solar E-panel and 7200wh of battleborn batteries.  Although this system is mounted in my house, I purposely designed it not to utilize any AC-in from my home's main breaker panel and kept it totally separate.  I'm using 900w of pv panel and a Victron 150/40 CC to recharge the batteries.  In keeping everything separate, I installed a separate ground rod for the inverter system and bonded everything in the E-panel to that ground.   I have a small breaker panel attached to the Magnum that supplies power via extension cords to various items near the inverter.  Everything works great.   

Now for the grounding issue.  Thinking that at some point in the future I may want to power a few circuits thru a gennie power inlet box and breaker panel interlok I had installed a while back, I installed a 30amp L14-30 plug on the inverter's breaker panel to mimic the gennie's setup.  I lifted the N-G bond from the E-panel before attaching the power cord and the inverter works fine back-feeding my home breaker panel.   After all was said and done though, I realized that I now actually had 2 separate grounding points while back-feeding the breaker panel.  One ground for the home's panel and one for the inverter's E-panel which were still connected at separate points.  After going thru all the literature from Magnum, Midnite solar, and at least 500 blog posts on grounding, I'm thinking I only should have used 1 ground rod and/or keep the 2 rods, but bond them together.  It would be easiest just to buy some 6awg and bond the 2 rods together.  They are only 40ft apart.  Or, am I overthinking this?  If I only use this arrangement in an emergency grid-down situation, would it matter for a few hours if I had 2 separate grounds?  I also was advised I could just disconnect the ground at the power inlet box and that in essence would allow the inverter to back-feed my panel via power cord without worrying about 2 grounds.      

Any input appreciated.   

BB.

Welcome to the forum Mish!

And your first question is one that I can type pages of detailed answers about... I will try to keep the Grounding Question answers short and sweet for the moment (I do not have a lot of time at the moment)--But certainly feel free to ask for clarifications and more questions as you wish.

First--What is the "Green Wire/Safety Ground" for? The first two basic/common answers are 1) to have a path for current shorted to metal electrical boxes, electric tools, sinks, etc. back to the source of the power--And trip a circuit breaker/fuse in the main panel for that branch circuit (lessen the chances of electrocution--I.e., a mixer falls in the sink and "electrifies the metal sink/dish water and electrocutes the person at the sink doing the dishes).

And 2), a quick/solid electrical circuit to "earth" -- To direct lightning to where it wants to go (rather than through your house wiring, telephone/cable wiring, or somebody using a wired phone, etc....

The detail for 1) --- First follow the NEC (or your electrical code) for wiring the green wire/bare copper wire/metal conduit/etc. grounding per code. We talk about "single point" grounding of the White/Neutral bus to the green wire/ground bus then to the local ground connection (a ground rod just outside the wall, metal water pipes, grounding to metal plumping pipes such as water, natural gas, etc.). The idea is that if the "Hot' (black or red, L1, L2, etc. of of the utility/inverter power "shorts" to any grounded metal in the home, an electrical connection is made from L1 back to Neutral--Tripping the fuse/breaker in the main panel). Note that the ground rod does not do much other than being "there" and sometimes a common grounding location (such as your AC mains, DC Battery negative bus for your off grid power system, etc.). The fact that the ground rod in in "dirt"--Does not do much as a "good ground rod" to "dirt" connection is anything less than 25 Ohms--I.e., 120VAC/25_Ohms=4.8amps -- Not enough to pop a 15 amp breaker).

In theory you are trying to avoid (for example) a Neutral+Ground bond in two or more locations (such as the Main breaker panel, and at the backup Genset--Many larger ones which default to tying N+G together at the Genset). In reality--Everything will usually work--But because of the parallel connections of Green wire ground and White Wire Neutral, you can have part of the normal load currents flowing in the White wire/neutral/return and another part in the Green Wire connection (for example between Genset "N+G" tie, and Main Panel "N+G" tie. We do not want "normal" load current flowing through the ground circuits--Only through the Neutral/normal AC power liines.

