Understanding inverters with 60 volts on hot and neutral

[Pepere65]

I used to be an ABYC certified marine electrician about 20 years ago. I understand electricity fairly well, however I have not been able to find much online regarding portable inverters using 60 volts on opposing phases on both hot and neutral.

The inverters we used in boat installations never had this mode of operation.

The owners manual I have for the inverter says it is not suitable for feeding in to an electrical distribution panel and to not bond ground and neutral or damage to the inverter may occur.

Ok, all of this is fine. I can use this one simply to run an item directly.

My query is to try to understand how the ground works.

What happens if you plug in a device that has a ground fault in to one of these inverters with an energized case sending current down the ground wire to the inverter?

Does the inverter get fried and that is your failsafe?

Just trying to understand how the ground functions here.

 

[hwy17]

I would have to imagine they contain no ground neutral bond an energized case would have nowhere to send the fault current to. Wouldn't really be able to shock you either

I think it's best to just not use these kind of inverters at all, in any application, but it sounds like you do already have one in hand.

 

[robbob2112]

Depends on the inverter.

I have one that says to not add a N-G bond to the output and do not connect the N or G from the input to the output or it will be damaged.

If you use a meter to ohm things out the N and G are connected when the wall plug is pulled, and when the wall plug is in they still show as connected.... I am assuming the transfer switch is doing its magic to prevent double N-G bonds.

note - have to use an AC based ohm meter that can take the voltage or it will be damaged.... best you can do with a generic DMM is connect the leads and test for voltage between N and G - it should be zero or as near zero as you can get.

 

[SparkyJJO]

I consider that type of inverter to be poorly designed and a potential hazard. In a mobile application where there is no actual ground, ok maybe on for short term temp usage but I would not like it.

 

[timselectric]

These cheap portable inverters are designed to be floating (no ground), and must remain floating, or will be damaged.
You are reading 60v to ground, because they are floating. This is normal.
You can connect a GFCI to them, but it may not function as any protection.
Just keep it and everything it's powering, ungrounded. And everything will be fine.

 

[robbob2112]

That is what the inverter is designed for - an RV - it does AC pass through or will run without shore power from battery.

OP,

what brand/model inverter do you have?

 

[Pepere65]

You can connect a GFCI to them, but it may not function as any protection.
Just keep it and everything it's powering, ungrounded. And everything will be fine

When working in the marine environment we never installed a GFCI device downstream of an inverter that was not specifically listed in the installation documentation as being compatible. Ie, dont use an undocumented brand of gfci outlet or there may not be protection and you have opened yourself up to significant liability…. On the inverters we installed that were designed and certified for use on boats and pretty pricey relatively speaking we had lots of documentation and specs.

I am aware there are low cost inverters with a hole for the ground plug to fit into that has no electrical connection with the inverter. Ie you are using them ungrounded…

The inverters I am questioning does indeed have electrical connection with the ground prong. I am assuming it must be there for some sort of protection…. I could easily understand some sort of cutout if the device noted significant current present on the ground shutting down the unit… and that would be fine…. But the fact that the owners manual says bonding the neutral and ground can damage the inverter has me wondering if frying the inverter is the faisafe in event of a ground fault in the device you are plugging into…

 

[timselectric]

But the fact that the owners manual says bonding the neutral and ground can damage the inverter has me wondering if frying the inverter is the faisafe in event of a ground fault in the device you are plugging into…

It's not designed as a failsafe, but might act as one.

 

[Pepere65]

It's not designed as a failsafe, but might act as one.

Do you know what the ground plug is designed to do? Ie, if someone plugs in a device that has a ground fault that developed, what is the inverter designed to do?

There is nothing in the documentation, and judging by the documentation there might be a significant language barrier if I reach out to customer service..

It does carry the ETL mark so presumably meets UL standards for portable inverters, standard 482 if my memory is correct.

I can easily enough use a grounding adapter and not connect it electrically, but I am curious as to how these are designed to work in a ground fault situation…

 

[timselectric]

In these cheap portable inverters, the "ground" is more like shielding.
They don't have very high quality circuitry. There's a lot of bleed through between the positive, negative, line, and neutral. And this is why they must remain floating. I used to have one of them on my work truck. Every once in a while I would fry one. It took a few to realize what I was doing differently, when they smoked. I used them to charge things that were on a trailer. I narrowed it down to the times when the thing being powered was grounded to the trailer and the trailer was connected to the truck. Which made a connection between the AC ground and DC negative. So, from that point forward. I always made sure to unhook the trailer first. And I never had another issue.

 

[Rednecktek]

The inverters we used in boat installations never had this mode of operation.

You work on pleasure craft don't ya? Out here in the commercial world our 120v ships are 60v/60v and no ground. The ground wires just go to the hull, so we don't have any kind of (intentional) neutral/ground bond.

The worst offenders for shunting power to ground are surge protectors. We make it a point to open those up and cut the ground wire inside then plug them into the 220v outlets and pop the internals turning them into regular power strips.

On the 120v ships usually cutting the ground wire or tearing the ground plug out works.

All our inverters are floating and as long as you have no neutral/ground bond they work fine into distribution panels. The only catch then is that every breaker has to be double pole so it can get both 60v lines down to the lights/outlets/equipment. Any kind of leakage to ground will show up on your ground meter and it'll be time to start throwing breakers to find it.

 

[timselectric]

All of the ocean tanker systems I built were 3-phase delta, floating.
All generators, no inverters.
3-phase 240v delta from the generators and stepped down through 3 single phase transformers to 3-phase 120v delta. Same thing with 2-pole breakers. But we installed ground detection lights at each distribution panel. A ground fault would be identified by the lights, and repair would be done at the next port.
But all of the river Tow Boats that I currently keep running, are standard 120v/ 208v WYE systems.

