Ground and Neutral bonding in mobile solar array.

Guisepi

Howdy all. 

I'm trying to work out some details here for a 3000W solar array mounted on a trailer. I am installing a Midnite Classic CC and Magnum MS4448-PAE inverter. This system will charge 8 x 224AH Fullriver 6V AGMs in series for a 48V battery bank. 

I'm having difficulties trying to figure out the grounding system, as well as the ground/neutral bonding for this system. The reason for this difficulty is because this trailer will be see various different circumstances. Sometimes there will be a generator to charge the batteries in the winter. Sometimes it will be plugged into the grid to charge the batteries. Sometimes it will be a standalone system with no grid. Sometimes it will power a house. Sometimes it will power an event. 

As per NEC, every AC system must have one place where neutral and ground are bonded. All the Magnum instructions have this happening in the AC main panel, which assumes my system will always be hooked up to an AC system. It won't always. Obviously, when I am hooked up to a main AC panel, I will have bonding in that panel. But when I am unhooked, I will then have no bonding. 

Many inverters have an automatic bonding of the ground and neutral when unplugged from an AC power source. This is what I would prefer, and honestly, I would have thought that for such an expensive inverter the Magnum would have this feature. 

Also, when hooked up to a generator, many generators have neutral/ground bonding. Will I just have to select one that has neutral bonding internally? I know some people make a bonding relay of some sorts to accomplish this too. 

So, where should I have my ground and neutral bonded?

Should I swap inverters for one that has automatic bonding? 

Also, I will potentially be driving a grounding rod when using this trailer at events (and not hooked up to the grid). This will be a pain in the butt, obviously, but I'd like it to be NEC compliant. I also may be using an existing grounding rod at an on-grid cabin where the trailer will usually be parked. Will this be a problem? Eventually this trailer may be the sole power source for the cabin. 

I truly appreciate any feedback on this. 

Estragon

AFAIK, MS series inverters by default do make/break the AC neutral-ground bond, (making the bond while inverting DC and breaking when passing AC through).

Guisepi

Thanks for the reply, Estragon.

Unfortunately the MS4448-PAE does not make and break the bond (as per the manual). I also read the manual for the MS4048 and it does not make and break the bond either. Perhaps there is a MS model that does. 

BB.

Guisepi,

Getting back to basics... What is grounded neutral used for.

1) make sure that all metal "things" in the home are ground bonded (plumbing, water heater, sinks, electrical boxes, etc.). And the Neutral is "ground bonded" in one place(sort of--further below) to the same grounds as the rest of the home (typically cold water pipe, 8 foot ground rod). Typically this is done in the main breaker box where the Neutral Bus Bar is tied to the metal box/green wire ground (to round rod).

If there is a short between the L2 and L2 (hot) wires and neutral, safety grounds, metal "stuff", there is a complete circuit which will trip a circuit breaker (or a "ganged" breakers for 240 VAC "pairs" of L1+L2--tripping one Lx turns off the other Ly).

Because the Neutral is already "ground bonded", you will not trip a breaker (or need a breaker/fuse on Neutral and why Neutral wires are "never switched") if Neutral is shorted to ground.

2) Neutral tied to ground is to help ensure a safe path to earth if there is a lightning strike and to ground static buildup in things (towers, solar frame on roof, etc. (natural earth's electric field, thunder cell passing overhead, etc.).

3) There are other ways of "protecting people and equipment" from shorts to ground problems. Those are typically GFI (ground fault interrupters in North America, or RCD residual current detector for Europe). These (GFI Breakers and GFI outlets) measure the current flowing "out" the Hot wire and "return" on the Neutral (or L1->L2 if 240 VAC North American Split Phase). If more than ~0.010 Amp difference between the "in and out" power circuit, it can be assumed there is a short from Hot to Ground, or a person is getting electrocuted Hot-Person-sink/wet ground/pool. If there is a GFI current difference detected, then it will open the Hot(s) circuits to stop the current flow.

Why the details... When you have your "solar trailer" (or any other power source), Ground Bonded Neutrals and GFIs are both present on many power sources. Your main panel, and almost any power source >~1,500 to 3,500 Watts (AC inverters, generators, etc.).

So, if you are using the trailer as "standalone" power source. It will probably have Neutral tied to the trailer frame ground (metal "stuff"). And probably any 120 VAC circuits that "leave the trailer" (power outlets) could have a GFI on the outlet (electric tools, lights, electronics, etc.). All is good. And you probably have (if your house is newer/remodeled) GFI outlets (and perhaps a breaker or two) for any circuits near water/sinks/outdoor outlets.

This is now where things get complicated... And is a similar issue for Recreational Vehicles where they have internal power sources (genset/AC inverters) and plug into shore power. Shore power is almost always has a Ground Bonded Neutral and can have a GFI protected outlet (for RV hookup). That means that the RV cannot have a ground bonded neutral inside--Because tying Neutral to Ground in the shore power, and tying Neutral to Ground inside the RV, you have parallel current path--Both through the Neutral and through the Safety Ground  wires. The GFI outlet on shore power will "trip" because some current flows through Neutral and ground (more than 0.010 Amps difference).

