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PTO Driven Generators
What Murf said. He put it in a nut shell. Well said.
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PTO Driven Generators
Thanks AC! The switches we're required to use here have 3-point main breakers so the service panel neutral would be disconnected from the panel when operating on generator. What I didn't know is if all transfer switches nowadays have 3-point disconnects. If so I don't know whether the transfer switch is bonded to the service panel so the generator would get its ground through the service panel ground bus or if the transfer switch has its own bonding to the service ground. My generator panel is bonded to the service panel through the ground bus so my generator would be grounded through the service ground. It uses 3-conductor plus ground line.
I was wondering how it would work with a 3-point transfer switch if the switch weren't bonded to ground in some way. I suppose the generator would have to have its own ground and then the generator neutral and ground should be bonded. I'm not certain the neutrals and grounds are bonded together on all generators but knowing for certain could be important for some hook ups. I think the neutral would float otherwise. The main point of all this is that people who install generator backups should understand grounding or have it done by an electrician.
The single-point grounding ideal is sort of ironic because almost be definition most people around can't have it. Codes require well casings, plumbing and metal septic system drains etc. to be connected to the panel ground bus. Any outbuildings that house livestock have to have supplemental grounds.
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For a generator, the ground connection should be connected to the generator frame and the neutral should be floating at the generator. They should be bonded at the Service panel.
Additional intentional and unintentional grounds are common. If you set the generator on dirt, it may get some grounding through the frame as well as being grounded at the service panel. You might see the same thing happen when you connect some equipment with grounded frames to GFI outlets on long electrical runs. If the ground potential is different enough between the local ground and the conductor bonded to the service panel it may trip off.
Auxilliary grounds on remote buildings are fine. The problems happen if you bond all the grounds and all the neutrals together in all the panels. If you just run the two hot wires and the neutral to the remote panel, you should be OK. If you run a local ground, you will be OK. You MAY run into problems if you bond the ground and neutral together in that remote panel. There is no safety problem if the ground and neutral are not bonded together in the remote panel as long as they are bonded at the main Service Panel. (Ten volts of voltage difference between the neutral and ground is not hazardous to humans, but could generate large currents in a low resistance conductor.)
Where I have seen these separated grounds become a big problem is with computer network and cable TV wiring. I have seen sparks fly when an Ethernet cable from one building is connected to equipment in another building and the the coax is grounded on both ends. This is because of different ground potentials and huge currents can flow. You have to separate the grounds and isolate the equipment. This can be done with transformers or optoisolators.
So, what happens if you do bond the neutral and ground together in the remote panel? You may have current flowing in the ground and neutral wiring because of the different ground potentials. This current has the potential to set up some long term problems with respect to corrosion of the building elements and structure. The local ground rod itself could corrode due to anodic action. If that local ground fails, then anchor bolts in moist concrete that are tied to a grounded metal building frame may carry the current and start corroding. This may take years to develop and does not happen if the bonding only happens at the main service panel. No extreme safety issue with respect to electrical shock should exist as long as the ground/neutral bond is still good at the main service panel.
Codes vary and change. You need to do what is defined locally as the standard to keep your local authorities and insurance companies happy.
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After reading all this stuff I elected to purchase a diesel generator in a weatherproof container. Who the hell wants to tie up a tractor to this kind of abuse anyways.
God knows what I was thinking-spend the extra money and do a stand alone and do it right.
http://www.winpowerinc.com/
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I went through the same drill and came to the same conclusion. Good call! Kubota makes some really nice stand alone gennys.
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AC: Thanks for the explanation. I never knew why sub-panels aren't bonded--just that it's one of the things inspectors always check. An inspector told me that the utility did some research and the even with proper installations, typical grounds start deteriorating after only five years in the soils around here.
I guess that a 3-point transfer switch breaks the neutral from the utility and makes it to the generator feed. The service ground is still on the panel side of the transfer switch so the generator neutral is connected to the service ground. My generator panel is a little different.
The important thing as I understand is to ensure that a generator has its neutral bonded to a good ground somewhere and ideally at only one point. A floating neutral can produce 120V of ground potential.
My generator does have a bonded neutral and ground as it has to for standalone operation. I connected the generator ground into the shed's supplemental ground and I leave it that way. I suppose that does give me two points of grounding and maybe some loss of efficiency through ground loops. However, then I'm sure the generator is always grounded when I use it as a standalone. Otherwise I'd have to make sure the feed line to the house was connected just for the ground connection.
I had a sideline sound and lighting buz for ten years and grounding can become a specialty all its own.
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Tom, a lot of what I was talking about relates to how the panels are supposed to be installed. From a practical standpoint, you have to ask a simple question...How often and how long am I going to be using the generator?
If you have a cabin and it is powered by the generator alone, then multiple grounding points are probably not going to apply. If you are using your generator for emergency power, then it is not going to be on for more than a few days at most, and is not going to cause the ground loop problems as long as it is completely removed from the electrical panel by the transfer switch when it is not being used. You may want to look for groundloops if you have problems with electronic equipment, but most of us are going to be more worried about refrigeration, heat, water pumping and so forth rather than computers and stereos.
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That's how I reasoned it as well. I wonder if the reason our utility company research found that good grounds don't last very long around here is that most of us have to have multiple point grounding by code?
When I installed our new service and panel I used two grounding rods or plates as required by code (which is not multiple grounds since they are on the same unbroken service ground line). I did extend the ground line to run to the opposite corner of the house where the soil stays damper due to drainage. Soil near the meter is sandy, dry and poorly compacted. The inspector liked the idea. He also said that the other benefit I mentioned of reducing the risk of lightening jumps inside the house was a rumor he had heard. I guess he was saying that it doesn't work.
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That kind of grounding extension is OK.
You should see what kind of grounding I do for my amateur radio towers. Each tower gets 9 to 12 eight foot ground rods. These rods are all buried below grade and the conductor connecting them is welded to each rod. But even these towers are tied to the single point ground at the incoming power service panel.
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I put in a 2nd panel and moved "critical" circuits to that panel. I have a natural gas generator, not PTO powered, rated at 9000 watts. We lost power for 7 days due to ice storm - needed a tractor to dig out and still wanted a warm house, TV, water, microwave + lights.
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Gas Generator Weather Protecti
06-06-2003, 10:17 
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