an argument about a ground rod for a guardian generator(13k) with an auto x-fer switch has arisen between two sparkys(what electricians arguing the code hmmmm).
Anyway, I am in the mindset that a standby generator is exactly that and not a seperately derived system.
Plus if memory serves(been awhile since I installed one)the ground and neutral are bonded together at the source by the factory and the transfer switch is fed with 4 conductors bonding it to the service panel.
So driving a seperate ground rod isn't required. Even though there is a ground lug on the frame for equipment grounding the generator is bonded via the factory whip to the x-fer switch(green conductor)which can easily be checked with a meter.
I think there is a lot of confusion over this point ... in no small part because the code seems to be worded as poorly as possible.
First, let's keep our issues apart. "Separately derived" simply means that the transfer switch opens / switches over the neutral. Strictly speaking, the ground network isn't part of this. I always recommend breaking the neutral ... electricity is trying to go 'home' to where it's made, so the genny electricity has no interest in the PoCo neutral. Let's remove one source of trouble by taking the PoCo neutral out of the system.
Even though the ground and neutral are bonded at some point, it's important to remember that the ground is normally NOT carrying current. It's there like a drip pan, to 'catch' any electricity that 'leaks' out. In the usual arrangement, you bond the ground to the PoCo neutral. Yet, with your transfer switch, you have removed the PoCo neutral from the picture. Therefore, you need to bond the ground to the genny neutral. This is usually done at the genny itself.
Now ... for the third element .... the ground rod.
Why is the ground rod there? It has nothing to do with clearing faults .... and we're not going to be using the earth itself as a conductor. The ground rod is there for lightning - that's all. What would we call the wires from the genny to the load center? They're not branch circuits ... they're now acting as your service drop. Let's look them that way.
The PoCo has a ground rod at their tranny, and you have one at the house, even if the tranny is but a few yards away. I suggest that you duplicate this arrangement. That is, if there is any way for the genny to be seen as a 'separate structure,' then it gets a ground rod. If the genny is actually inside the building, then you need only tie it to the buildings' grounding network. If it's 100 yards 'out back,' in a shed, it needs a rod.
Apply the same thoughts to any source of power ... be it a transformer, a wind turbine, a bank of solar panels, whatever. Keep these three (over-simplified) principles in mind: - Electricity wants to go back where it was made; - Ground rods are for lightning; and, - The ground wires, etc., are not normally part of a circuit. They're for 'just in case' to make sure the breaker trips.
If this generator is the same unit I have a 13KW Generac, the neutral is not bonded to the frame of the generator from the factory. The automatic transfer switch that comes with the generator does not switch the neutral and the system bonding happens at the Main Service. The Manufacturer specifies that a ground rod be installed and based on that specification it should be done.
Right up there with needing a ground rod at a light pole in the parking lot. Manufacturer's Spec or Design Professional recommendation. Maybe we should also install ground rod at the condensing units for residential air conditioners.
If this unit is manufactured and sold by Kohler- the neutral is bonded to the frame at the factory and it becomes a field application issue.
irregardless is a generater is bonded or not does not determine if a ground rod is required, just as a ground rod is not required for light poles. Remember that a grounding electrode system is on the line side of a service. The Bonding main is on the load side. Two, often confused topics. As a back up power supply does not generally require a ground rod since it supplements an existing system. If the generator is used to supply power lets say for a construction site that is not serviced by a utility, then it would need a ground rod. It not wrong to install a ground rod with a genset providing it is tied to the grounding electrode system for zero potential.
There are two acceptable ways to deal with the neutral and bonding. The key concept here is that the nuetral must be bonded to ground once, and exactly once, regardless of what else you do. Speaking as an engineer, pick one of these two:
1) Float the neutral at the generator, and bond it at the main panel or ATS. No additional ground rod required.
2) Switch the neutral at the ATS, and bond it to ground separately at the generator and service disconnect. You can run the ground wires right back to the same main bond if you size and bond the conductor properly.
Also note that for the generator, the ground is functioning only as a safety ground and not a parallel conductor for neutral current returning to the substation.