ground wiring on inverter install

ws9876ws9876 Solar Expert Posts: 385 ✭✭✭
this is about installing a small inverter in a car.300 watt.
the wiring diagram shows the ac output ground as being spliced into the white neutral wire and from there to the big main DC ground wire. I assume this means you can just run the bare ground wire in the Romex from the ground screw on the standard plug ac receptacle to any good solid bare metal.Right..????..:roll:

Comments

  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: ground wiring on inverter install

    if we could see the diagram we could possibly confirm what you are saying, but initially it sounds like you could go to the dc ground. i caution on saying for sure as my inverter does not allow this, but yours may not be like mine. to be on the safe side why not just wire everything to the neutral and be done with it?
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: ground wiring on inverter install

    This is for a car installation, right? Where's "ground"?

    Regards,
    Jim / crewzer
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: ground wiring on inverter install

    jim,
    you should know that on most cars and trucks today that the chassis is consider a ground point for electrical stuffs even though it does not go to earth ground. they call it chassis ground.
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: ground wiring on inverter install

    niel,

    I need to ponder this...

    Jim / crewzer
  • ws9876ws9876 Solar Expert Posts: 385 ✭✭✭
    Re: ground wiring on inverter install

    its a hassle to splice into the neutral. its a morningstar 300.
    I am not an EE but I thought if the big dc ground is to the chassis then any ground can go there..??? why not..??
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: ground wiring on inverter install

    that may be true, but you have to watch as some sections may have more resistivity than others, especially in older cars. the metel shell of most cars are at the negative battery potential. some rusting can occur when contact is made with the electrical wires and of course any areas that have rusting going on will ultimately have more resistance. this is why certain places are usually reserved for electrical use.
  • ws9876ws9876 Solar Expert Posts: 385 ✭✭✭
    Re: ground wiring on inverter install

    how do I post a schematic in Paint or Notepad files??
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: ground wiring on inverter install

    Follow up...

    I've never been comfortable with the idea of vehicle chassis common being considered "ground". It seems to me that if this configuration was fundamentally OK, then we wouldn't bother with grounding an RV via its shore power cable. I'm not worried about a vehicle's 12 V system. But a 120 VAC inverter rattling' around inside of a vehicle is another matter -- to me, anyway.

    I suppose I'll have to look into this issue a bit further...

    Regards,
    Jim / crewzer
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: ground wiring on inverter install

    ok jim. looking forward to hearing what you dig up.
  • BB.BB. Super Moderators, Administrators Posts: 29,634 admin
    Re: ground wiring on inverter install

    My 2 cents worth...

    "Ground" is intended for safety. If there are any "shorts", those are intended to go to a solid ground and short/trip the fuse/breaker/protective device.

    "Ground" in a car is the sheet metal itself and in many/most cases, it intended to be a current conductor (radio, lights, lighter socket, etc.). And since 12 VDC is 10x the current of 120 VAC, I would not worry too much about (in general). If the ground can manage the high current of 12 VDC, there is nothing to worry about the 120 VAC current. However, notice that the inverter wants a 4 awg 12 VDC ground wire (PDF Inverter Manual)--that is capable of blowing a hole in any thin sheetmetal in a car--let alone what it would do with trim, electronic devices, etc.

    So, back to grounding as a safety device. Assuming that the case is metal (don't know), or that there are other DC related voltages (i.e., communications port, aux on/off switch), then if there were any internal shorts, you would want the high current to go to "safety ground" (4 AWG to ground) wire instead of energizing the exposed metal parts (case, comm wires, aux switch). That ground wire needs to go to any solid ground (metal chassis, frame, negative terminal of battery/ground connection point) and needs to able to handle the current available through the +12 VDC fuse/harness.

    Since normal large diameter wiring is not very flexible, I would suggest that the inverter/wiring be carefully handled and probably bolted down in place with the wiring tied down too. The fuse for the 12 VDC circuit should be as close to the source of the power as possible and not located at the inverter itself (fuse to protect the wire run against shorts too).

    The output of the inverter itself is fully isolated (per manual) and there is no defined "neutral/hot" leg. It is a transformer isolated output.

    Normally, in home wiring (in the US) we ground one end of the 120 VDC / 240 VDC wiring (the transformer "center tap") to limit the ability of the entire wiring system from rising more than 120 VAC above ground. With an isolated output, a primary to secondary short could, in theory for home wiring, raise the overall voltage to the primary voltage (~12,000 VDC or so)--one reason for a ground referenced "low voltage" output into our homes.

