Inverter grounding - avoiding ground loops, etc

[Gerald55]

I am in the final stages of installing a Xantrex Prowatt SW inverter. The inverter will be hardwired into the coach's AC system through a transfer switch (the other AC source is shore power and is wired into the other switch input).

This inverter has its AC neutral permanently bonded to AC ground.

The in addition to its 3-prong AC out, and DC +ve and -ve in, the inverter has a "chassis ground" screw. The manual says this should be connected via a (preferably green) 8 AWG wire to the vehicle chassis.

I'm struggling to understand the purpose of this wire, and whether it is appropriate on my system, and if it is, if wiring it to the battery negative is just as good as the chassis.

What I'm confused about is that this chassis ground seems internally bonded in the inverter to the AC ground and AC neutral. On my vehicle, the AC ground is already electrically connected to the chassis (at least according to a continuity tester). Since AC ground is connected to this grounding terminal on my inverter, won't adding a grounding cable to the chassis create a ground loop?

Similarly, the DC -ve of my coach's electrical system is already connected to the chassis (as I expect is normal/required on RVs). Hence, can't I ground this cable to the DC battery negative, rather than the chassis? Those two are electrically connected already, so the only question is that of how current will flow in the various failure modes:

It seems like for DC-side failure, the ground-to-battery-negative connection is strictly better, as that is the most direct return path so keeping the chassis out of it is useful.

For an AC-side failure, it is less clear, since I don't even know how the ground wire helps here. It seems like the return path is back to the inverter, and whatever AC-side fuse or breaker exists there should kick in.

I've searched quite a bit, but I don't find any consistent advice.

[westend]

The separate chassis ground from the inverter is for safety in the event that the inverter case becomes energized in a fault situation. It has nothing to do with the circuitry of the inverters operation, hence, it can not create any type of ground loop. Any power to the separate inverter ground wire will only occur if the inverter fails and something internal connects to the case of the inverter. In that limited possibility, the ground is connected to the vehicle chassis and anyone or anything in contact with the inverter case will not be harmed.
That ground connection is for safety and should be connected to the vehicle frame, not to the battery (-) phase. In the event that the inverter case is energized it may pass 120V to the ground wire and that could lead to other problems with anything connected to the 12V system or the battery, should anyone choose to connect the inverter case ground to the 12v battery ground,

I'd suggest to follow the Mfg's instructions and connect that separate ground wire to the frame of your RV. If you get an 8 gauge wire with an insulated ring terminal, it is an easy operation to use a self tapping screw through the new ground ring terminal and into the frame.

Good luck with your inverter installation and the ground. FWIW, I have the same inverter and believe it's a wonderful product. I use mine to power a residential fridge and 120V entertainment gear. It handles everything well. I've never heard the fan come on while in use.

[time2roll]

Gerald55 wrote:

Ungrounded electrical equipment leaves open the possibility that YOU become the ground.

As I mentioned above, the unit is already grounded, via the AC ground wire.

What happens when you unplug?

The idea for installed equipment is to have a separate permanent ground connection independent of all power connections.

[WyoTraveler]

The chasis is grounded for safety. You may never encounter a problem. It is there for safety. I wear seat belts when I drive my car. I have never needed them. That doesn't mean I should remove them. If you encounter a gound loop later deal with the problem i.e hum or buzzing in your electronic equipment.

[SoundGuy]

RoyB wrote:
This included PDF associated with an POWER INVERTER describes the requirement for grounding the CHASSIS as well as the DC RETURN cables to be compliant with the NEC.

Perhaps your POWER INVERTER MANUAL does not go into much detail as listed in this specific PDF...

Read POWER INVERTER Install procedure section...

Although specific to a particular model of Samlex inverter this owner's manual contains a lot of information about inverter installation in general and is certainly far more informative than the pathetic manual which came with my own Motomaster branded inverter from Canadian Tire. It too has a chassis ground lug but the manual makes zero reference to it so last year when I wired the inverter to power our trailer "whole house" I didn't bother with it. I never had any issues I know of but this spring I plan to ditch the whole house scheme and wire the inverter output to dedicated duplex receptacles in the camper and this time intend to wire the inverter chassis ground lug to the frame of the trailer with 8 AWG wire. Thanks for that link Roy! 😄

[brulaz]

Gerald55 wrote:

...
What I'm confused about is that this chassis ground seems internally bonded in the inverter to the AC ground and AC neutral. On my vehicle, the AC ground is already electrically connected to the chassis (at least according to a continuity tester).
...

I don't suppose the inverter chassis ground is also connected to it's DC negative terminal?
EDIT: on second thought I can't imagine anybody ever doing that.

Like yours, my Can. Tire 1000W inverter's AC ground is connected to the inverter chassis, and via the 10ga AC cord to the RV frame. The inverter has only a tiny bolt presumably for grounding but with no documentation. I never checked if the inverter's DC neg terminal was also connected to its chassis.

[RoyB]

This included PDF associated with an POWER INVERTER describes the requirement for grounding the CHASSIS as well as the DC RETURN cables to be compliant with the NEC.

Perhaps your POWER INVERTER MANUAL does not go into much detail as listed in this specific PDF...

Read POWER INVERTER Install procedure section...


