ground wiring on inverter install

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