There are several major reasons we do not want this... First is that if you were (worst case) bring L1 in through one conduit knockout in the breaker panel and N through a second knockout in the main panel... The sheet metal between the L1 and N wiring acts like a transformer--Causing current to flow in the main panel sheet metal--And could overheat the sheet metal of the electrical box.

The second is that some gensets have been known to blow their AVR (automatic voltage regulator) circuitry when you have shared current in the ground connection (not sure--But that seemed to happen a few times here).

Third is the GFI outlet that is present on many generators (and any home 120 VAC outlets near sinks, outside, etc.). The GFI (ground fault interrupter breaker or outlet) "measures" the total curren between L1 and N... Normally L1 is 180 degrees out of phase with N -- The total current flow is Zero Amps. If the total is not zero (or more than ~10 milliamps), the GFI will trip.

If you have two N+G bonds in your home (aka genset and main panel), the "shared" current will look like a short from L1 to Ground somewhere in the system at the Genset's GFI breaker/outlet and it will kill your AC power (as a safety measure).

The second major reason for Ground Rods are to "send lightning energy" directly to ground (instead of through various wires and things in the home). You have multiple ground rods in your system... For example one at the main electrical panel earth ground, and a second one from the solar array racking/solar panel frames to a local ground rod... Lightning does not "like" to follow electrical wiring very far--It is always looking for the low impedance path to earth ground (suggest 6 AWG minimum ground wire, "smooth bends"/not sharp corners", etc.).

Also--When you have multiple ground rods (main panel, solar array, genset frame, etc.)--I also (and code may require?) that you connect each ground rod (separate grounding) together with 6 AWG cable... This tying of all the grounds/racking/genset frame/well caseing, etc.) together is so that if there is a short from L1/L2 (aka "Hot") to "local metal" (such as the array racking), that there is a "return" path back to the main panel where the branch circuit breaker/fuse can be tripped... You don't want a "metal thing" (such as solar racking) to become "hot" by L1 to metal short--And somebody walk by on a wet day/wet grass and get electrocuted.

That is the 50,000 foot view for a "quick" discussion of grounding--Why and how... There are a lot of details that I have not discussed... And there is always lots of code and "best practices" (such as for lightning control)--Which matter a lot in Florida, but not so much for me near San Francisco and little chance of ground strikes.

Does the above make sense?

-Bill

mish

Hey Bill,

Thanks for the detailed response.  Makes sense and I was aware of the N-G bonding issue when connecting an inverter or gen set to my breaker panel.  At no point would I have two N-G bonds when using the power cord to connect my inverter system to the house breaker panel interlok.  But, what I would have at that point in time, is 2 separate grounding electrode locations.  1 from the E-panel ground bus and 1 from the house's breaker panel, now enjoined via the power cord (for what I would anticipate a brief period during an outage).  So, what I was having a difficult time figuring out if the term "single point" grounding meant only having 1 grounding electrode for 2 different systems.  

Basically, what in my mind I'm trying to accomplish is using my off-grid inverter system exactly as I would use a portable generator set.  If/when an outage occurs, instead of connecting a gennie, I'd have the option of connecting my inverter system via the 30amp power cord and power inlet box (I would disconnect the N-G bond in the E-panel prior to connecting since that bond is at the home breaker panel).  As you can see, If I do this, I now have 2 distinct ground electrode points connected together (home panel GE and E-panel GE). It would seem to make sense and easier to bond both the house and E-panel electrodes together to create 1 grounding point, but I wasn't sure if that would cause any issues when the inverter wasn't connected to the house panel.  Which would be the vast majority of time.   Also, lightning is not an issue in my area at all.     

I know this isn't exactly the way this style Magnum inverter was designed to be used.  It's supposed to either be hard-wired as a battery based backup connected to the house's main breaker panel, or, as a completely off-grid system with no grid AC input.  I was trying to utilize it as both (if possible) but grounded it as a totally off-grid arrangement using it's own GE.  

BB.

Everyone "talks" about single point grounding... But it is not what everyone thinks it is...

For example, we "single point" bond Neutral to Earth in the main panel of the home, and then run a cable from the main panel to a ground rod/cold water pipe. "Single point" grounding...