But, when the RV is running from Genset or AC inverter, then the RV needs its own Neutral to Ground bond (metal stuff, like RV frame, plumbing, etc.). And if they have "good size" generator and/or AC inverter, they will probably come from the factory with Neutral/Ground bonds internal to each unit. One way around this is for the RV to have Relay that can switch from L1/L2/Neutral/Ground (internal power) to L1/L2/Neutral (+ green wire safety ground) for shore power.

The typical solution would be:

1) For standalone power trailer, tie Neutral+Trailer Frame ground together. And you could use a ground rod (or ground plate)--Especially if you have lighting in the area. And use GFI for connections to power tools (protection against electric shock).

2) For powering your home, open the Neutral to Frame Ground, and run L1/L2/Neutral+Safety ground to the home (use a generator type transfer switch). If you do not have lightning in your area--You probably do not need a ground rod/plate.

If you have lightning possibilities, then a ground rod/plate at the trailer (tied to frame, can even tie to neutral, use Lightning arrestors from L1/L2/Neutral(?) to frame ground).

Tying Neutral to Ground rod in multiple places "before the main panel and in the main panel in the house" is what your utility does (one neutral to ground at the pole transformer, one at the main panel to ground rod in your house, and at the 4 other homes or so that are sharing your 120/240 VAC feeds to their homes. That is the "exception" to "single point" grounding.

What you don't want and cannot have, is Neutral to Ground bonding in sub panels (i.e., main panel bonded, then sends a circuit to a sub panel--That sub panel DOES NOT ground bond neutral). You don't want the Green Wire (from main to sub panel) to share Neutral/White wire current.

And if you have a GFI outlet with the input AC circuit Neutral/Ground bonded. And it went to a Neutral/Ground bonded sub panel (or your main panel at the house), the shared current flow in Neutral/Green Wire ground would trip the GFI.

Anyway, you will have to also comply with the NEC too (at least you should). And use a true transfer switch (or a combination of main panel lockouts). You don't want to "cheat" and use (for example) a standard main panel with main breaker, and a second breaker (or ganged breaker) tied to the trailer circuit (if both are accidently turned on at the sametime, you can have some excitement (or possibly energy power lines on the street that a lineman may be working on).

I will stop here--I hope this gives you some help to understand the "why" behind these (complex) grounding issues.

-Bill

Estragon

Guisepi said:

Thanks for the reply, Estragon.

Unfortunately the MS4448-PAE does not make and break the bond (as per the manual). I also read the manual for the MS4048 and it does not make and break the bond either. Perhaps there is a MS model that does. 

That's unfortunate but not really surprising.  Mobile applications tend to be smaller and lower DC voltage.  A 4kw 48v inverter is pretty big for mobile.  As Bill said, an external relay/transfer switch may be the best solution.

burnin_daylight

One solution I didn’t see mentioned was using an isolation transformer. Since the Iso doesn’t bring neutral on board, it allows for the N-G bond to be done on Iso output. It would be wired:

Shore power inlet to Iso Xformer
Iso Xformer to Transfer switch(auto or manual)
Genset to same transfer switch 
Transfer switch to inverter
Inverter to trailers main AC panel
(N-G bond is made on output of Iso or trailer main panel). 

The caveat is that you must ensure the inverter, genset or ANY other power source doesn’t also have a N-G bond. 

I have done this setup on boats, and has passed electrical testing by certified surveyers. I also showed them the electrical diagrams which they did not have any issue with. The real concern is making sure you have only 1 N-G bond anywhere at any time, and that it is reliable. 

It can also be easily done with a simple AC DPDT contactor. Just have the N-G connect anytime there is not AC present on the shore power inlet. BUT, this approach gets a bit more complicated when you bring a genset into the mix. You have to know if your genset has N-G bond to determine where you need to get your contactor coil input from(before or after the transfer switch, and from which source). So once you have your system in place, it requires any genset to either have a bond or not, you can’t mix/match.

Iso is my preferred method, because it doesn’t matter if the shore power and/or genset are bonded or not. It creates its own neutral. But I have used both approaches. 

Kudos to you for taking the time and being concerned with safety. It is regrettably not that common these days. 

BB.

Using an Isolation Transformer is also a possibility... Of course, you have the expense and losses of a transformer.

And for boats, where galvanic currents (small current between shore power neutral/ground and boat in water/salt water, even small amounts of leakage current can corrode and ruin parts in motor/bearings/prop/etc. And if enough leakage current (things done wrong between shore power and boat), you can create an electrocution hazard around the boat/in the water.

BD, do you have a source for isolation transformers you like?

From that little I have seen, larger transformers are not cheap or small (5kVA version is something like 80lbs and USD$900):

https://canadatransformers.com/5-kva-isolation-transformer-sc5k-k.html

-Bill

burnin_daylight

I always use an ELCI to make sure there is no current leakage. It can be a huge pain, as most, if not all boats have something wired incorrectly and cause it to trip. It takes time isolate each circuit to find the culprit. But between the isolation xformer and the ELCI I feel very confident there won’t be any shock hazard from my boats. 

Iso xformers aren’t cheap, until some type of shore power or genset is wired wrong and fries a panel or appliances. Seen too many marina and RV pedestals in my day that scare the heck out of me. But definitely might be overkill for this install. 

I have only used the Charles for the 120/240 split phase boats, and the Victron on the 120 single phase boats. That link looks interesting, I’ll definitely look more into those. 

Didn’t want to overload the OP with info, just wanted to offer another possible solution. 

Good info in this forum, quite fun to follow some of these threads.