    The other reason, in the US, of the safety ground, is to prevent any open metal items from becoming energized in the event of an internal short (say the case of an old metal drill motor) electrocuting/starting a fire, when being used around and/or drilling a grounded piece of metal.

    So, for use in a stand alone metal structure (like a car), what should be done with the isolated AC output. Well, in a ship (at least the old ex-US Navy ship I interned one summer on--not claiming expert status here), they don't ground reference either side of the 120 VAC line... But they float them both, and use two filament 120 VAC lights tied from line to ground. With a properly running electrical circuit, both lights from a connection between line 1 and line 2, and so each sees 1/2 of 120 VAC or 60 VAC each--and each light just glows.

    Now, if one side of the 120 VAC electrical circuit is grounded, one light will burn at full intensity and the other will be out. There is no fuse blown in this case, but an obvious condition is created where a short is indicated and corrective measures performed to fix the problem.

    From what I understood, they did not want any current to flow in the metal hull of the ship to (probably) prevent corrosion problems as well as other possible issues (that I can foresee in a navel vessel).

    So, back to the car, if the inverter is used only inside of the car, floating the outputs or grounding one of them will not really impact safety. If one side of the floating output is grounded (shorted), it would still take a second insulation failure to create a short. If one side is safety grounded (creating a bonded neutral circuit), the same conditions would still exist. There is no high side primary voltage (or lighting source) available to raise the common mode voltage in the inverter's output to cause any problems. And no reason to ground the car since the inverter is used inside the car.

    However, if they inverter output is intended to be used outside of the car (i.e., to power a microphone/amplifier for a stage), then it would be required to ground both the neutral and vehicle chassis to conform with standards electrical codes and to prevent weird problems of having different ground potentials (if the inverter has a 3 wire plug, its ground should be the inverter ground which is the chassis ground).

    If the car ground is isolated, an another AC power source is also used in the stage setup (generator, AC mains, another inverter, etc.), it would be possible to create an issue were a fault ("hot" to un-earthed safety ground short) could energize the safety ground in the cord back to the car, and the car's body could be energized with 120/240 VAC--person touches the door handle while getting in the car, and happens to be stepping in a puddle--zap. Or a singer/musician could get a fault where the metal guatar strings could be energized and a grounded mike stand (or any combination) and zap...

    So, my suggestion. Connect to the wiring diagram in the manual. However, grounding the AC output of the inverter to the car chassis is really only necessary when following NEC code and/or there will be an extension cord used and then--ideally, the car chassis or an electrical box with bonded earth safety ground + neutral is installed at the point of use. And this would only be necessary if this is part of a large/complex outdoor setup (then you are talking about ground fault interrupters which protect against shock when you are using a bonded to earth safety grounded/neutral bonded system).

    But, in general, an isolated AC output (like this inverter) is actually safer because it cannot cause a short/shock to ground. It can only create line to line shorts/shocks. It can only create line to ground (chassis, safety, earth) shocks/shorts when the one side of the output is grounded.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: ground wiring on inverter install

    Bill,

    Thank you for your thoughtful analysis and comments. I have to admit that I had kind of forgotten about the isolation transformer issue.

    My main concern remains the issue of electrical shock hazard to someone standing outside of an inverter equipped vehicle that's not grounded. I've always been nervous about that, and, when RV'ing, I always try to ground the chassis before firing up a generator or inverter.

    Perhaps it's an irrational fear.

    Thanks again,
    Jim / crewzer
  • System2System2 Posts: 6,290 admin
    Re: ground wiring on inverter install

    I may not understand the discussion but it seams to me your talking about grounding the inverters 120v ground wire to the car frame which is in fact how the 12v is grounded. It doesn't seam like a good ideal to me to possibably push 120v through your 12v system in the rare case your 120v appliance shorts and goes to ground. wouldn't it be better to leave that ground alone?
  • BB.BB. Super Moderators, Administrators Posts: 29,634 admin
    Re: ground wiring on inverter install

    Jayven,

    The voltage is usually not the issue with grounding--it is the current. In general, for an equal amount of power, a 12 VDC circuit will have 10x the current of a 120 VAC circuit.

    Also, as has been discussed here before (to death...) DC circuits actually sustain an arc much better than AC (which switches "off" for part of every cycle). So--in general, for an inverter the DC grounding issues are much larger than the AC grounding issues.

