Confused about inverter ground

[lfhoward]

I am confused about whether or not to ground my inverter to the trailer chassis (it is a military trailer with a steel floor). It is a Xantrex 1000 watt pure sine wave inverter, and the directions say to ground it to the chassis. But before I get into it, let me describe my setup.

First, the trailer's tail lights are grounded to the trailer frame, and are run of course from the tow vehicle's battery. The tow vehicle's battery is not connected at all to the trailer's electrical system. (I did not set it up to charge the trailer's batteries, so there is no isolation switch.)

That being the case, if I install a ground strap from the inverter's grounding tab to the trailer chassis, and from the trailer frame to the trailer battery's negative terminal (as it says to do in the inverter's manual), am I not potentially mixing up the tow vehicle's ground with the trailer's ground, in effect connecting their negative terminals together? I am thinking this would not be a good situation. What do you think?

Second, I have an automatic transfer switch for switching between shore AC power vs. the inverter's AC power. When connected to shore power, I should be grounded via the campground's electrical system (in theory). If on inverter power, there would be no grounding rod in the equation (although the trailer does have a steel jacking foot). What is the advantage to grounding the inverter to the chassis, and then to the battery's negative terminal? Are we not talking about 120 volt AC being grounded, not 12 Volt DC? Which kind of current is the inverter's grounding tab actually for?

Also, I have a GFCI outlet in the inverter, and a GFCI outlet just downstream from my shore power inlet. So both sides of my 120 VAC power system are fault protected. My 120 VAC system is fused at 15 amps just like a simple household circuit.

So, does it make sense to ground the trailer's inverter & batteries to the trailer chassis or not, given the considerations above?

Thanks for your help!

[Dale M.]

Ground is relative to device (circuit).... For instance if tow vehicle lights and battery are grounded to trailer and you have internal battery for lighting in trailer, the ground(s) are common BUT the the positive side of the circuit is NOT common to trailer battery or tow vehicle so you do not have completer circuit so there is no problem.... Adding inverter and grounding it to trailer chassis is not a issue becasue it a protective or safety ground and designed to protect inverter and as long as out put of inverter does not become "Connected" to the ground all is well....... The idea of grounding the inverter and other devices (one side of circuit) is so you do not get a accidental voltage path between devices created by you body and you becoming part of the circuit and possible getting shocked or electrocute.... In theory the "ground" is at ZERO potential and should only show a voltage to the circuit that is providing the positive side of the circuit that is in use by specific battery or inverter or device the uses its specific power source... The common ground theory holds true in about 99% of the time, and you should not fear grounding inverter to chassis, or instructions would not say to do it...

One thing you shoudl be extremely aware of is when plugged into shore power you ground path from trailer to shore power plug and mother earth is good.... This keeps any stray electrons from trying to create a path from your trailer to mother earth (through yourbody) also test you GFI outlets occasionally to be sure the are functioning properly....

One should always test "unknown" shore power plug to be sure its wired correctly before plugging in to it... One of these can be your best friend....



Dale

[lfhoward]

Dale, thank you for the explanation.

Just to clarify, if the trailer's battery bank is grounded to the trailer chassis via the negative battery post, and I am towing it with my Jeep... And I hit the brakes, electrons from my Jeep's battery will flow though the positive line in the tow harness to the tail lights, out into the chassis, and...
A) back to the Jeep battery's negative post
or
B) take a detour to the trailer battery?

I know your post indicated (A) but I just wanted to lay it out there.

The other option could also be a question: The inverter, powered by the trailer battery, has a fault that sends electrons into the trailer chassis. These electrons will:
A) Return to the trailer battery negative terminal via the ground cable
or
B) travel to the Jeep's battery's negative post via the tow harness?

In other words, how do the electrons know where they came from and which battery to go back to?

Thanks for your help.

[capnTelescope]

Let me try.

First, ground the trailer to the tow vehicle through the wiring connector. They're going to be grounded to each other through the hitch/coupler anyway, so don't fight it. Just make it a good solid ground with wire. The lighting ground wire should be connected to both the TV chassis/frame and to the trailer chassis on their respective side of the connector plug.

Connect the trailer battery ground post, by its own wire, to the lighting ground wire.