The reality is that there is a pole transformer, and 5 homes connected to the 120/240 VAC output of that transformer. So, that "single point" is now, at least, 6 points of N+G bonding. Most of the current flows through the drops from the pole to the home--Very little flows through the "ground" and ground rod connections. Classic example of multipoint N+G to earth to manage lightning strike energy (want to go to nearest ground, not into homes/power lines). And don't worry about current flow through the "earth" in parallel with the neutral cable.

In fact, there have been cases where the electrician has connected the L1 or L2 to the ground bar in the main panel... And then to the ground rod... And the way they found the problem? Somebody wondered why the snow was melting around the ground rod. Of course, there was a few amps of 120 VAC flowing through the ground rod ("shorting" Lx to earth ground back to the pole transformer ground+neutral bond)--So they also had an elevated power bill because of the "soft short circuit".

We do the "single point" N+G bond so that the Neutral wiring is never above (approximatly) zero volts. That means that we don't have to put a fuse/breaker on the neutral line. For truly "floating" outputs--A "floating" 120/240 VAC split phase system would need to have breakers on both Hot and Neutral connections. For example, say L1 was shorted to chassis (floating inverter output). Now, we have a L1+G bond and the output is no longer floating---But L1=zero volts, N=120 volts, and L2=240 volts... And the Neutral now needs a breaker per branch circuit to prevent over currents from subsequent short circuits causing an neutral wire to overheat and start a fire.

Recreational vehicles (and boats) are classic examples of the "single point" ground issue... Plug into shore power--That circuit has a N+G bond and probably a GFI breaker. If the RV has a N+G bond in its main AC panel--Then that will trip the GFI. So for shore power, the RV needs to have a floating N and G system.

Now--off shore power, your AC inverter output would need a N+G bond (to conform to NEC requirements). Need to switch in N+G bond.

Now--AC inverter "off" and using genset instead--Need N+G bond somewhere. If a portable Genset--For OSHA, it needs to have N+G bond--So trailer N+G bond needs to be broken. If the genset is part of the RV (installed in RV), then you could have the N+G in the main panel--And just make sure the genset does NOT have a N+G connection.

There are transfer switches that have L1+L2+N switching... Allows you to configure where the N+G takes place and under what circumstances (I think--I would need a better schematic or the part in my hands to confirm-Or call our host (NAWS) engineers and confirm):

https://www.solar-electric.com/pomaxpmautrs.html
https://www.solar-electric.com/lib/wind-sun/PMTS-Install.pdf

Also, many AC inverters with an AC input for genset/shore power also have transfer switches that can switch the N+G connection as needed.

The short answer--There is no really short answer. You need to look at the needs (both electrical and code) to see what is required.

From what I have seen, smaller gensets (say less than 3kW?) seem to have floating 120/240 VAC outputs. And larger gensets seem to (usually) come pre-wired with Neutral bonded to Frame Ground. Also gensets for OSHA job sites, also have N+FG bonded.

You are left with the choice of floating the N/G connection when connecting the AC inverter to the home panel, or switching N/G "safely".

Or, you can "float" The N/G in the source, and use GFI outlets (or breakers) to (help) protect the end user from electrocution (the GFI will trip if there is >~0.010 amps of 120 VAC current flowing "elsewhere" vs the Lx+N wiring).

What model of Magnum inverter are you using. Some inverters have an AC input that is configured to use with Line or "Genset" input--And it may actually already have the ability to do the N+G as needed. GFI's "fail" to work properly if there is a N+G connection on both the input and the output of the GFI... If you have no N+G or just one N+G (input or output side)--Then they work as expected.

A suggestion on using GFI's... I like to have a GFI per circuit... Not one single GFI circuit for everything down stream of the inverter/genset... For example, if you have a GFI trip (blender falls in sink)--A single GFI will cut power to blender output and your lights--Putting you in the dark. If you have 2x GFI... The blender outlet is turned off, but you still have lights to see what you are doing (wrong).

-Bill

A little more detail on floating gensets/sources:

https://cinematography.com/index.php?/topic/50236-do-honda-portable-generators-have-to-be-grounded/