Floating neutral generator

[dave1949]

Joshua, it doesn't matter what you are powering, it only matters that it is plugged into a receptacle mounted on the portable generator, and that the grounding terminal of the receptacle and the metal parts of the generator are bonded to the generator frame. Dave, a GFCI works by comparing the current in the hot and neutral wires. They must add up to zero. Say you only turn on your water heater. You would have about 20 amps going out one hot wire and the same 20 coming back on the other hot wire, with nothing on the neutral, so the sum is zero. If you only turn on a 120 volt light you may have 1 amp going out on a hot and 1 amp coming back on the neutral, with nothing on the other hot. The sum is still zero. Turn them both on: 21 out on one hot, 20 back in on the other hot, 1 back in on the neutral... you get the idea. If more than a few thousandths of an amp is unaccounted for in the three wires, something is wrong... current is going somewhere it shouldn't. When you connect your backfeed breaker to your 4-wire generator receptacle, you create a parallel circuit because the neutral and green wires are connected at both ends. Current divides in parallel circuits, so with the green connected, the currents in the neutral and 2 hot wires no longer add up to zero because of the current in the parallel green / white circuit you created. Current in the green wire is unaccounted for and interpreted as a ground fault.

Clear as mud?

The creation of the parallel circuit does make sense, thanks! All things being equal, half of the current in the neutral would be "missing" as seen by the GFCI.

 

[cqaigy2]

Joshua, it doesn't matter what you are powering, it only matters that it is plugged into a receptacle mounted on the portable generator, and that the grounding terminal of the receptacle and the metal parts of the generator are bonded to the generator frame. Dave, a GFCI works by comparing the current in the hot and neutral wires. They must add up to zero. Say you only turn on your water heater. You would have about 20 amps going out one hot wire and the same 20 coming back on the other hot wire, with nothing on the neutral, so the sum is zero. If you only turn on a 120 volt light you may have 1 amp going out on a hot and 1 amp coming back on the neutral, with nothing on the other hot. The sum is still zero. Turn them both on: 21 out on one hot, 20 back in on the other hot, 1 back in on the neutral... you get the idea. If more than a few thousandths of an amp is unaccounted for in the three wires, something is wrong... current is going somewhere it shouldn't. When you connect your backfeed breaker to your 4-wire generator receptacle, you create a parallel circuit because the neutral and green wires are connected at both ends. Current divides in parallel circuits, so with the green connected, the currents in the neutral and 2 hot wires no longer add up to zero because of the current in the parallel green / white circuit you created. Current in the green wire is unaccounted for and interpreted as a ground fault.

Clear as mud?

Wow you made that pretty clear actually.

 

[MikeInMaine]

I think there are other reasons to consider grounding the frame of a portable generator. Sorry, I'm not going to look it up, but doesn't the NEC require installing equipment as per instructions? So if the instructions say, "bond the frame to ground" then the NEC would require you to follow those instructions.

I would also think that grounding the frame would equalize the electrical potential of the unit to ground, reducing the likelihood of shock.

Finally, if you are connecting the generator via 4-wire plug to your house electrical system, then you are effectively grounding the unit, as the green wire is the equipment ground. I think that is why GenTrans instruct you to connect your generator to the GenTran before starting it up.

 

[heviduty]

I would also think that grounding the frame would equalize the electrical potential of the unit to ground, reducing the likelihood of shock.

Actually, there is no potential to ground unless you do ground it. There are two ways to prevent shock: equipotential bonding and isolation. Equipotential bonding is what we are used to because that is what our power system uses. It works because metal you touch is all connected together and therefore all at the same potential. Isolation means there are no paths between potentials because nothing is bonded. No current path, no shock. Germany, until 20 or 30 years ago, allowed receptacles ...230 volt receptacles, in bathrooms to be placed right above sinks. 230 volt receptacles and no GFCI!. They did this by incorporating an isolation transformer right into the receptacle. You see, the secondary of a transformer is electrically isolated, a new source of electricity, so to speak, just like a generator. The 2 wires have a potential between them, but not between them and anything else because there is no return path. Don't try this at home, but you can touch either of the secondary wires of an isolation transformer as long as neither of them is grounded. They are floating potentials and the electricity in them only wants to get back to the other one. Grounding one of them turns it into a "neutral" and the other one into a "hot" wire.

 

[hr3]

I think there are other reasons to consider grounding the frame of a portable generator. Sorry, I'm not going to look it up, but doesn't the NEC require installing equipment as per instructions? So if the instructions say, "bond the frame to ground" then the NEC would require you to follow those instructions.

I would also think that grounding the frame would equalize the electrical potential of the unit to ground, reducing the likelihood of shock.

Finally, if you are connecting the generator via 4-wire plug to your house electrical system, then you are effectively grounding the unit, as the green wire is the equipment ground. I think that is why GenTrans instruct you to connect your generator to the GenTran before starting it up.

Mike is right,, The instructions that come with the unit is what you are supposed to do.. They are UL approved units and UL is the last word..

 

[Tig]

Just my experience, but I have run into that situation. I once had a large "portable" diesel generator backfeeding my electrical service through the dryer receptacle. When I turned on the lights, they looked real bright. Too bright, and then one went POP! I found that the legs were not even. I had something like 160V and 70V. I drove a "ground rod" (taken from my horseshoe pit )and that straightened things out. I was glad to send that unit back as soon as I could.

 

[DarkBlack]

Some good information here. I need to add one missing saftey tip. Only isolation type transformers are isolated on the secondary. There are many tranformers out there that are not isolation type.

 

[heviduty]

Some good information here. I need to add one missing saftey tip. Only isolation type transformers are isolated on the secondary. There are many tranformers out there that are not isolation type.

Right you are, DarkBlack. That is why I included the "Don't try this at home" caveat in the posting. You can end up very dead experimenting with electricity. It isn't prejudiced and it doesn't care who you are, it will bite you.

 

[dave1949]

Speaking of floating electrical, the next time I go grocery shopping, I'm taking my voltmeter with me.

Ever since they did a major re-model, I get zinged every time I touch a metal shelf. I asked the cashier if others had mentioned this, he said one other person had made that comment to him.

I have experienced this before where touching a commercial metal door frame gives you a little jolt.

 

[Tig]

Ever since they did a major re-model, I get zinged every time I touch a metal shelf. I asked the cashier if others had mentioned this, he said one other person had made that comment to him.

Something very wrong there. To the point that touching two pieces of equipment at the same time could be lethal. That flaw needs to be reported and fixed.
I've seen this situation in large computer systems.