Connect the trailer battery ground to the trailer chassis ground. Yes, it's a redundant connection. It's for reliability.

Connect the inverter + and - to it's 12V power source, either the trailer battery or the TV battery.

(edit)(omitted)(/edit)

If the inverter feeds a circuit also connected to shore power, keep the shore power and inverter output power separated from each other by switch or relay. (edited)

Done this way, the answer to your first question is A. Done any other way, the answer is "maybe A."

I think the answer to your second question is C) blow a fuse.

The answer to your third question is the electrons are lazy, ignorant and don't care. They'll follow the path of least resistance.

If I got something wrong, somebody say so and I will correct it. If I'm just full of baloney, make your case and I'll take the whole post down.

HTH

[Dale M.]

Um.... We need a little clarity here..... Do not ground the 110 Volt NEUTRAL to chassis, but if inverter has ground tab for "safety ground" (upside down "U" shaped prong on plug or "green" wire), yes connect it trailer chassis... Lets not confuse safety ground with the inverters output NEUTRAL side of circuit (which may be grounded in inverter by design which then it will be correct)....

Many circuits can have connection common GROUND POINT but the problem only arises if two different types of circuits find a unintentional path ( side of circuit opposite ground) through a circuit in which it was never intended to be connected too .... Only exception is in a circuit which has a floating ground by design.... BUT you instructions for your inverter states to ground it so do it , its internal design should have no effect on "other" 12 volts DC wiring schemes (internal lights and accessory plugs and tow lights) ..

IN your scenario of a fault... The electron are going to flow through any path where there is a electrical loop to form a full circle of current flow.... This can not happen with only one side of a circuit(s) connected common.... The only way it can happen is if circuit " A" finds a intended path through circuit "B" to to form a completer loop or short to cause current flow.... You can have circuits "C, D, E, F, and more") which have common ground point and these will not be effected unless they also cause a complete "loop" for current flow....

Dale

[capnTelescope]

In the interest of further clarity:

Yes, of course you should follow the inverter's installation instructions. If there's a ground tab, ground it to the trailer chassis directly or by wire.

NEUTRAL refers to the 110V wire that is typically white in house wiring. It's also the wire connected to the wider hole/blade on your outlet/appliance cord. This should NOT be connected to ground on your trailer in any way.

Dale, am I correct in saying that one should connect the 110V ground to the trailer chassis, which is also the 12V ground? I bow to your expertise on that. It does make sense. I'll correct that in my first post.

[lfhoward]

Dale & Capn, thank you both for your detaied explanations. It does make sense and I will be connecting the grounding tab on the back of the inverter to the metal trailer, and also running a strap from the trailer to the negative post on my battery. I will not ground the 120 volt AC to the trailer.

I think it's problem solved.

[MtnDon]

I believe I am correct in stating that new "store bought" RV's all have the green / ground wire in the AC circuits connected to the metal framing.

The reason to do so is the same as why the NEC states that in residential or commercial wiring where metal boxes or conduit is used, the ground must be connected to the metal box / conduit. If a hot wire somehow makes a connection to the metal (wire breaks, insukation scrapes through or some shoddy workmanship) the metal can become energized. If there is a connection between the third wire, the ground, and the metal box, frame or conduit then the breaker will trigger, or the fuse blow. If the frame does not have a 120 VAC ground wire connection dog, whatever can receive a shock if they touch the metal railer and step onto damp ground, for example.

The trailer system will work fine if the ground is not connected to the trailer metal frame, and if all the wiring is done properly. The ground connection is a safety thing, in case something wears, breaks or if someone does something stupid a few years from now.


Follow the inverter manufacturer directions for mobile grounds.

Never connect an AC neutral and ground together inside an RV system. That is only done in the campground or house wiring.

And carry one of those testers and use it whenever plugging into a campground. If it signals a fault ask for a different spot.

[H.A.]

I believe I am correct in stating that new "store bought" RV's all have the green / ground wire in the AC circuits connected to the metal framing.

Yes they do or at least should to be safe !

A trailer of RV is no different than a Skilsaw or refrigerator. The green grounding conductor must be connected to the metallic structure. The other two conductors are current carrying and therefore dangerous and must never be allowed connection the the metallic structure of any appliance, trailer, lamp, whatever.

