Yanche wrote:If your local earth ground is correctly bonded to the neutral, and the two hot feeds are disconnected I fail to see how that's possible.
I guess the key word is correctly. I'm no expert, but we had a thread running last year in which an electrician stated the neutral must be transferred too or you risk killing someone. I'm not sure what all this means, but it looks like the same thing to me. http://members.rennlist.org/warren/genfaq.html
"Using a generator with bonded neutral and ground in a system which does not switch neutral in the main panel leads to a potential safety issue. Neutral and ground will be connected at both ends of the extension cord, thus making them one conductor. As wire has a certain finite resistance, a voltage will develop when current flows through. This voltage will appear at the chassis and want to find a path to ground. Touch the chassis and you will be providing such a path. Normally this voltage is very low, but if there are problems elsewhere, it could become dangerously high. Safe operation can no longer be assured. Wired properly the neutral wire would be isolated at one end, preventing ground wire current and ensuring the chassis remained at zero volts." http://en.allexperts.com/q/Electrical-W ... bond-1.htm
"Once you leave the main service, the neutral and ground are separate. So your sub-panel will have 2 hot legs a neutral and ground. The neutral bar is NOT to be bonded to the panel via the green 10/24 screw. Now for the reason.... If you were to bond the neutral to the box, then you will created a parallel neutral. The grounded conductor, or neutral, is grounded at the service. The ground is separate in the field for safety of a ground fault.Now by creating the second (parallel) path for the neutral, the egc becomes essentially another neutral, so that any current that is supposed to be travelling on the neutral is now shared on the egc, energizing any conduits, boxes, ceiling grids, metal studs, or metal parts on the way back to the source. the situation becomes worse if the neutral gets damaged, loose or broken, then all the neutral current is now on the egc, and because all the equipment is still functioning properly no one finds out until someone is killed or injured, perhaps an electrician taking apart a piece of conduit or box who unknowingly becomes part of the circuit. the egc is designed to carry fault current only in order to trip ocpd's, not to be a grounded conductor.
EGC = equipment grounding conductors
So to answer your question you can install a separate ground rod at those panels make sure the ground and neutral are separate.
If there is no grounding conductor in the branch circuits and the neutral is isolated from the case in the sub-panel, then leave it that way. Any attempt to "improve" the system by trying to use the neutral as an equipment ground would actually introduce additional hazards. Many of the early electrical systems did not include equipment grounding conductors. The problem is when we try to "upgrade" the system and mix grounded circuits with ungrounded panels. Either leave it alone or start over, there is no halfway patch."