    Regarding "shocks" from a sine wave inverter with a full isolation transformer--assuming that there are no other energy supplies (AC Mains, shorted isolation transformer in the inverter)--it is impossible (actually I am not telling the truth here) to be shocked by by an inverter by touching only one lead and "ground"... Just like touching one end of a battery--there is no return path for current to flow--so no shock.

    Now, for the truth about getting shocked... The inverter may be perfectly isolated, but when you plug in an external load, things may happen.

    1. The most obvious, a grounded appliance with a Hot/Ground short--On a regular grounded neutral home electrical system, this would blow a fuse/breaker. In an Isolated AC system, it now ground references on of the output leads. You will not get shocked just using the appliance--but can get shocked if you touch a metal ground and the other (now hot) "what was Isolated" output. --but this is really no different than touching the Hot Lead with one hand and the neutral/ground of any normal US AC distribution system.

    2. There is another source of "pain" that most people are not aware of... One of the ways to pass FCC EMI regulations is to filter the AC input. And one way to do this is using a small a pair of Line to ground AC capacitors. What happens is that this AC couples about 1/2 line (~60 VAC) to the chassis ground of the electronic device (say a desktop computer chassis). This is not fatal--but can give a nasty surprise if you touch an ungrounded chassis (or a failed ground return in your AC wall outlet) and a grounded piece of metal (happened to me in a computer lab with a faulty ground AC outlet and an expensive piece of test gear). UL requirements limit the size of the capacitor (without special warnings/wiring connections for the appliance) so that the amount of current available is kept at a safe level.

    But in the end, an ungrounded RV with a an external outlet (TV, tools, etc.) is pretty much shock proof (other than touching line to line inverter/generator power)--unless there is some extenuating circumstances (like a lighting strike, connected to an AC mains source for charging/AC power, mixed AC power to a "stage", etc.) and there is one or more faults in the system somewhere.

    In real life, grounding the RV actually increases the risk for shock of external users--but no worst than using the same appliance inside of the normal home. If you want to be very careful, just install a GFI (ground fault interrupter) outlet or purchase an inline GFI for use with extension cords. This will prevent Hot to Ground shorts/shocks (will not prevent line to line shocks/shorts).

    And that is the reason UL/NEC do require safety grounding--it is to limit the hazards associated with one or more faults.

    However, given all of the above--I would not be driving a ground rod (with chassis ground lead) next to my RV.

    If you are really paranoid, get a GFI breaker/outlet/extension cord. Usually required by code if the outlet is outside or around water (sinks, tubs, pools, etc.).

    About the only real problem with an ungrounded RV would be around lightening--and you should not be outside using power tools/appliances anyway. And if you are using shore power, a properly wired RV/Shore Power installation will connect the RV chassis ground to earth ground through the shore power plug.

    -Bill

    PS: The RV is well isolated being on rubber tires--however, you don't want to rely on this for isolation--somebody lays a metal tool against the chassis, chain on the ground, metal stairs touching earth, and even a kid in a wet swim suit and bare feet sitting on the bumper, etc. can all provide inadvertent chassis to earth connections. Most of the time the resistance will be so high that a dangerous current cannot flow--but that is how accidents happen. The person leaning against RV did not know that somebody on the other side was using an electric drill and working on an AC mains panel nearby (drill hits AC main, energizing the drill's ground, goes back to RV and energizes chassis, person in bare feet/wet grass grabs metal handle climbing inside--electrocution).

    That is the problem with isolated metal objects--is they can become energized. However, it takes quite a round-a-bout way to create this hazard in the first place. In a house, well worth safety ground to earth. The average RV with no hookups in the wilderness--grounding the RV chassis is not an issue (other than possible static electricity shocks in dry climates).

    -BB
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ws9876ws9876 Solar Expert Posts: 385 ✭✭✭
    Re: ground wiring on inverter install

    thanks for the detailed comments. those are the flat out most extensive and complete I have ever had. I wish I understood more
    about the issues. I am learning slowly about electrons but have been Very interested for about 6 years now.
    This inverter will be used for running a Dremel outside of the car
    mostly. I will ground the Romex ground wire to the chassis.And the big dc ground will go to the block. And the big dc neg wire will go to the neg post.Via crimped lugs 3/8. Was going to get Optima with dual posts but skip that. Wont use it enough.I have used tools with square waved invs and was ok but I want a pure sine to use later in a house. too bad it doesnt do 12/24 v .
    I am going to print out all these comments and study them over time. thanks.;)
  • BB.BB. Super Moderators, Administrators Posts: 29,634 admin
    Re: ground wiring on inverter install

    To be honest, I would ground the DC as shown and simply leave the AC floating. I presume that you are an artist doing work at a table outside the car (or equivalent).