I have separately grounded both my POWER INVERTER Chassis connections as well as my CONVERTER Chassis connections in my installations to meet the safety requirements of the specific NEC regulations.

The NEC added requirements to me is because the POWER INVERTERs, CONVERTER/CHARGERs as well as SOLAR POWER SYSTEMs all fall under ALTERNATE POWER SOURCES. Its an effort to make things safe to use when things fail...

There are also safety regulations telling us not to stand under a tree during heavy lightning storms. I don't question those safety regulations as well to save a few dollars. This is well above my paygrade to be questioning things...

Roy Ken

[Gerald55]

smkettner wrote:
There is no ground loop because the neutral-ground bond is in just one place, the inverter. No other neutral-ground bond should exist. Any bond at an onboard generator would be broken with the transfer switch same as the inverter when not in use.

Ground loops don't only (or even usually) occur due to multiple GN bonds. That is one type of loop that can occur, but a classic ground loop is entirely within the ground conductor and doesn't involve the neutral connector.

The classic loops occurs when the ground conductor paths form a loop, e.g., in this case because there is a path from the inverter through AC ground into the RV into the chassic, and also from the inverter directly into the chassis. It's not clear if this loop is actually problematic, however.

The ground is for internal failure on either the 120v or 12v side. Needs to be direct to the frame so that no matter what fuses or switches you have on either side the inverter chassis remains connected to the frame in all conditions. This makes the inverter chassis just a safe as touching the frame.

Yes, I understand how grounding works in general, but I'm trying to understand this particular case and the manual recommendations.

In particular, 120v failures are already protected since the chassis is already connected to ground via the AC grounding wire! So the inverter chassis gets the same grounding protection as all the other 120v appliances in the RV.

For 12V failures, I don't see how an 8 AWG cable is going to help, when the inverter is itself connected with 1/0 AWG cables, so 8 AWG is going to be woefully inadequate. Furthermore, if the goal is to protect against DC shorts, it would make much more sense to connect the cable back to the battery negative terminal, since that's a much more reliable connection than trying to push dead short current through the vehicle body.

Ungrounded electrical equipment leaves open the possibility that YOU become the ground.

As I mentioned above, the unit is already grounded, via the AC ground wire.

Go down to Home Depot and get some #8 green wire and make the connection.

I've already done it (yes, at HD even) - but I want to understand exactly the reasoning behind it. It's not magic - the reasoning behind grounding rules, for e.g. in house wiring, is open for scrutiny and makes a lot of sense. I'm looking for something similar for this grounding terminal.

[time2roll]

There is no ground loop because the neutral-ground bond is in just one place, the inverter. No other neutral-ground bond should exist. Any bond at an onboard generator would be broken with the transfer switch same as the inverter when not in use.

The ground is for internal failure on either the 120v or 12v side. Needs to be direct to the frame so that no matter what fuses or switches you have on either side the inverter chassis remains connected to the frame in all conditions. This makes the inverter chassis just a safe as touching the frame. Ungrounded electrical equipment leaves open the possibility that YOU become the ground.

Go down to Home Depot and get some #8 green wire and make the connection.

[Golden_HVAC]

Gerald55 wrote:

smkettner wrote:
The chassis ground should be to the RV frame. Simple and direct.
Not to the battery or the negative 12v input.

The instructions are following NEC.

Why? I'm trying to understand in what types of failures an 8 AWG wire from the inverter chassis to the vehicle chassis is useful.

Lets say that the wire to the inverter shorted to the chassis, and did not trip the circuit breaker. You could touch the inverter case, and the metal in the RV or be on wet grass, and then get shocked.

Why question the installation over a couple of feet of #8 wire?

#8 wire it not that expensive, and it will complete your installation, make it safe, ect.

Fred.

[Gerald55]

smkettner wrote:
The chassis ground should be to the RV frame. Simple and direct.
Not to the battery or the negative 12v input.

The instructions are following NEC.

Why? I'm trying to understand in what types of failures an 8 AWG wire from the inverter chassis to the vehicle chassis is useful.

[Gerald55]

DrewE wrote:
Is this a charger/inverter or just a straight inverter?

It is an inverter only.

[time2roll]

The chassis ground should be to the RV frame. Simple and direct.
Not to the battery or the negative 12v input.

The instructions are following NEC.

[DrewE]

Is this a charger/inverter or just a straight inverter?

If it's a charger, the chassis bond wire is necessary to safely carry the charge current should the battery negative wire fault. Otherwise, the full charge current could be carried by the AC safety ground, which is undersized for that task. At least with a charger, the chassis bond is required by the NEC. It also provides some protection for other wiring should there be an internal DC fault in the inverter.

I don't think you need to worry much about ground loops in this particular situation. Absolute purity and noiselessness of the AC waveform is not needed (nor present at an inverter output, even a pure sine inverter output). Ground loops are more of a concern with things like audio or video signals.

The chassis ground tie should be directly bonded to the chassis, not the battery negative terminal. The battery negative to the inverter could be tied either directly to the battery or to the chassis, though going directly to the battery is often preferable as a lower impedance connection. The neutral line should, of course, be switched via the transfer switch along with the hot, since the RV should not have neutral and ground bonded together when powered via the shore power cord.