Floating neutral generator

[joshuabardwell]

If I have a portable generator with a floating neutral (e.g. it has a grounding lug on the frame, which is supposed to be run to a ground rod) what are the implications of not running a ground rod? I have done a bunch of web searching on this, and I still don't feel like I have a straight answer. Near as I can tell, nobody actually pounds in a ground rod when they use a portable generator. It's portable! Duh! If I have to put a 6' rod in and pull it out every time I use it, then it's not portable! But I know that electricity is Serious Business and don't like the idea of leaving something that is supposed to be grounded, ungrounded.

 

[heviduty]

The frame of a portable generator is not required to be connected to Earth (ground rod, water pipe, etc.) if it has receptacles mounted on the generator panel and the receptacles have equipment grounding terminals bonded to the generator frame.
NEC 250.34(A 1,2)

 

[joshuabardwell]

Thanks. And of course the receptacles are mounted to the panel and (presumably) are bonded to the frame. So under what circumstances would that grounding lug need to be used?

If I understand correctly, this means that the generator frame is providing ground for the entire electrical system plugged into it. Is it sufficient ground to accomplish that? Presumably, or they wouldn't have designed it that way. Could a situation occur where voltage potential existed between the generator's frame and earth ground, and a person touching the generator frame got shocked?

 

[hr3]

This is what the National Electrical Code says about it,,,

250.34 Generators – Portable and Vehicle-Mounted

Revision clarifies when portable and vehicle-mounted generators are not required to be grounded to a grounding electrode.


(A) Portable Generators. The frame of a portable generator isn’t required to be grounded to a grounding electrode as defined in 250.52 if: Figure 250-13
(1) The generator only supplies equipment or receptacles mounted on the generator, and
(2) The metal parts of the generator and the receptacle terminal for the connection of the equipment grounding (bonding) conductor are bonded to the generator frame.

(B) Vehicle-Mounted Generators. The frame of a vehicle-mounted generator isn’t required to be grounded to a grounding electrode as defined in 250.52 if: Figure 250-14
(1) The generator frame is bonded to the vehicle frame,
(2) The generator only supplies equipment or receptacles mounted on the vehicle or generator, and
(3) The metal parts of the generator and the receptacle terminal for the connection of the equipment grounding (bonding) conductor are bonded to the generator frame

 

[joshuabardwell]

So it sounds like the generator would be expected to be grounded if it was supplying power to a home via a transfer switch or backfeed breaker, since the receptacles are not mounted on the generator.

 

[DarkBlack]

Joshua, There's reality, and there's the NEC. NEC is written to provide a right/wrong answer to every situation in the world. If you understand electricity very well, you soon find out that for the more uncommon situations, the "one size fits all" code is a hinderance at best for these situations. For non electrical people- ground and neutral are the same thing. it has to do where they are physically split, and the distance from the split sets up a "potential" voltage difference if there was a lot of current flow.Either one can carry current back to earth. Since it's a portable genny, it can not produce enough current to produce a voltage drop that would cause the conductor you call neutral or ground to elevate to a voltage that would be a risk. Your house will have the center tap of the pole split at your entrance. That point is where the physical center tap is "split" to what is called neutral, and ground. Both the same thing. There must be a groundrod at that point. If you keep your genny connection point close to the service entrance, you will have no safety issues regarding grounding,

 

[sparc]

if you set up your generator at one place often enough why remove the ground. just leave it driven in the ground and then you can connect to it any time you need to use your generator to power the house.

 

[dave1949]

My generator has a grounding lug, I have never used it. My grounding rod is not far from where it runs for power outages, but ...

I did have an issue. My generator has a four wire plug: 120v-120v-Neutral-Ground. It gets plugged into a receptacle that is wired to a backfeed 220v breaker. This breaker panel is my primary service entry, so the neutral and ground busses are tied together.

If I am recalling this correctly, if the green ground wire in the receptacle was used, the ground-fault breakers would trip immediately on the generator plug panel. That's without any load plugged into the generator but with the backfeed breaker closed. I thought maybe something else connected to that panel could be the cause, but with all other breakers opened, it still tripped the ground-fault.

I looked around for any stray ground potentials with my voltmeter and found nothing. Checked the continuity of the ground wire and isolated each end and checked for shorting. Nothing. So, I put a wire nut on the unused ground wire and forgot about it-- based on the electrical theory that if I can't find the problem, it isn't broken. :laughing:

I do not remember testing to see if grounding my generator frame to the ground rod would cure the issue, but I may have.

 

[heviduty]

So it sounds like the generator would be expected to be grounded if it was supplying power to a home via a transfer switch or backfeed breaker, since the receptacles are not mounted on the generator.

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.

dave1949

Re: Floating neutral generator
My generator has a grounding lug, I have never used it. My grounding rod is not far from where it runs for power outages, but ...

I did have an issue. My generator has a four wire plug: 120v-120v-Neutral-Ground. It gets plugged into a receptacle that is wired to a backfeed 220v breaker. This breaker panel is my primary service entry, so the neutral and ground busses are tied together.

If I am recalling this correctly, if the green ground wire in the receptacle was used, the ground-fault breakers would trip immediately on the generator plug panel. That's without any load plugged into the generator but with the backfeed breaker closed. I thought maybe something else connected to that panel could be the cause, but with all other breakers opened, it still tripped the ground-fault.

I looked around for any stray ground potentials with my voltmeter and found nothing. Checked the continuity of the ground wire and isolated each end and checked for shorting. Nothing. So, I put a wire nut on the unused ground wire and forgot about it-- based on the electrical theory that if I can't find the problem, it isn't broken.

I do not remember testing to see if grounding my generator frame to the ground rod would cure the issue, but I may have.

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?

 

[joshuabardwell]

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.

I appreciate your bearing with me, but I have to say I'm still confused. I am having trouble imagining any scenario where the NEC (or, alternatively, good common sense) would mandate the use of the grounding lug on the generator. I mean anything being powered by the generator is going to be plugged into a receptacle mounted on the generator, and that receptacle is always going to be bonded to the frame, right? So why does the generator even come with a grounding lug?

EDIT: Here is what one person on the Internet says.

So why ground an otherwise floating ground generator? Radio interference. Generators are by definition a source of 60 hertz energy. They also can have noisy brushes, harmonics of the 60 hertz signal, and have cords act as antennas. Slap a ground on the case and most of that goes away.