Inverter Ground Connections

[createthis]

Hello,

I've been looking at inverters for my portable camping solar generator lately. One thing I notice is that most of the pure sine wave inverters feature a ground connection and caution you to use it in the user manual.

Cheap modified sine wave inverters, like those you find at BestBuy, do not feature a ground connection. In addition, my Yeti 400 does not feature a ground connection, despite having a pure sine wave inverter built-in.

What's the deal here? Why are the ground connections necessary on some inverters but not others?

Thanks!

[inetdog]

Hello,

I've been looking at inverters for my portable camping solar generator lately. One thing I notice is that most of the pure sine wave inverters feature a ground connection and caution you to use it in the user manual.

Cheap modified sine wave inverters, like those you find at BestBuy, do not feature a ground connection. In addition, my Yeti 400 does not feature a ground connection, despite having a pure sine wave inverter built-in.

What's the deal here? Why are the ground connections necessary on some inverters but not others?

Thanks!

Under the NEC, ground connections are required for most systems with any voltages over 50V, either on the AC or DC side.
The biggest issue with most MSW inverters is that the negative battery terminal is expected to be grounded and often connected to the chassis. But the AC output has two wires that are each 60V but of opposite polarity with respect to ground. If you take the white wire of the AC output, which is NOT actually a neutral wire in this case, and you connect it to ground you will burn out the inverter.
If the case of the unit is plastic and there are no exposed metal parts the NEC and UL may not require the use of a ground wire or Equipment Grounding Conductor (EGC.)

[Mike90250]

mod sine inverters have incredibly cheap wiring and circuitry inside them, and if a battery terminal is grounded (like in a car to the chassis) and one of the inverter outputs gets grounded, the smoke is let out of the inverter, never connect a mod-sine inverter to a grounded household electrical system, because you will fly the inverter, and likely any 12V gear on the battery

[createthis]

Under the NEC, ground connections are required for most systems with any voltages over 50V, either on the AC or DC side.
The biggest issue with most MSW inverters is that the negative battery terminal is expected to be grounded and often connected to the chassis. But the AC output has two wires that are each 60V but of opposite polarity with respect to ground. If you take the white wire of the AC output, which is NOT actually a neutral wire in this case, and you connect it to ground you will burn out the inverter.
If the case of the unit is plastic and there are no exposed metal parts the NEC and UL may not require the use of a ground wire or Equipment Grounding Conductor (EGC.)

So... how does the Yeti 400 get around this, I wonder? It's got exposed metal bolts and an aluminum heatsink. No grounding requirement.

[createthis]

never connect a mod-sine inverter to a grounded household electrical system, because you will fly the inverter, and likely any 12V gear on the battery

I'm having trouble picturing how one would even go about doing that. Are you talking about wiring the ground prong of the inverter AC output into the ground prong of the household electrical system?

[inetdog]

I'm having trouble picturing how one would even go about doing that. Are you talking about wiring the ground prong of the inverter AC output into the ground prong of the household electrical system?

Actually, he is talking about the white wire (wide prong) of the mod-square being connected to the house neutral which is in turn connected to earth ground if the building is wired in the normal way for an on-grid building. That will make the internals of the inverter and the battery bank bounce around at about 60VAC relative to earth ground.
If you then ground the negative of the battery bank you will end up with a short circuit through the grounding system.

As for the Yeti, if the equipment is double insulated (any exposed metal has at least two insulating elements between it and energized parts) then UL allows it to be operated on a two wire cord without a ground wire.

[jflorey2]

So... how does the Yeti 400 get around this, I wonder? It's got exposed metal bolts and an aluminum heatsink. No grounding requirement.

It is cheaper to not add grounding to an inverter. And in the case of the Yeti, since there is no access to the DC input terminals, not all that important. An internal fault cannot return current to ground, so cannot create a dangerous situation where the case of the device you are using is at a dangerous potential. (As long as the heatsink is isolated, which it should be in a well designed inverter.)

[createthis]

Actually, he is talking about the white wire (wide prong) of the mod-square being connected to the house neutral which is in turn connected to earth ground if the building is wired in the normal way for an on-grid building. That will make the internals of the inverter and the battery bank bounce around at about 60VAC relative to earth ground.
If you then ground the negative of the battery bank you will end up with a short circuit through the grounding system.

Is that a common error when migrating a circuit from grid to battery? What's the proper way to connect it?

As for the Yeti, if the equipment is double insulated (any exposed metal has at least two insulating elements between it and energized parts) then UL allows it to be operated on a two wire cord without a ground wire.

Maybe it's double insulated, I'm not sure. I made a teardown video a while back: https://youtu.be/FWYIQDA-QCU
I've got the inverter ripped out and sitting on my back deck right now. Looks like the heat sink is isolated from the PCB with electrical tape. Shrug.

