See www.metatek.org for power systems.

http://www.mikeholt.com/forum/Forum1/HTML/000541.html –

Section 250-20(d) in the '99 NEC Handbook has a good description and diagrams of both 3-pole and 4-pole transfer switches. If you have a 3-pole transfer switch: a) you do not have a separately derived system b) the [generator] neutral should not be grounded [at the generator] - i.e. should not be bonded to the EGC or the generator frame (check the generator to make sure this is not done internally by the manufacturer) c) an EGC must be run from the service equipment to the 3-pole transfer switch and from the transfer switch to the generator frame

If you have a 4-pole transfer switch and the neutral is connected to the switching contacts (not always the case because I generally always use 4-pole switches but sometimes only utilize 3 sets of contacts): a) this is a separately derived system b) the neutral must be grounded - i.e. connected to a GEC and grounding electrode and must be connected to the generator frame (assuming this is a 3-phase, 4 wire wye)

Both installations are safe, but you have to know if you have a neutral to ground connection at the generator. An incorrectly wired 3-pole transfer switch with a neutral to ground connection at the generator will allow return current to flow back to the generator on the EGC. It will also provide a shock potential in the service equipment since the neutral from the 3-pole transfer switch to the service equipment is carrying return current back to the main bonding jumper (neutral to ground connection in the service equipment) and then back on the EGC to the transfer switch and out to the generator frame. An incorrectly wired 4-pole transfer switch (with the neutral switched) without a neutral to ground connection at the generator will not trip the OCPD (branch circuit or otherwise) in the event of a fault between a phase conductor and the equipment grounding system (EGC, conduits, etc.) because there is not a fault path from the equipment grounding system back to the neutral of the generator.

A properly wired 3-pole transfer switch should not put current back on the utility even if the neutral is broken at the generator because the utility neutral still wouldn't have a path back to the generator neutral connection. However, it would cause high or low voltages on the different phases, depending on the unbalance between phases.