Floating Neutral Generator to Service Panel Question

Mountain Don · February 2016

A friend is taking over a ranch from a relative. There is an out building that sees seasonal use. There is a compressor, a water pump, and various other tools that are electrically powered. It's too remote for grid tie. Use is too sporadic to make PV sensible. So a generator has been used for years.

The generator has 240 VAC output. The generator has labels that indicate a floating ground. It is located in a small "generator" shed about 50 feet from the barn / bunkhouse; 4 conductors, hot - hot - neutral and ground. At the barn the power line is connected to a small Square D QO service panel. The neutral and the ground are bonded with the green screw in the panel. The service panel is grounded / earthed to a rod in the ground.

Here comes the question... There is also a ground rod at the generator. The generator frame lug is connected to the ground rod there with a length of bare copper wire. Is that ground wire at the generator needed? Or is it even a potential problem? As the system is wired there is a ground connection in the service cable from generator to service panel.

Thanks for the assistance.

vtmaps · February 2016

If you don't locally ground the generator frame, then if the ground or neutral wires in the generator faulted and touched the generator frame, the frame could be at the potential of the tool shed ground, not the local generator ground.

That could be dangerous... if you are standing on the ground by the generator and touch the frame of the generator, you want that frame to be at the same potential as your feet. In other words, leave the frame grounded.

Of course, you need to make sure that inside the generator, neutral is NOT bonded to the frame/ground because you already have a neutral-ground bond at the tool shed... You did say that the generator had labels indicating a floating ground.

And of course, it doesn't hurt to put some Midnite surge protectors on both ends of the power line between the generator and the tool shed. Another improvement would be to run a bare, solid copper cable in a trench between the two ground rods.

--vtMaps

Ethan Brush · February 2016

Don,

Here is my take on it. Your generator would be a separately derived system in NEC jargon, as such you would bond the neutral bus, the equipment grounding bus, and the grounding electrode conductor together at one single point between the source and the first disconnecting means. Note that the grounding electrode conductor connection must be made "at the some point" as the N-G bond. I assume the generator has a breaker so I would consider that the first disconnecting means. It seems the bond is not made there, and it seems if it was, the GEC is not made at the some point. If you were to correct it and bond in the generator shed, you would remove the N-G bond in the barn (2005 code IIRC was the last code cycle that allowed three wire feeder and re-bonding the neutral at the second structure, but it is still allowed for existing installations). Note that you can bond a separately derived system at BOTH the source and the first disconnecting means if doing so does not create a parallel path for the grounded conductor.

With all that said, since the installation doesnt seem totally correct, its hard to say anything code wise about that rod to the generator, but the NEC does allow "auxiliary electrodes" so perhaps we could call it that. I dont see much need for it, the generator is earthed through the EGC to the barn.

VT,

I dont think the rod at the generator will do much. Due to the high resistance of dirt, we cant lower touch potential due to faults by earthing a metal object, even if that electrode is close. If you look at the voltage gradient as you move away from an electrode, you will find that one pretty much has to be standing nearly right on the electrode for there to be a small potential difference between the hot object and the patch of dirt a person is standing on. I guess the argument can be made that "it wont hurt", but the lowering of touch potential would be minimal. This is a very prevalent grounding myth - I used to believe it, It is in many books, one of the guys at midnite said it, mike holt admits to believing it for years.....

Ethan Brush · February 2016

here is a good video from mike holt on grounding myths. Its almost a half hour long and has some of the more basic myths but also some of the less clear ones dealing with step and touch potential.

https://www.youtube.com/watch?v=qNZC782SzAQ

starting at 3:25 talks about touch and step potential. 15:10 actually touches on rods at generators.

Mountain Don · February 2016

Thanks for that. Thanks very much!!

vtmaps · February 2016

Mountain Don said:

Thanks for that. Thanks very much!!

Me too! Thank you Ethan for posting that. --vtMaps

Raj174 · February 2016

Then, should a PV array be connected to a additional grounding electrode or to the equipment ground only? What is the latest NEC requirement?

Dave Angelini · February 2016

Yes! A remote array should have it's own local ground system with an SPD and then be connected with the smallest required conductor to the home. Oversizing this conductor to the home is a mistake!

Raj174 · February 2016

Thanks Dave

Dave Angelini · February 2016

You are welcome but many will disagree. I have been doing this too long successfully to change....

Ethan Brush · February 2016

There are some subtleties and debates about having a grounding electrode system at each detached structure. Generally, the NEC requires a GES at each structure but there are some exceptions: 1) a structure fed by a single branch circuit 2) A remote PV array depending on the code cycle 3) perhaps a creative situation with a structure fed by transformer secondary conductors (or another type of SDS) 4) A structure such as a meter pedestal or a building with a meter on it and service conductors running on the outside but not feeding the building - Thats all I can think of at the moment. Here is a link to a thread from the mike holt forum on grounding electrodes at detached structures for anyone interested in reading further on the topic. I have been meaning to read and digest it but haven't gotten around to it so the views expressed therein do not necessarily reflect my opinions

http://forums.mikeholt.com/showthread.php?t=172203

Floating Neutral Generator to Service Panel Question

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