Grounding of generators

[chris_r]

My guess is that a the neutral of a 3 phase 4 wire generator would only to be bonded to ground at the unit if this is the primary source of power. Otherwise for standby duty it remains isolated and is grounded at the main hydro service.

Is this correct?

[electric-ed]

The grounding and bonding of a stand-by generator depends on whether it is configured as a Separately Derived System (SDS) or not.

In an SDS installation, the neutral is switched by the 4-pole transfer switch, which requires the generator neutral to be both (earth) grounded, and bonded to the generator frame. Diagram B

In the non-SDS installation, the neutral is not switched by the (3-pole, solid neutral) transfer switch, so it should not be re-grounded at the generator, and must be isolated from the generator frame. Diagram A

Ed

[electric-ed]

They are available, but pricey. [img]eek.gif[/img]

This is from the Square D catalog.

Ed

[chris_r]

great explanation. How common are 4 pole transfer switches? Every one I have installed has been 2 or 3 pole.