Grounding a generator

[John Z]

Should the gound lug on a genset be run to the bus frame? If so, what does this accomplish?

[prevost82]

The engine / generator should be grounded to the bus frame but the ground lug (if I understand you) the lug that is inside the generator with the other lugs (neutral / 2 hots / ground) should not be grounded. You need a transfer switch so when you are on shore power the ground and neutral are not bonded together, when you are on generator the neutral and ground are bonded together.
Ron

[Stan]

The ground lug on a genset is the safety ground, serving the same purpose as the as the green wire connection to shore power.

[Ncbob]

The generator frame should be bonded to the coach by a heavy cable or metal strap.  It serves no good purpose to isolate it from frame ground.  The green wire (sometimes referred to as equipment ground) should also go to bus frame which serves the same function as earth ground.

The neutral (white wire) goes (electrically) back to the center of the generator winding to separate the two 120V windings from each other and should never be common with earth ground or chassis ground.

NCbob

[Stan]

quote "The neutral (white wire) goes (electrically) back to the center of the generator winding to separate the two 120V windings from each other and should never be common with earth ground or chassis ground." unquote

Is this a requirement of the NEC? That seems to be the opposite of common practice with the neutral tied to ground at the power source. The wiring diagrams that I have for gensets (Onan & Kohler) show the neutral tied to ground.

[Sean]

Code (and good, safe practice) requires that the generator frame be bonded to the vehicle frame with a heavy bonding strap.  The green safety ground wire in the generator is already internally bonded to the generator frame, but, nevertheless, should also be wired to the safety ground of the main electrical panel, which, in turn, should be bonded to both the panel and the vehicle frame.

The neutral wire, which is (confusingly, to some) referred to in the code as the grounded conductor (vs. safety ground, referred to as the grounding conductor), is generally bonded to the frame and safety ground on most generators made for contractor, residential, or RV use.  Again, the code requires neutral and ground to be bonded within the coach system, but only when such a system is NOT connected to a shore power system (where neutral and ground are bonded in the transformer or main panel).

For most of us, the fact that there is a permanent neutral-ground bond in the generator is not a problem, because transfer switches made for the RV market switch the neutral along with the hots.  So the generator neutral will be disconnected when the shore neutral is connected, and vice-versa.  (The whole issue, spoken of here many times, regarding automatic neutral-ground bonding pertains to inverter operation only -- without an inverter, bonding happens at the main power source, whether shore or generator).

It is generally inadvisable to remove the neutral-ground bonding strap in the generator.  However, if you have set up a system that can use power from both the shore system and generator system at the same time (as opposed to completely separating them with a transfer switch), then you must take precautions to ensure that the system is not bonded in both the generator and shore system simultaneously.  Design of such a system is best left to professional engineers, as the code and safety issues are complex.

-Sean
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