Ground and Neutral bonding in mobile solar array.

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