I wish the engineering world would come up with more generally understood terms to describe these conductors.
The vernacular phrase of 'Hot, Neutral & Ground" does a better job but still too many folks somehow believe "ground" & "neutral" to be the same thing.

[MtnDon]

I believe the Brits have it more right when they refer to the wire being earthed, when we call it ground.

Too much confusion stems from our tendency to call the negative wire in a DC system the ground. IMO, we might have less confusion if DC systems had their wires named positive and negative. And always run a negative wire, rather than use the frame as a conductor for the negative.

[capnTelescope]

I wish the engineering world would come up with more generally understood terms to describe these conductors.

Amen to that. The whole thing is a mishmash hodgepodge mess. The AC convention is Hot black, Neutral white, Ground green/copper. Why'd they make Hot hard-to-see black? To make it easier for you to electrocute yourself in the dark?

DC isn't any better. The average layman understands red-positive, black- negative. Automotive wiring generally follows that, but add pink & brown. Until you get to the trailer wiring. Then it's black positive, white ground, and red is backup lights or something. Sheesh.

The vernacular phrase of 'Hot, Neutral & Ground" does a better job but still too many folks somehow believe "ground" & "neutral" to be the same thing.

Even more folks won't know a thing about AC wiring and wonder why there is no red wire. I think we should be careful, when we answer someone's question, to make it clear in the beginning not just hot/neutral/ground, but black/white/copper. Especially when the question is very elementary.

[/rant]

[MtnDon]

IF you take your lead from the NEC, then black is always hot. AC wiring. The NEC also states that the second hot wire (if any) will be red and that the neutral will be white or grey.

If you take your lead from automobile conventions, red is often positive at the battery but ranges all over the rainbow when connected to various devices. The DC negative is often black, but depending on make can be brown, black with yellow stripe, or white in the case of my trailer connector. Confusing. More confusing when we combine automotive DC with 120 VAC as we have in an RV.

In residential solar power the NEC approved color codes are black for a DC positive with red accepted by most inspectors. Negative DC though can never be black under NEC. Black is reserved for a hot wire. White is seen as neutral if AC, so in DC white (or grey) becomes the color for negative wires.

That how my cabin solar is and it was blessed by the inspector. My trailer follows the same "rules". White is either AC neutral or DC negative. Both start with letter N. Black is then AC hot or DC positive in my trailer. Unless the positives are color coded to differentiate one + from another in a wire bundle. Just as in a car.

I have a schematic with color codes for my trailer. Like the car mfg do. Helps to remember what is what.

[H.A.]

IF you take your lead from the NEC, then black is always hot. AC wiring. The NEC also states that the second hot wire (if any) will be red and that the neutral will be white or grey.

To split NEC hairs & totally escape the OP inquiry...
An alternating current "hot" or more correctly,"ungrounded conductor" can be any color other than green, white or natural grey. (Single phase circuits...)

In residential solar power the NEC approved color codes are black for a DC positive with red accepted by most inspectors. Negative DC though can never be black under NEC. Black is reserved for a hot wire. White is seen as neutral if AC, so in DC white (or grey) becomes the color for negative wires.

This is correct if its negative grounded system.
If its ungrounded DC, then its preferred red & black(Or any color one desires excepting white,natural gray or green)
A positive grounded systen, white or natural grey is positive.
The point being grounded current carrying conductors are ALWAYS white or natural grey no matter if AC or DC.

[Dale M.]

Ahhh... To confuse issue more another proper term for DC ground is "return".... But lets not go into that here...

Dale

[Dale M.]

Even more folks won't know a thing about AC wiring and wonder why there is no red wire. I think we should be careful, when we answer someone's question, to make it clear in the beginning not just hot/neutral/ground, but black/white/copper. Especially when the question is very elementary.

Umm...240 AC Circuits have a red wire.... And if you are wiring a 3 way switch (120 V AC) the "traveler" is Red...

Think its better to stay with convention and educate people to recognize if its a DC or AC circuit... Trying to redefined what already exists will cause so much confusion.. Then there is people that go off the reservation and use just any old color wire and it only makes since to them...

For Alternating Current systems...

http://ask-the-electrician.com/wiringdiagrams.html

I know its for a HOUSE but CONCEPT is same, TD is simply a little house...

Dale