Maybe the inverters I'm looking at just feature GFCI and that's why they require grounds? Seems like they would just say that in the user manual though, instead of talking about risk of electric shock.

[inetdog]

Is that a common error when migrating a circuit from grid to battery? What's the proper way to connect it?

It is a very common problem when dealing with an RV that will have both an MSW inverter and a shore connection. The simplest way to avoid it is to get a transfer switch that switches the neutral too. That way the grid neutral can be bonded to ground as the NEC requires while the inverter neutral is isolated from ground.

If you are just running off the inverter at all times, then the best solution is to avoid the MSW inverters with that problem.
Or wire your space with an ungrounded neutral, which is technically an NEC violation.

[createthis]

It is a very common problem when dealing with an RV that will have both an MSW inverter and a shore connection. The simplest way to avoid it is to get a transfer switch that switches the neutral too. That way the grid neutral can be bonded to ground as the NEC requires while the inverter neutral is isolated from ground.

If you are just running off the inverter at all times, then the best solution is to avoid the MSW inverters with that problem.
Or wire your space with an ungrounded neutral, which is technically an NEC violation.

Interesting. When I read the original comment, I missed the MSW reference. So, pure sine wave inverters don't have this problem? Isn't it a really bad idea to feed power back into the grid without a grid tie permit/inspection anyway? The transfer switch seems like the way to go there.

Back on the topic of pure sine inverters and grounding... I ended up buying this Go Power 300W inverter: http://www.amazon.com/gp/product/B001539BCE

I'll do a review video on it in the next day or two, I think. My initial impressions are good. It's significantly heavier than the old inverter. Probably a good sign for longevity.

When it arrived, I wired it up and tested it without a ground connection. To my pleasant surprise it worked properly, even though the front sockets feature GFCI. I didn't get shocked when I touched the case carefully with a knuckle either. I'll wire up a cable to run the house ground to the case, eventually, but it's nice to know it operates like my old yeti 400 pure sine inverter without a ground if needed. Makes it a bit more portable that way. I didn't want to have to pack a 6 foot grounding rod and pound it into the ground every time I go camping.

[SunEagle]

Interesting. When I read the original comment, I missed the MSW reference. So, pure sine wave inverters don't have this problem? Isn't it a really bad idea to feed power back into the grid without a grid tie permit/inspection anyway? The transfer switch seems like the way to go there.

Back on the topic of pure sine inverters and grounding... I ended up buying this Go Power 300W inverter: http://www.amazon.com/gp/product/B001539BCE

I'll do a review video on it in the next day or two, I think. My initial impressions are good. It's significantly heavier than the old inverter. Probably a good sign for longevity.

When it arrived, I wired it up and tested it without a ground connection. To my pleasant surprise it worked properly, even though the front sockets feature GFCI. I didn't get shocked when I touched the case carefully with a knuckle either. I'll wire up a cable to run the house ground to the case, eventually, but it's nice to know it operates like my old yeti 400 pure sine inverter without a ground if needed. Makes it a bit more portable that way. I didn't want to have to pack a 6 foot grounding rod and pound it into the ground every time I go camping.

That inverter can be used only in a totally isolated system from your home grid power so there is no need to worry about back feeding into the grid.

Like your Yeti the Go Power has a place to plug in the loads not provide power to your house panel which would be illegal and unsafe.

It can be used on a RV but might required a transfer switch as inetdog suggested but IMO would require some type of equipment grounding system and a rod if used in the field.

[createthis]

That inverter can be used only in a totally isolated system from your home grid power so there is no need to worry about back feeding into the grid.

Like your Yeti the Go Power has a place to plug in the loads not provide power to your house panel which would be illegal and unsafe.

It can be used on a RV but might required a transfer switch as inetdog suggested but IMO would require some type of equipment grounding system and a rod if used in the field.

I'm not planning to grid tie it to the house system, but I was planning to attach the ground on the back of the inverter to the house ground, which is the point of this thread.

[SunEagle]

I'm not planning to grid tie it to the house system, but I was planning to attach the ground on the back of the inverter to the house ground, which is the point of this thread.

What are you going to use as a "ground" if you take that inverter and use it somewhere not close to your home?

[createthis]

What are you going to use as a "ground" if you take that inverter and use it somewhere not close to your home?

I won't. That's why it's nice that it doesn't require a ground. However, when it's at my house, having a ground protected GFCI is nice.

[Sunking]

Mobile inverters do not need or require any ground. Nor should you use one. All you need and it built into the inverter is the GFCI, and GFCI needs no ground. Your Inverter cannot possible produce enough current to operate a Breaker unless it is extremely small breaker like 1 or 2 amps.