    You will be less likely to get shocked or have other issues with the AC not grounded. In any case, the hazards are small either way (no worst and probably much better than using the same tool at home with much more power available from a wall outlet).

    Don't over think the issue and just make sure that the DC harness is well routed (no sharp edges, vibration at joints, fuse on + lead near battery).

    Another issue is how much power are you going to use... The standard car battery would run an 80 watt 120 VAC load for about 2-3 hours continuous before you would start to shorten the life of the battery (assuming that you fully recharge the battery between uses).

    Also, make sure that you have a good (fairly new) battery when you start using the setup. You could probably get a storage battery for your car and run it about 4-6 hours at a clip and still get decent battery life.

    More power than that (work lamps, buffer, soldering iron, etc.), you should look at a Honda eu1000i or eu2000i (900/1,600 watt) inverter/generator. Pretty quiet and don't use a lot of fuel (eu2000i will run about 15 hours 1.1 gallons of fuel with a 400 watt load).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: ground wiring on inverter install

    "To be honest, I would ground the DC as shown and simply leave the AC floating."

    i fully agree with bill and this is what i was trying to get at in my responce to you before. do as the inverter manufacturer recommends is a better way to put it, but you don't need to tie it to the 12v negative ground and that may not be possible with the electronics in the inverter. the subject of the earth ground was also covered very well and stated when you should need that as well.
    again, but more directly: very well said bill.
  • ws9876ws9876 Solar Expert Posts: 385 ✭✭✭
    Re: ground wiring on inverter install

    the rep from Morningstar said to ground it. I guess I should
    although I see the point in keeping a surge of current out of the walls of the car and all that they touch.
    If I run the bare ground wire in the romex directly to the neg post, and all grounds to the post and dont use any
    other points or chassis,then that should be ok.

    **********The DC ground wire from the inverter can be connected to the negative
    battery terminal if the battery is already grounded, or directly to the
    vehicle chassis, which ever is more accessible. One of the AC output wires
    (doesn't matter which) should also be tied in with the DC ground wire from
    the inverter. This will make one side of the AC output the neutral side,
    and the other will be the hot side.

    Best regards,

    Noah Sindermann
    MS Technical Support
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: ground wiring on inverter install

    if you posed this to them and they read it like you do, then do as they say, but do be very sure of what they said or meant.
  • BB.BB. Super Moderators, Administrators Posts: 29,634 admin
    Re: ground wiring on inverter install

    Where you ground it does not matter too much other than to make sure it is a solid ground (battery/chassis).

    My recommendation is to ground it where the battery ground wire connects to the chassis--that is how they do it with UL--separate safety ground connection that does nothing else (grounding at the battery post makes it more difficult to service the battery, possible getting acid in the wire twist, flexing, etc.). There is no good reason to attach to the battery post/connector directly for your ground/safety ground.

    Grounding to other parts of the car can be OK, but you have to make sure that the point you pick is not isolated by rubber mounts (obviously, no ground connection back to the battery) or has a ground return through a moving part (you can ruin bearings and bushings by sending current through them).

    If you are using "house" wiring type cable (instead of very flexible fine braid welding type cable), ground to the fixed chassis where the engine/battery bond is--don't connect to the engine bonding point--The motion of the engine will work harden the cable and cause it to fail.

    Also, I would probably keep the safety ground cable insulated (instead of stripping the insulation) when running it around the car--more likely to help prevent short/ground any other wiring/circuits in the area.

    Again, the surge currents in your DC harness are 10x larger than the AC harness (plus the DC harness wires are much heavier--allowing larger surge currents). Any AC "surges" will not affect your car's chassis.

    Again, don't over think this (yea--after I did several two page posts on grounding). Adding a "neutral/ground" connection just makes the AC harness more difficult to install and more likely to fail/cause other issues. There is no fuse in the AC circuit, the current is "low" (only 300 watts or ~3 amps of AC) and having a grounded neutral really performs no safety option.

    Using an Isolated output sine wave inverter (not grounding one AC wire) is actually, a bit, safer in virtually any normal use that you would have. Grounding the "neutral" adds virtually nothing to the safety and, in fact, makes shocks more likely (before bonding the neutral, you could touch the metal car chassis and either "hot lead" and not get shocked; after bonding the neutral, touching the chassis and the hot lead will shock you).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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