 

[SparkyJJO]

Are you telling me you don't just stick a long metal pole out the bottom of your boat as a "ground" rod?
Guess you could call it a water rod.

 

[timselectric]

Are you telling me you don't just stick a long metal pole out the bottom of your boat as a "ground" rod?
Guess you could call it a water rod.

A couple of years ago, I had a safety inspector ask me where the ground rod was, on a river dredge. lol

 

[Q-Dog]

A couple of years ago, I had a safety inspector ask me where the ground rod was, on a river dredge. lol

I think I would have pointed to the shore, and not said anything.

 

[Pepere65]

I am still trying to understand what function the ground conductor has on these inverters. If you plug in an appliance that has a ground fault, what happens? What is the inverter designed to do with that current coming back via the ground wire?

It would be nice if it simply tripped off the inverter from an overload…

But the fact that the manual says that bonding neutral to ground can damage the inverter is not encouraging…

And based on everything I have read here and other places on the internet, either nobody knows or it is classified…

 

[SparkyJJO]

It literally is nothing. Just a spot for the prong to fit in.

 

[Pepere65]

It literally is nothing. Just a spot for the prong to fit in.

If that is the case, why bother with the expense of having electrical contact on the ground plug?

There are inverters where the ground plug contacts nothing electrically. Just air. That is not the case here. If it was just a place for the prong to fit in why does the manual state to not bond ground and neutral and that doing so might damage the inverter?

Is that an opinion or informed fact?

Again, what happens if you plug in an appliance with a ground fault? How is the inverter designed to respond?

 

[timselectric]

I am still trying to understand what function the ground conductor has on these inverters. If you plug in an appliance that has a ground fault, what happens? What is the inverter designed to do with that current coming back via the ground wire?

It would be nice if it simply tripped off the inverter from an overload…

But the fact that the manual says that bonding neutral to ground can damage the inverter is not encouraging…

And based on everything I have read here and other places on the internet, either nobody knows or it is classified…

It could do nothing, or it could fry the inverter.
I don't think that anyone has done it on purpose, to see what happens.
In either case, it won't provide any protection, because it's not an actual ground.
And therefore it wouldn't be a ground fault. It would just be a short to nothing, or possibly to the inverter case.

 

[upnorthandpersonal]

I deleted an earlier comment, because I wanted to clarify some things in more detail. The ground on these kinds of inverters is not useless. In fact, even with a floating Neutral on these, they still work with an RCD for protection. The reason this is the case is that the output has a double capacitor (one between line and 'ground' and another between neutral and 'ground'). Since both capacitors are equal, and form a voltage divider, you now see 1/2 of the voltage on Line and the other 1/2 on Neutral. Something like this:



When you now connect this center-point together with all your metal cases etc. and turn that into your actual 'ground' you basically created a Circuit Protective Conductor or CPC. This is not the same as a low impedance actual ground, which is why you need an RCD for safety. The impedance created between Line/Neutral and the 'ground' by those capacitors is in the order hundreds of kiloOhm (big enough to make this safe to touch). Any connection between Line and CPC or Neutral and CPC will cause the RCD to trip because of the leakage current through this impedance.

 

[Pepere65]

When you now connect this center-point together with all your metal cases etc. and turn that into your actual 'ground' you basically created a Circuit Protective Conductor or CPC. This is not the same as a low impedance actual ground, which is why you need an RCD for safety. The impedance created between Line/Neutral and the 'ground' by those capacitors is in the order hundreds of kiloOhm (big enough to make this safe to touch). Any connection between Line and CPC or Neutral and CPC will cause the RCD to trip because of the leakage current through this impedance.

And the RCD could also serve benefit to protect the inverter as well?

At least if the fault was on the hot presumably, or does GCFI break both poles or just trip the hot?

 

[Pepere65]

Ok, just read up. GCFI outlets are double pole.. GCFI breakers of course only trip the hot…

So, conceivable an inline GCFI, if it reliably trips on modified sine wave could protect the inverter by quickly tripping when leakage is detected.

I have ordered a plug in GCFI and a GCFI tester to ensure the outlet trips under modified sine wave inverter…

Yes they are cheap inverters, but they can still be handy…

 

[mikefitz]

'ground' by those capacitors is in the order hundreds of kiloOhm (big enough to make this safe to touch). Any connection between Line and CPC or Neutral and CPC will cause the RCD to trip because of the leakage current through this impedance.

Perhaps I am being stupid, but how can the RCD trip at 15 mA if there is a high impedance return path?

 

[OffGridInTheCity]

These cheap portable inverters are designed to be floating (no ground), and must remain floating, or will be damaged.
You are reading 60v to ground, because they are floating. This is normal.
You can connect a GFCI to them, but it may not function as any protection.
Just keep it and everything it's powering, ungrounded. And everything will be fine.

A long time ago (8yrs ago, didn't even have my 1st small solar system up yet / zero understanding of things), I tried to 'ground' the 60v middle tap of my Reliable 120v Inverter and it fried it! The correct approach (for Reliables) is to ignore it

 

[upnorthandpersonal]

Perhaps I am being stupid, but how can the RCD trip at 15 mA if there is a high impedance return path?

The actual impedance will be lower, there will be more than 15mA, but it's not a high current like you would see which can trip a breaker. A 10mA RCD can trip even at 5mA.

I have this set up in the lab, I need to find out what the actual impedance is and do some other measurements... I do know the test button for example indicates that it works.